iPhoneAppProgrammingGuide

iPhoneAppProgrammingGuide
App Programming
Guide for iOS
Contents
About iOS App Architecture 7
At a Glance 7
Apps Are Expected to Support Key Features 8
Apps Follow Well-Defined Execution Paths 8
Apps Must Run Efficiently in a Multitasking Environment 8
Communication Between Apps Follows Specific Pathways 8
Performance Tuning is Important for Apps 9
How to Use This Document 9
Prerequisites 9
See Also 9
Expected App Behaviors 10
Providing the Required Resources 10
The App Bundle 11
The Information Property List File 14
Declaring the Required Device Capabilities 15
App Icons 15
App Launch (Default) Images 16
Supporting User Privacy 16
Internationalizing Your App 19
The App Life Cycle 21
The Main Function 21
The Structure of an App 22
The Main Run Loop 24
Execution States for Apps 26
App Termination 29
Threads and Concurrency 29
Background Execution 31
Executing Finite-Length Tasks 31
Downloading Content in the Background 33
Implementing Long-Running Tasks 34
Declaring Your App’s Supported Background Tasks 35
2014-09-17 | Copyright © 2014 Apple Inc. All Rights Reserved.
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Contents
Tracking the User’s Location 36
Playing and Recording Background Audio 37
Implementing a VoIP App 38
Fetching Small Amounts of Content Opportunistically 39
Using Push Notifications to Initiate a Download 40
Downloading Newsstand Content in the Background 40
Communicating with an External Accessory 41
Communicating with a Bluetooth Accessory 41
Getting the User’s Attention While in the Background 42
Understanding When Your App Gets Launched into the Background 43
Being a Responsible Background App 44
Opting Out of Background Execution 46
Strategies for Handling App State Transitions 47
What to Do at Launch Time 47
The Launch Cycle 48
Launching in Landscape Mode 52
Installing App-Specific Data Files at First Launch 53
What to Do When Your App Is Interrupted Temporarily 53
Responding to Temporary Interruptions 55
What to Do When Your App Enters the Foreground 56
Be Prepared to Process Queued Notifications 57
Handle iCloud Changes 58
Handle Locale Changes 59
Handle Changes to Your App’s Settings 59
What to Do When Your App Enters the Background 59
The Background Transition Cycle 60
Prepare for the App Snapshot 62
Reduce Your Memory Footprint 62
Strategies for Implementing Specific App Features 64
Privacy Strategies 64
Protecting Data Using On-Disk Encryption 64
Identifying Unique Users of Your App 65
Supporting Multiple Versions of iOS 66
Preserving Your App’s Visual Appearance Across Launches 67
Enabling State Preservation and Restoration in Your App 68
The Preservation and Restoration Process 68
What Happens When You Exclude Groups of View Controllers? 79
Checklist for Implementing State Preservation and Restoration 82
2014-09-17 | Copyright © 2014 Apple Inc. All Rights Reserved.
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Contents
Enabling State Preservation and Restoration in Your App 83
Preserving the State of Your View Controllers 83
Preserving the State of Your Views 87
Preserving Your App’s High-Level State 90
Tips for Saving and Restoring State Information 90
Tips for Developing a VoIP App 91
Configuring Sockets for VoIP Usage 92
Installing a Keep-Alive Handler 94
Configuring Your App’s Audio Session 94
Using the Reachability Interfaces to Improve the User Experience 94
Inter-App Communication 96
Supporting AirDrop 96
Sending Files and Data to Another App 96
Receiving Files and Data Sent to Your App 97
Using URL Schemes to Communicate with Apps 98
Sending a URL to Another App 98
Implementing Custom URL Schemes 98
Displaying a Custom Launch Image When a URL is Opened 103
Performance Tips 105
Reduce Your App’s Power Consumption 105
Use Memory Efficiently 107
Observe Low-Memory Warnings 107
Reduce Your App’s Memory Footprint 108
Allocate Memory Wisely 108
Tune Your Networking Code 109
Tips for Efficient Networking 109
Using Wi-Fi 110
The Airplane Mode Alert 110
Improve Your File Management 111
Make App Backups More Efficient 111
App Backup Best Practices 111
Files Saved During App Updates 113
Move Work off the Main Thread 113
Document Revision History 114
2014-09-17 | Copyright © 2014 Apple Inc. All Rights Reserved.
4
Figures, Tables, and Listings
Expected App Behaviors 10
Table 1-1
Table 1-2
A typical app bundle 11
Data protected by system authorization settings 18
The App Life Cycle 21
Figure 2-1
Figure 2-2
Figure 2-3
Table 2-1
Table 2-2
Table 2-3
Listing 2-1
Key objects in an iOS app 22
Processing events in the main run loop 25
State changes in an iOS app 28
The role of objects in an iOS app 23
Common types of events for iOS apps 25
App states 27
The main function of an iOS app 21
Background Execution 31
Table 3-1
Listing 3-1
Listing 3-2
Background modes for apps 35
Starting a background task at quit time 32
Scheduling an alarm notification 42
Strategies for Handling App State Transitions 47
Figure 4-1
Figure 4-2
Figure 4-3
Figure 4-4
Figure 4-5
Table 4-1
Launching an app into the foreground 49
Launching an app into the background 51
Handling alert-based interruptions 55
Transitioning from the background to the foreground 56
Moving from the foreground to the background 61
Notifications delivered to waking apps 57
Strategies for Implementing Specific App Features 64
Figure 5-1
Figure 5-2
Figure 5-3
Figure 5-4
Figure 5-5
Figure 5-6
Table 5-1
A sample view controller hierarchy 70
Adding restoration identifies to view controllers 73
High-level flow interface preservation 75
High-level flow for restoring your user interface 77
Excluding view controllers from the automatic preservation process 80
Loading the default set of view controllers 81
Configuring stream interfaces for VoIP usage 93
2014-09-17 | Copyright © 2014 Apple Inc. All Rights Reserved.
5
Figures, Tables, and Listings
Listing 5-1
Listing 5-2
Listing 5-3
Creating a new view controller during restoration 85
Encoding and decoding a view controller’s state. 87
Preserving the selection of a custom text view 89
Inter-App Communication 96
Figure 6-1
Figure 6-2
Table 6-1
Listing 6-1
Listing 6-2
Launching an app to open a URL 100
Waking a background app to open a URL 101
Keys and values of the CFBundleURLTypes property 99
Displaying an activity sheet on iPhone 96
Handling a URL request based on a custom scheme 102
Performance Tips 105
Table 7-1
Table 7-2
Tips for reducing your app’s memory footprint 108
Tips for allocating memory 109
2014-09-17 | Copyright © 2014 Apple Inc. All Rights Reserved.
6
About iOS App Architecture
Apps need to work with the iOS to ensure that they deliver a great user experience. Beyond just a good design
for your app’s design and user interface, a great user experience encompasses many other factors. Users expect
iOS apps to be fast and responsive while expecting the app to use as little power as possible. Apps need to
support all of the latest iOS devices while still appearing as if the app was tailored for the current device.
Implementing all of these behaviors can seem daunting at first but iOS provides the help you need to make
it happen.
This document highlights the core behaviors that make your app work well on iOS. You might not implement
every feature described in this document but you should consider these features for every project you create.
Note: Development of iOS apps requires an Intel-based Macintosh computer with the iOS SDK
installed. For information about how to get the iOS SDK, go to the iOS Dev Center.
At a Glance
When you are ready to take your ideas and turn them into an app, you need to understand the interactions
that occur between the system and your app.
2014-09-17 | Copyright © 2014 Apple Inc. All Rights Reserved.
7
About iOS App Architecture
At a Glance
Apps Are Expected to Support Key Features
The system expects every app to have some specific resources and configuration data, such as an app icon
and information about the capabilities of the app. Xcode provides some information with every new project
but you must any resource files and you must make sure the information in your project is correct before
submitting your app.
Relevant Chapter: Expected App Behaviors (page 10)
Apps Follow Well-Defined Execution Paths
From the time the user launches an app to the time it quits, apps follow a well-defined execution path. During
the life of an app, it can transition between foreground and background execution, it can be terminated and
relaunched, and it can go to sleep temporarily. Each time it transitions to a new state, the expectations for the
app change. A foreground app can do almost anything but background apps must do as little as possible. You
use the state transitions to adjust your app’s behaviors accordingly.
Relevant Chapter: The App Life Cycle (page 21), Strategies for Handling App State Transitions (page
47)
Apps Must Run Efficiently in a Multitasking Environment
Battery life is important for users, as is performance, responsiveness, and a great user experience. Minimizing
your app’s usage of the battery ensures that the user can run your app all day without having to recharge the
device, but launching and being ready to run quickly are also important. The iOS multitasking implementation
offers good battery life without sacrificing the responsiveness and user experience that users expect, but the
implementation requires apps to adopt system-provided behaviors.
Relevant Chapters: Background Execution (page 31), Strategies for Handling App State
Transitions (page 47)
Communication Between Apps Follows Specific Pathways
For security, iOS apps run in a sandbox and have limited interactions with other apps. When you want to
communicate with other apps on the system, there are specific ways to do so.
2014-09-17 | Copyright © 2014 Apple Inc. All Rights Reserved.
8
About iOS App Architecture
How to Use This Document
Relevant Chapter: Inter-App Communication (page 96)
Performance Tuning is Important for Apps
Every task performed by an app has a power cost associated with it. Apps that drain the user’s battery create
a negative user experience and are more likely to be deleted than those that appear to run for days on a single
charge. So be aware of the cost of different operations and take advantage of power-saving measures offered
by the system.
Relevant Chapter: Performance Tips (page 105)
How to Use This Document
This document is not a beginner’s guide to creating iOS apps. It is for developers who are ready to polish their
app before putting it in the App Store. Use this document as a guide to understanding how your app interacts
with the system and what it must do to make those interactions happen smoothly.
Prerequisites
This document provides detailed information about iOS app architecture and shows you how to implement
many app-level features. This book assumes that you have already installed the iOS SDK, configured your
development environment, and understand the basics of creating and implementing an app in Xcode.
If you are new to iOS app development, read Start Developing iOS Apps Today . That document offers a
step-by-step introduction to the development process to help you get up to speed quickly. It also includes a
hands-on tutorial that walks you through the app-creation process from start to finish, showing you how to
create a simple app and get it running quickly.
See Also
If you are learning about iOS, read iOS Technology Overview to learn about the technologies and features you
can incorporate into your iOS apps.
2014-09-17 | Copyright © 2014 Apple Inc. All Rights Reserved.
9
Expected App Behaviors
Every new Xcode project comes configured to run right away in iOS Simulator or on a device. But simply being
able to run on a device does not mean that your app is ready to ship on the App Store. Every app requires
some amount of customization to ensure a good experience for the user. Customizations can range from
providing an icon for your app to making architectural-level decisions about how your app presents and uses
information. This chapter describes the behaviors that all apps are expected to handle and that you should
consider early in the planning process.
Providing the Required Resources
Every app you create must have the following set of resources and metadata so that it can be displayed properly
on iOS devices:
●
An information property-list file. The Info.plist file contains metadata about your app, which the
system uses to interact with your app. Xcode creates this file for you automatically based on your project’s
configuration and settings. If you want to view or modify the contents of this file directly, you can do so
from the Info tab of your project. For information about editing this file and for recommendations about
what keys you should include, see The Information Property List File (page 14).
●
A declaration of the app’s required capabilities. Every app must declare the hardware capabilities or
features that it requires to run. The App Store uses this information to determine whether or not a user
can run your app on a specific device. You can edit your app’s list of requirements using the Required
device capabilities entry in the Info tab of your project. For information on how to configure this key, see
Declaring the Required Device Capabilities (page 15).
●
One or more icons. The system displays your app icon on the home screen of a user’s device. The system
may also use other versions of your icon in the Settings app or when displaying the results of a search.
For information about how to specify app icons, see App Icons (page 15).
●
One or more launch images. When an app is launched, the system displays a temporary image until the
app is able to present its user interface. This temporary image is your app’s launch image and it provides
the user with immediate feedback that your app is launching and will be ready soon. You must provide
at least one launch image for your app and you may provide additional launch images to address specific
scenarios. For information about creating your launch images, see App Launch (Default) Images (page
16).
2014-09-17 | Copyright © 2014 Apple Inc. All Rights Reserved.
10
Expected App Behaviors
Providing the Required Resources
These resources are required for all apps but are not the only ones you should include. There are many keys
that Xcode does not include in your app’s Info.plist file by default. Most of the additional keys are important
only if you incorporate specific features into your app. For example, an app that uses the microphone should
include the NSMicrophoneUsageDescription key and provide the user with information about how the
app intends to use it.
The App Bundle
When you build your iOS app, Xcode packages it as a bundle. A bundle is a directory in the file system that
groups related resources together in one place. An iOS app bundle contains the app executable file and
supporting resource files such as app icons, image files, and localized content. Table 1-1 lists the contents of
a typical iOS app bundle, which for demonstration purposes is called MyApp. This example is for illustrative
purposes only. Some of the files listed in this table may not appear in your own app bundles.
Table 1-1
A typical app bundle
File
Example
Description
App
executable
MyApp
The executable file contains your app’s compiled
code. The name of your app’s executable file is the
same as your app name minus the .app extension.
This file is required.
The
information
property list
file
Info.plist
App icons
Icon.png
The Info.plist file contains configuration data
for the app. The system uses this data to determine
how to interact with the app.
This file is required and must be called
Info.plist. For more information, see The
Information Property List File (page 14).
[email protected]
Icon-Small.png
[email protected]
Your app icon is used to represent your app on the
device’s Home screen. Other icons are used by the
system in appropriate places. Icons with @2x in
their filename are intended for devices with Retina
displays.
An app icon is required. For information about
specifying icon image files, see App Icons (page
15).
2014-09-17 | Copyright © 2014 Apple Inc. All Rights Reserved.
11
Expected App Behaviors
Providing the Required Resources
File
Example
Description
Launch images
Default.png
The system uses this file as a temporary
background while your app is launching. It is
removed as soon as your app is ready to display
its user interface.
Default-Portrait.png
Default-Landscape.png
At least one launch image is required. For
information about specifying launch images, see
App Launch (Default) Images (page 16).
Storyboard
files (or nib
files)
MainBoard.storyboard
Storyboards contain the views and view controllers
that the app presents on screen. Views in a
storyboard are organized according to the view
controller that presents them. Storyboards also
identify the transitions (called segues) that take
the user from one set of views to another.
The name of the main storyboard file is set by
Xcode when you create your project. You can
change the name by assigning a different value to
the UIMainStoryboardFile key in the
Info.plist file.) Apps that use nib files instead
of storyboards can replace the
UIMainStoryboardFile key with the
NSMainNibFile key and use that key to specify
their main nib file.
The use of storyboards (or nib files) is optional but
recommended.
Ad hoc
distribution
icon
iTunesArtwork
If you are distributing your app ad hoc, include a
512 x 512 pixel version of your app icon. This icon
is normally provided by the App Store from the
materials you submit to iTunes Connect. However,
because apps distributed ad hoc do not go through
the App Store, your icon must be present in your
app bundle instead. iTunes uses this icon to
represent your app. (The file you specify should be
the same one you would have submitted to the
App Store, if you were distributing your app that
way.)
The filename of this icon must be iTunesArtwork
and must not include a filename extension. This
file is required for ad hoc distribution but is
optional otherwise.
2014-09-17 | Copyright © 2014 Apple Inc. All Rights Reserved.
12
Expected App Behaviors
Providing the Required Resources
File
Example
Description
Settings
bundle
Settings.bundle
If you want to expose custom app preferences
through the Settings app, you must include a
settings bundle. This bundle contains the property
list data and other resource files that define your
app preferences. The Settings app uses the
information in this bundle to assemble the
interface elements required by your app.
This bundle is optional. For more information about
preferences and specifying a settings bundle, see
Preferences and Settings Programming Guide .
Nonlocalized
resource files
sun.png
Subdirectories
for localized
resources
en.lproj
mydata.plist
fr.lproj
es.lproj
Nonlocalized resources include things like images,
sound files, movies, and custom data files that your
app uses. All of these files should be placed at the
top level of your app bundle.
Localized resources must be placed in
language-specific project directories, the names
for which consist of an ISO 639-1 language
abbreviation plus the .lproj suffix. (For example,
the en.lproj, fr.lproj, and es.lproj
directories contain resources localized for English,
French, and Spanish.)
An iOS app should be internationalized and have
a language .lproj directory for each language it
supports. In addition to providing localized versions
of your app’s custom resources, you can also
localize your app icon, launch images, and Settings
icon by placing files with the same name in your
language-specific project directories.
For more information, see Internationalizing Your
App (page 19).
For more information about the structure of an iOS app bundle, see Bundle Programming Guide . For information
about how to load resource files from your bundle, see Resource Programming Guide .
2014-09-17 | Copyright © 2014 Apple Inc. All Rights Reserved.
13
Expected App Behaviors
Providing the Required Resources
The Information Property List File
Xcode uses information from the General, Capabilities, and Info tabs of your project to generate an information
property list (Info.plist) file for your app at compile time. The Info.plist file is a structured file that
contains critical information about your app’s configuration. It is used by the App Store and by iOS to determine
your app’s capabilities and to locate key resources. Every app must include this file.
Although the Info.plist file provided by Xcode includes default values for all of the required entries, most
apps require some changes or additions. Whenever possible, use the General and Capabilities tabs to specify
the configuration information for your app. Those tabs contain the most common configuration options
available for apps. If you do not see a specific option on either of those tabs, use the Info tab.
For options where Xcode does not provide a custom configuration interface, you must provide appropriate
keys and values. The Custom iOS Target Properties section of the Info tab contains a summary of the entries
to be included in the Info.plist file. By default, Xcode displays human-readable descriptions of the intended
feature but each feature actually corresponds to a unique key in the Info.plist file. Most keys are optional
and used infrequently, but there are a handful of keys that you should consider when defining any new project:
●
Declare your app’s required capabilities in the Info tab. The Required device capabilities section contains
information about the device-level features that your app requires to run. The App Store uses the
information in this entry to determine the capabilities of your app and to prevent it from being installed
on devices that do not support features your app requires. For more information, see Declaring the Required
Device Capabilities (page 15).
●
Apps that require a persistent Wi-Fi connection must declare that fact. If your app talks to a server
across the network, you can add the Application uses Wi-Fi entry to the Info tab of your project. This entry
corresponds to the UIRequiresPersistentWiFi key in the Info.plist file. Setting this key to YES
prevents iOS from closing the active Wi-Fi connection when it has been inactive for an extended period
of time. This key is recommended for all apps that use the network to communicate with a server.
●
Newsstand apps must declare themselves as such. Include the UINewsstandApp key to indicate that
your app presents content from the Newsstand app.
●
Apps that define custom document types must declare those types. Use the Document Types section
of the Info tab to specify icons and UTI information for the document formats that you support. The system
uses this information to identify apps capable of handling specific file types. For more information about
adding document support to your app, see Document-Based App Programming Guide for iOS .
●
Apps can declare any custom URL schemes they support. Use the URL Types section of the Info tab to
specify the custom URL schemes that your app handles. Apps can use custom URL schemes to communicate
with each other. For more information about how to implement support for this feature, see Using URL
Schemes to Communicate with Apps (page 98).
2014-09-17 | Copyright © 2014 Apple Inc. All Rights Reserved.
14
Expected App Behaviors
Providing the Required Resources
●
Apps should provide usage descriptions for certain app features. Whenever there is a privacy concern
about an app accessing a user’s data or a device’s capabilities, iOS frameworks prompt the user and request
permission for your app to use the feature. Apps that use these features should provide privacy usage
descriptions that explain what your app plans to do with the corresponding data. For information about
the features that require user permission, see Table 1-2 (page 18).
For detailed information about the keys and values you can include in the Info.plist file, see Information
Property List Key Reference .
Declaring the Required Device Capabilities
All apps must declare the device-specific capabilities they need to run. Xcode includes a Required device
capabilities entry in your project’s Info tab and populates it with some minimum requirements. You can add
values to this entry to specify additional requirements for your app. The Required device capabilities entry
corresponds to the UIRequiredDeviceCapabilities key in your app’s Info.plist file.
The value of the UIRequiredDeviceCapabilities key is either an array or dictionary that contains additional
keys identifying features your app requires (or specifically prohibits). If you specify the value of the key using
an array, the presence of a key indicates that the feature is required; the absence of a key indicates that the
feature is not required and that the app can run without it. If you specify a dictionary instead, each key in the
dictionary must have a Boolean value that indicates whether the feature is required or prohibited. A value of
true indicates the feature is required and a value of false indicates that the feature must not be present on
the device. If a given capability is optional for your app, do not include the corresponding key in the dictionary.
For detailed information on the values you can include for the UIRequiredDeviceCapabilities key, see
Information Property List Key Reference .
App Icons
Every app must provide an icon to be displayed on a device’s Home screen and in the App Store. An app may
actually specify several different icons for use in different situations. For example, an app can provide a small
icon to use when displaying search results and can provide a high-resolution icon for devices with Retina
displays.
New Xcode projects include image asset entries for your app’s icon images. To add icons, assign the
corresponding image files to the image assets of your project. At build time, Xcode adds the appropriate keys
to your app’s Info.plist file and places the images in your app bundle.
For information about designing your app icons, including the sizes of those icons, see iOS Human Interface
Guidelines .
2014-09-17 | Copyright © 2014 Apple Inc. All Rights Reserved.
15
Expected App Behaviors
Supporting User Privacy
App Launch (Default) Images
When the system launches an app for the first time on a device, it temporarily displays a static launch image
on the screen. This image is your app’s launch image and it is a resource that you specify in your Xcode project.
Launch images provide the user with immediate feedback that your app has launched while giving your app
time to prepare its initial user interface. When your app’s window is configured and ready to be displayed, the
system swaps out the launch image for that window.
When a recent snapshot of your app’s user interface is available, the system prefers the use of that image over
the use of your app’s launch images. The system takes a snapshot of your app’s user interface when your app
transitions from the foreground to the background. When your app returns to the foreground, it uses that
image instead of a launch image whenever possible. In cases where the user has killed your app or your app
has not run for a long time, the system discards the snapshot and relies once again on your launch images.
New Xcode projects include image asset entries for your app’s launch images. To add launch images, add the
corresponding image files to the image assets of your project. At build time, Xcode adds the appropriate keys
to your app’s Info.plist file and places the images in your app bundle.
For information about designing your app’s launch images, including the sizes of those images, see iOS Human
Interface Guidelines .
Supporting User Privacy
Maintaining user privacy should be an important consideration when designing your app. Most iOS devices
contain user and device data that users might not want to expose to apps or external entities. Remember that
the user might delete your app if it uses data in an inappropriate way.
Access user or device data only with the user’s informed consent obtained in accordance with applicable law.
In addition, take appropriate steps to protect user and device data and be transparent about how you use it.
Here are some best practices that you can take:
●
Review guidelines from government or industry sources, including the following documents:
●
The Federal Trade Commission’s report on mobile privacy: Mobile Privacy Disclosures: Building Trust
Through Transparency.
●
The EU Data Protection Commissioners’ Opinion on data protection for Mobile Apps: http://ec.europa.eu/justice/data-protection/article-29/documentation/opinion-recommendation/files/2013/wp202_en.pdf
●
The Japanese Ministry of Internal Affairs and Communications’ Smartphone Privacy Initiatives:
●
Smartphone Privacy Initiative (2012):
English: http://www.soumu.go.jp/main_sosiki/joho_tsusin/eng/presentation/pdf/Initiative.pdf
2014-09-17 | Copyright © 2014 Apple Inc. All Rights Reserved.
16
Expected App Behaviors
Supporting User Privacy
Japanese: http://www.soumu.go.jp/main_content/000171225.pdf
●
Smartphone Privacy Initiative II (2013):
English: http://www.soumu.go.jp/main_sosiki/joho_tsusin/eng/presentation/pdf/Summary_II.pdf
Japanese: http://www.soumu.go.jp/main_content/000247654.pdf
●
The California State Attorney General’s recommendations for mobile privacy: Privacy on the Go: Recommendations for the Mobile Ecosystem
These reports provide helpful recommendations for protecting user privacy. You should also review these
documents with your company’s legal counsel.
●
Request access to user or device data that is protected by the iOS system authorization settings at the
time the data is needed. Consider supplying a usage description string in your app’s Info.plist file
explaining why your app needs that data. Data protected by iOS system authorization settings includes
location data, contacts, calendar events, reminders, photos, and media; see Table 1-2 (page 18). Provide
reasonable fallback behavior in situations where the user does not grant access to the requested data.
●
Be transparent with users about how their data is going to be used. For example, you should specify a
URL for your privacy policy or statement with your iTunes Connect metadata when you submit your app,
and you might also want to summarize that policy in your app description.
For more information about providing your app’s privacy policy in iTunes Connect, see Adding an App in
iTunes Connect.
●
Give the user control over their user or device data. Provide settings so that the user can disable access
to certain types of sensitive information as needed.
●
Request and use the minimum amount of user or device data needed to accomplish a given task. Do not
seek access to or collect data for non obvious reasons, for unnecessary reasons, or because you think it
might be useful later.
●
Take reasonable steps to protect the user and device data that you collect in your apps. When storing
such information locally, try to use the iOS data protection feature (described in Protecting Data Using
On-Disk Encryption (page 64)) to store it in an encrypted format. And try to use HTTPS when sending user
or device data over the network.
●
If your app uses the ASIdentifierManager class, you must respect the value of its
advertisingTrackingEnabled property. And if that property is set to NO by the user, then use the
ASIdentifierManager class only for Limited Advertising Purposes. “Limited Advertising Purposes”
means frequency capping, attribution, conversion events, estimating the number of unique users, advertising
fraud detection, debugging for advertising purposes only, and other uses for advertising that may be
permitted by Apple in Documentation for the Ad Support APIs.
2014-09-17 | Copyright © 2014 Apple Inc. All Rights Reserved.
17
Expected App Behaviors
Supporting User Privacy
●
If you have not already done so, stop using the unique device identifier (UDID) provided by the
uniqueIdentifier property of the UIDevice class. That property was deprecated in iOS 5.0, and the
App Store does not accept new apps or app updates that use that identifier. Instead, apps should use the
identifierForVendor property of the UIDevice class or the advertisingIdentifier property of
the ASIdentifierManager class, as appropriate.
●
If your app supports audio input, configure your audio session for recording only at the point where you
actually plan to begin recording. Do not configure your audio session for recording at launch time if you
do not plan to record right away. In iOS 7, the system alerts users when apps configure their audio session
for recording and gives the user the option to disable recording for your app.
Table 1-2 lists the types of data authorizations supported by iOS. Using the services listed in this table causes
an alert to be displayed to the user requesting permission to do so. You can determine if the user authorized
your app for a service using the API listed for each item. You should view this table as a starting point for your
app’s own privacy behaviors and not as a finite checklist. The contents of this table may evolve over time.
Table 1-2
Data protected by system authorization settings
Data
System authorization support
Location
The current authorization status for location data is available from the
authorizationStatus class method of CLLocationManager. In requesting
authorization in iOS 8 and later, you must use the requestWhenInUseAuthorization or requestAlwaysAuthorization method and include the
NSLocationWhenInUseUsageDescription or NSLocationAlwaysUsageDescription key in your Info.plist file to indicate the level of
authorization you require.
Photos
The authorization status for photo data is available from the authorizationStatus
method of ALAssetsLibrary. To inform the user about how you intend to use this
information, include the NSPhotoLibraryUsageDescription key in your
Info.plist file.
Music, video,
and other
media assets
The authorization status for media assets is available from the
authorizationStatus method of ALAssetsLibrary.
Contacts
The authorization status for contact data is available from the
ABAddressBookGetAuthorizationStatus function. To inform the user about
how you intend to use this information, include the NSContactsUsageDescription
key in your Info.plist file.
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18
Expected App Behaviors
Internationalizing Your App
Data
System authorization support
Calendar data
The authorization status for calendar data is available from the
authorizationStatusForEntityType: method of EKEventStore. To inform
the user about how you intend to use this information, include the
NSCalendarsUsageDescription key in your Info.plist file.
Reminders
The authorization status for reminder data is available from the
authorizationStatusForEntityType: method of EKEventStore. To inform
the user about how you intend to use this information, include the
NSRemindersUsageDescription key in your Info.plist file.
Bluetooth
peripherals
The authorization status for Bluetooth peripherals is available from the state property
of CBCentralManager. To inform the user about how you intend to use Bluetooth,
include the NSBluetoothPeripheralUsageDescription key in your Info.plist
file.
Microphone
In iOS 7 and later, the authorization status for the microphone is available from the
requestRecordPermission: method of AVAudioSession. To inform the user
about how you intend to use the microphone, include the NSMicrophoneUsageDescription key in your Info.plist file.
Camera
In iOS 7 and later, the authorization status for the camera is available in
deviceInputWithDevice: error: method of AVCaptureDeviceInput. To
inform the user about how you intend to use the camera, include the
NSCameraUsageDescription key in your Info.plist file.
Internationalizing Your App
Because iOS apps are distributed in many countries, localizing your app’s content can help you reach many
more customers. Users are much more likely to use an app when it is localized for their native language. When
you factor your user-facing content into resource files, localizing that content is a relatively simple process.
Before you can localize your content, you must internationalize your app in order to facilitate the localization
process. Internationalizing your app involves factoring out any user-facing content into localizable resource
files and providing language-specific project (.lproj) directories for storing that content. It also means using
appropriate technologies (such as date and number formatters) when working with language-specific and
locale-specific content.
For a fully internationalized app, the localization process creates new sets of language-specific resource files
for you to add to your project. A typical iOS app requires localized versions of the following types of resource
files:
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19
Expected App Behaviors
Internationalizing Your App
●
Storyboard files (or nib files)—Storyboards can contain text labels and other content that need to be
localized. You might also want to adjust the position of interface items to accommodate changes in text
length. (Similarly, nib files can contain text that needs to be localized or layout that needs to be updated.)
●
Strings files—Strings files (so named because of their .strings filename extension) contain localized
versions of the static text that your app displays.
●
Image files—You should avoid localizing images unless the images contain culture-specific content.
Whenever possible, you should avoid storing text directly in your image files. For images that you load
and use from within your app, store text in a strings file and composite that text with your image-based
content at runtime.
●
Video and audio files—You should avoid localizing multimedia files unless they contain language-specific
or culture-specific content. For example, you would want to localize a video file that contained a voice-over
track.
For information about the internationalization and localization process, see Internationalization and Localization
Guide . For information about the proper way to use resource files in your app, see Resource Programming
Guide .
2014-09-17 | Copyright © 2014 Apple Inc. All Rights Reserved.
20
The App Life Cycle
Apps are a sophisticated interplay between your custom code and the system frameworks. The system
frameworks provide the basic infrastructure that all apps need to run, and you provide the code required to
customize that infrastructure and give the app the look and feel you want. To do that effectively, it helps to
understand a little bit about the iOS infrastructure and how it works.
iOS frameworks rely on design patterns such as model-view-controller and delegation in their implementation.
Understanding those design patterns is crucial to the successful creation of an app. It also helps to be familiar
with the Objective-C language and its features. If you are new to iOS programming, read Start Developing iOS
Apps Today for an introduction to iOS apps and the Objective-C language.
The Main Function
The entry point for every C-based app is the main function and iOS apps are no different. What is different is
that for iOS apps you do not write the main function yourself. Instead, Xcode creates this function as part of
your basic project. Listing 2-1 shows an example of this function. With few exceptions, you should never change
the implementation of the main function that Xcode provides.
Listing 2-1
The main function of an iOS app
#import <UIKit/UIKit.h>
#import "AppDelegate.h"
int main(int argc, char * argv[])
{
@autoreleasepool {
return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate
class]));
}
}
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21
The App Life Cycle
The Structure of an App
The only thing to mention about the main function is that its job is to hand control off to the UIKit framework.
The UIApplicationMain function handles this process by creating the core objects of your app, loading
your app’s user interface from the available storyboard files, calling your custom code so that you have a chance
to do some initial setup, and putting the app’s run loop in motion. The only pieces that you have to provide
are the storyboard files and the custom initialization code.
The Structure of an App
During startup, the UIApplicationMain function sets up several key objects and starts the app running. At
the heart of every iOS app is the UIApplication object, whose job is to facilitate the interactions between
the system and other objects in the app. Figure 2-1 shows the objects commonly found in most apps, while
Table 2-1 lists the roles each of those objects plays. The first thing to notice is that iOS apps use a
model-view-controller architecture. This pattern separates the app’s data and business logic from the visual
presentation of that data. This architecture is crucial to creating apps that can run on different devices with
different screen sizes.
Figure 2-1
Key objects in an iOS app
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22
The App Life Cycle
The Structure of an App
Table 2-1
The role of objects in an iOS app
Object
Description
UIApplication
The UIApplication object manages the event loop and other high-level app
behaviors. It also reports key app transitions and some special events (such as
incoming push notifications) to its delegate, which is a custom object you define.
Use the UIApplication object as is—that is, without subclassing.
object
App delegate
object
The app delegate is the heart of your custom code. This object works in tandem
with the UIApplication object to handle app initialization, state transitions,
and many high-level app events. This object is also the only one guaranteed to
be present in every app, so it is often used to set up the app’s initial data structures.
Documents and
data model
objects
Data model objects store your app’s content and are specific to your app. For
example, a banking app might store a database containing financial transactions,
whereas a painting app might store an image object or even the sequence of
drawing commands that led to the creation of that image. (In the latter case, an
image object is still a data object because it is just a container for the image data.)
Apps can also use document objects (custom subclasses of UIDocument) to
manage some or all of their data model objects. Document objects are not required
but offer a convenient way to group data that belongs in a single file or file
package. For more information about documents, see Document-Based App
Programming Guide for iOS .
View controller
objects
View controller objects manage the presentation of your app’s content on screen.
A view controller manages a single view and its collection of subviews. When
presented, the view controller makes its views visible by installing them in the
app’s window.
The UIViewController class is the base class for all view controller objects. It
provides default functionality for loading views, presenting them, rotating them
in response to device rotations, and several other standard system behaviors. UIKit
and other frameworks define additional view controller classes to implement
standard system interfaces such as the image picker, tab bar interface, and
navigation interface.
For detailed information about how to use view controllers, see View Controller
Programming Guide for iOS .
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23
The App Life Cycle
The Main Run Loop
Object
Description
UIWindow object
A UIWindow object coordinates the presentation of one or more views on a screen.
Most apps have only one window, which presents content on the main screen,
but apps may have an additional window for content displayed on an external
display.
To change the content of your app, you use a view controller to change the views
displayed in the corresponding window. You never replace the window itself.
In addition to hosting views, windows work with the UIApplication object to
deliver events to your views and view controllers.
View objects,
control objects,
and layer objects
Views and controls provide the visual representation of your app’s content. A view
is an object that draws content in a designated rectangular area and responds to
events within that area. Controls are a specialized type of view responsible for
implementing familiar interface objects such as buttons, text fields, and toggle
switches.
The UIKit framework provides standard views for presenting many different types
of content. You can also define your own custom views by subclassing UIView
(or its descendants) directly.
In addition to incorporating views and controls, apps can also incorporate Core
Animation layers into their view and control hierarchies. Layer objects are actually
data objects that represent visual content. Views use layer objects intensively
behind the scenes to render their content. You can also add custom layer objects
to your interface to implement complex animations and other types of
sophisticated visual effects.
What distinguishes one iOS app from another is the data it manages (and the corresponding business logic)
and how it presents that data to the user. Most interactions with UIKit objects do not define your app but help
you to refine its behavior. For example, the methods of your app delegate let you know when the app is
changing states so that your custom code can respond appropriately.
The Main Run Loop
An app’s main run loop processes all user-related events. The UIApplication object sets up the main run
loop at launch time and uses it to process events and handle updates to view-based interfaces. As the name
suggests, the main run loop executes on the app’s main thread. This behavior ensures that user-related events
are processed serially in the order in which they were received.
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24
The App Life Cycle
The Main Run Loop
Figure 2-2 shows the architecture of the main run loop and how user events result in actions taken by your
app. As the user interacts with a device, events related to those interactions are generated by the system and
delivered to the app via a special port set up by UIKit. Events are queued internally by the app and dispatched
one-by-one to the main run loop for execution. The UIApplication object is the first object to receive the
event and make the decision about what needs to be done. A touch event is usually dispatched to the main
window object, which in turn dispatches it to the view in which the touch occurred. Other events might take
slightly different paths through various app objects.
Figure 2-2
Processing events in the main run loop
Many types of events can be delivered in an iOS app. The most common ones are listed in Table 2-2. Many of
these event types are delivered using the main run loop of your app, but some are not. Some events are sent
to a delegate object or are passed to a block that you provide. For information about how to handle most
types of events—including touch, remote control, motion, accelerometer, and gyroscopic events—see Event
Handling Guide for iOS .
Table 2-2
Common types of events for iOS apps
Event type
Delivered to…
Notes
Touch
The view object in
which the event
occurred
Views are responder objects. Any touch events not
handled by the view are forwarded down the
responder chain for processing.
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25
The App Life Cycle
Execution States for Apps
Event type
Delivered to…
Notes
Remote control
First responder object
Remote control events are for controlling media
playback and are generated by headphones and other
accessories.
The object you
designate
Events related to the accelerometer, magnetometer,
and gyroscope hardware are delivered to the object
you designate.
Location
The object you
designate
You register to receive location events using the Core
Location framework. For more information about using
Core Location, see Location and Maps Programming
Guide .
Redraw
The view that needs
the update
Redraw events do not involve an event object but are
simply calls to the view to draw itself. The drawing
architecture for iOS is described in Drawing and
Printing Guide for iOS .
Shake motion
events
Accelerometer
Magnetometer
Gyroscope
Some events, such as touch and remote control events, are handled by your app’s responder objects. Responder
objects are everywhere in your app. (The UIApplication object, your view objects, and your view controller
objects are all examples of responder objects.) Most events target a specific responder object but can be passed
to other responder objects (via the responder chain) if needed to handle an event. For example, a view that
does not handle an event can pass the event to its superview or to a view controller.
Touch events occurring in controls (such as buttons) are handled differently than touch events occurring in
many other types of views. There are typically only a limited number of interactions possible with a control,
and so those interactions are repackaged into action messages and delivered to an appropriate target object.
This target-action design pattern makes it easy to use controls to trigger the execution of custom code in your
app.
Execution States for Apps
At any given moment, your app is in one of the states listed in Table 2-3. The system moves your app from
state to state in response to actions happening throughout the system. For example, when the user presses
the Home button, a phone call comes in, or any of several other interruptions occurs, the currently running
apps change state in response. Figure 2-3 (page 28) shows the paths that an app takes when moving from
state to state.
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26
The App Life Cycle
Execution States for Apps
Table 2-3
App states
State
Description
Not running
The app has not been launched or was running but was terminated by the system.
Inactive
The app is running in the foreground but is currently not receiving events. (It may be
executing other code though.) An app usually stays in this state only briefly as it
transitions to a different state.
Active
The app is running in the foreground and is receiving events. This is the normal mode
for foreground apps.
Background
The app is in the background and executing code. Most apps enter this state briefly
on their way to being suspended. However, an app that requests extra execution time
may remain in this state for a period of time. In addition, an app being launched directly
into the background enters this state instead of the inactive state. For information
about how to execute code while in the background, see Background Execution (page
31).
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27
The App Life Cycle
Execution States for Apps
State
Description
Suspended
The app is in the background but is not executing code. The system moves apps to
this state automatically and does not notify them before doing so. While suspended,
an app remains in memory but does not execute any code.
When a low-memory condition occurs, the system may purge suspended apps without
notice to make more space for the foreground app.
Figure 2-3
State changes in an iOS app
Most state transitions are accompanied by a corresponding call to the methods of your app delegate object.
These methods are your chance to respond to state changes in an appropriate way. These methods are listed
below, along with a summary of how you might use them.
●
application:willFinishLaunchingWithOptions:—This method is your app’s first chance to
execute code at launch time.
●
application:didFinishLaunchingWithOptions:—This method allows you to perform any final
initialization before your app is displayed to the user.
●
applicationDidBecomeActive:—Lets your app know that it is about to become the foreground app.
Use this method for any last minute preparation.
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28
The App Life Cycle
App Termination
●
applicationWillResignActive:—Lets you know that your app is transitioning away from being the
foreground app. Use this method to put your app into a quiescent state.
●
applicationDidEnterBackground:—Lets you know that your app is now running in the background
and may be suspended at any time.
●
applicationWillEnterForeground:—Lets you know that your app is moving out of the background
and back into the foreground, but that it is not yet active.
●
applicationWillTerminate:—Lets you know that your app is being terminated. This method is not
called if your app is suspended.
App Termination
Apps must be prepared for termination to happen at any time and should not wait to save user data or perform
other critical tasks. System-initiated termination is a normal part of an app’s life cycle. The system usually
terminates apps so that it can reclaim memory and make room for other apps being launched by the user, but
the system may also terminate apps that are misbehaving or not responding to events in a timely manner.
Suspended apps receive no notification when they are terminated; the system kills the process and reclaims
the corresponding memory. If an app is currently running in the background and not suspended, the system
calls the applicationWillTerminate: of its app delegate prior to termination. The system does not call
this method when the device reboots.
In addition to the system terminating your app, the user can terminate your app explicitly using the multitasking
UI. User-initiated termination has the same effect as terminating a suspended app. The app’s process is killed
and no notification is sent to the app.
Threads and Concurrency
The system creates your app’s main thread and you can create additional threads, as needed, to perform other
tasks. For iOS apps, the preferred technique is to use Grand Central Dispatch (GCD), operation objects, and
other asynchronous programming interfaces rather than creating and managing threads yourself. Technologies
such as GCD let you define the work you want to do and the order you want to do it in, but let the system
decide how best to execute that work on the available CPUs. Letting the system handle the thread management
simplifies the code you must write, makes it easier to ensure the correctness of that code, and offers better
overall performance.
When thinking about threads and concurrency, consider the following:
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29
The App Life Cycle
Threads and Concurrency
●
Work involving views, Core Animation, and many other UIKit classes usually must occur on the app’s main
thread. There are some exceptions to this rule—for example, image-based manipulations can often occur
on background threads—but when in doubt, assume that work needs to happen on the main thread.
●
Lengthy tasks (or potentially length tasks) should always be performed on a background thread. Any tasks
involving network access, file access, or large amounts of data processing should all be performed
asynchronously using GCD or operation objects.
●
At launch time, move tasks off the main thread whenever possible. At launch time, your app should use
the available time to set up its user interface as quickly as possible. Only tasks that contribute to setting
up the user interface should be performed on the main thread. All other tasks should be executed
asynchronously, with the results displayed to the user as soon as they are ready.
For more information about using GCD and operation objects to execute tasks, see Concurrency Programming
Guide .
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30
Background Execution
When the user is not actively using your app, the system moves it to the background state. For many apps,
the background state is just a brief stop on the way to the app being suspended. Suspending apps is a way of
improving battery life it also allows the system to devote important system resources to the new foreground
app that has drawn the user’s attention.
Most apps can move to the extended state easily enough but there are also legitimate reasons for apps to
continue running in the background. A hiking app might want to track the user’s position over time so that it
can display that course overlaid on top of a hiking map. An audio app might need to continue playing music
over the lock screen. Other apps might want to download content in the background so that it can minimize
the delay in presenting that content to the user. When you find it necessary to keep your app running in the
background, iOS helps you do so efficiently and without draining system resources or the user’s battery. The
techniques offered by iOS fall into three categories:
●
Apps that start a short task in the foreground can ask for time to finish that task when the app moves to
the background.
●
Apps that initiate downloads in the foreground can hand off management of those downloads to the
system, thereby allowing the app to be suspended or terminated while the download continues.
●
Apps that need to run in the background to support specific types of tasks can declare their support for
one or more background execution modes.
Always try to avoid doing any background work unless doing so improves the overall user experience. An app
might move to the background because the user launched a different app or because the user locked the
device and is not using it right now. In both situations, the user is signaling that your app does not need to be
doing any meaningful work right now. Continuing to run in such conditions will only drain the device’s battery
and might lead the user to force quit your app altogether. So be mindful about the work you do in the
background and avoid it when you can.
Executing Finite-Length Tasks
Apps moving to the background are expected to put themselves into a quiescent state as quickly as possible
so that they can be suspended by the system. If your app is in the middle of a task and needs a little extra time
to complete that task, it can call the beginBackgroundTaskWithName:expirationHandler: or
beginBackgroundTaskWithExpirationHandler: method of the UIApplication object to request some
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31
Background Execution
Executing Finite-Length Tasks
additional execution time. Calling either of these methods delays the suspension of your app temporarily,
giving it a little extra time to finish its work. Upon completion of that work, your app must call the
endBackgroundTask: method to let the system know that it is finished and can be suspended.
Each call to the beginBackgroundTaskWithName:expirationHandler: or
beginBackgroundTaskWithExpirationHandler: method generates a unique token to associate with
the corresponding task. When your app completes a task, it must call the endBackgroundTask: method with
the corresponding token to let the system know that the task is complete. Failure to call the
endBackgroundTask: method for a background task will result in the termination of your app. If you provided
an expiration handler when starting the task, the system calls that handler and gives you one last chance to
end the task and avoid termination.
You do not need to wait until your app moves to the background to designate background tasks. A more
useful design is to call the beginBackgroundTaskWithName:expirationHandler: or
beginBackgroundTaskWithExpirationHandler: method before starting a task and call the
endBackgroundTask: method as soon as you finish. You can even follow this pattern while your app is
executing in the foreground.
Listing 3-1 shows how to start a long-running task when your app transitions to the background. In this example,
the request to start a background task includes an expiration handler just in case the task takes too long. The
task itself is then submitted to a dispatch queue for asynchronous execution so that the
applicationDidEnterBackground: method can return normally. The use of blocks simplifies the code
needed to maintain references to any important variables, such as the background task identifier. The bgTask
variable is a member variable of the class that stores a pointer to the current background task identifier and
is initialized prior to its use in this method.
Listing 3-1
Starting a background task at quit time
- (void)applicationDidEnterBackground:(UIApplication *)application
{
bgTask = [application beginBackgroundTaskWithName:@"MyTask" expirationHandler:^{
// Clean up any unfinished task business by marking where you
// stopped or ending the task outright.
[application endBackgroundTask:bgTask];
bgTask = UIBackgroundTaskInvalid;
}];
// Start the long-running task and return immediately.
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32
Background Execution
Downloading Content in the Background
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0),
^{
// Do the work associated with the task, preferably in chunks.
[application endBackgroundTask:bgTask];
bgTask = UIBackgroundTaskInvalid;
});
}
Note: Always provide an expiration handler when starting a task, but if you want to know how
much time your app has left to run, get the value of the backgroundTimeRemaining property of
UIApplication.
In your own expiration handlers, you can include additional code needed to close out your task. However, any
code you include must not take too long to execute because, by the time your expiration handler is called,
your app is already very close to its time limit. For this reason, perform only minimal cleanup of your state
information and end the task.
Downloading Content in the Background
When downloading files, apps should use an NSURLSession object to start the downloads so that the system
can take control of the download process in case the app is suspended or terminated. When you configure an
NSURLSession object for background transfers, the system manages those transfers in a separate process
and reports status back to your app in the usual way. If your app is terminated while transfers are ongoing,
the system continues the transfers in the background and launches your app (as appropriate) when the transfers
finish or when one or more tasks need your app’s attention.
To support background transfers, you must configure your NSURLSession object appropriately. To configure
the session, you must first create a NSURLSessionConfiguration object and set several properties to
appropriate values. You then pass that configuration object to the appropriate initialization method of
NSURLSession when creating your session.
The process for creating a configuration object that supports background downloads is as follows:
1.
Create the configuration object using the backgroundSessionConfigurationWithIdentifier:
method of NSURLSessionConfiguration.
2.
Set the value of the configuration object’s sessionSendsLaunchEvents property to YES.
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33
Background Execution
Implementing Long-Running Tasks
3.
if your app starts transfers while it is in the foreground, it is recommend that you also set the
discretionary property of the configuration object to YES.
4.
Configure any other properties of the configuration object as appropriate.
5.
Use the configuration object to create your NSURLSession object.
Once configured, your NSURLSession object seamlessly hands off upload and download tasks to the system
at appropriate times. If tasks finish while your app is still running (either in the foreground or the background),
the session object notifies its delegate in the usual way. If tasks have not yet finished and the system terminates
your app, the system automatically continues managing the tasks in the background. If the user terminates
your app, the system cancels any pending tasks.
When all of the tasks associated with a background session are complete, the system relaunches a terminated
app (assuming that the sessionSendsLaunchEvents property was set to YES and that the user did not
force quit the app) and calls the app delegate’s
application:handleEventsForBackgroundURLSession:completionHandler: method. (The system
may also relaunch the app to handle authentication challenges or other task-related events that require your
app’s attention.) In your implementation of that delegate method, use the provided identifier to create a new
NSURLSessionConfiguration and NSURLSession object with the same configuration as before. The system
reconnects your new session object to the previous tasks and reports their status to the session object’s
delegate.
Implementing Long-Running Tasks
For tasks that require more execution time to implement, you must request specific permissions to run them
in the background without their being suspended. In iOS, only specific app types are allowed to run in the
background:
●
Apps that play audible content to the user while in the background, such as a music player app
●
Apps that record audio content while in the background
●
Apps that keep users informed of their location at all times, such as a navigation app
●
Apps that support Voice over Internet Protocol (VoIP)
●
Apps that need to download and process new content regularly
●
Apps that receive regular updates from external accessories
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Background Execution
Implementing Long-Running Tasks
Apps that implement these services must declare the services they support and use system frameworks to
implement the relevant aspects of those services. Declaring the services lets the system know which services
you use, but in some cases it is the system frameworks that actually prevent your application from being
suspended.
Declaring Your App’s Supported Background Tasks
Support for some types of background execution must be declared in advance by the app that uses them. In
Xcode 5 and later, you declare the background modes your app supports from the Capabilities tab of your
project settings. Enabling the Background Modes option adds the UIBackgroundModes key to your app’s
Info.plist file. Selecting one or more checkboxes adds the corresponding background mode values to that
key. Table 3-1 lists the background modes you can specify and the values that Xcode assigns to the
UIBackgroundModes key in your app’s Info.plist file.
Table 3-1
Background modes for apps
Xcode
UIBackgroundModes value
Description
audio
The app plays audible content to the user or
records audio while in the background. (This
content includes streaming audio or video content
using AirPlay.)
background
mode
Audio and
AirPlay
The user must grant permission for apps to use
the microphone prior to the first use; for more
information, see Supporting User Privacy (page
16).
Location updates
location
The app keeps users informed of their location,
even while it is running in the background.
Voice over IP
voip
The app provides the ability for the user to make
phone calls using an Internet connection.
Newsstand
downloads
newsstand-content
The app is a Newsstand app that downloads and
processes magazine or newspaper content in the
background.
External
accessory
communication
external-accessory
The app works with a hardware accessory that
needs to deliver updates on a regular schedule
through the External Accessory framework.
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35
Background Execution
Implementing Long-Running Tasks
Xcode
UIBackgroundModes value
Description
Uses Bluetooth
LE accessories
bluetooth-central
The app works with a Bluetooth accessory that
needs to deliver updates on a regular schedule
through the Core Bluetooth framework.
Acts as a
Bluetooth LE
accessory
bluetooth-peripheral
The app supports Bluetooth communication in
peripheral mode through the Core Bluetooth
framework.
background
mode
Using this mode requires user authorization; for
more information, see Supporting User
Privacy (page 16).
Background
fetch
fetch
The app regularly downloads and processes small
amounts of content from the network.
Remote
notifications
remote-notification
The app wants to start downloading content when
a push notification arrives. Use this notification to
minimize the delay in showing content related to
the push notification.
Each of the preceding modes lets the system know that your app should be woken up or launched at appropriate
times to respond to relevant events. For example, an app that begins playing music and then moves to the
background still needs execution time to fill the audio output buffers. Enabling the Audio mode tells the system
frameworks that they should continue to make the necessary callbacks to the app at appropriate intervals. If
the app does not select this mode, any audio being played or recorded by the app stops when the app moves
to the background.
Tracking the User’s Location
There are several ways to track the user’s location in the background, most of which do not actually require
your app to run continuously in the background:
●
The significant-change location service (Recommended)
●
Foreground-only location services
●
Background location services
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Implementing Long-Running Tasks
The significant-change location service is highly recommended for apps that do not need high-precision
location data. With this service, location updates are generated only when the user’s location changes
significantly; thus, it is ideal for social apps or apps that provide the user with noncritical, location-relevant
information. If the app is suspended when an update occurs, the system wakes it up in the background to
handle the update. If the app starts this service and is then terminated, the system relaunches the app
automatically when a new location becomes available. This service is available in iOS 4 and later, and it is
available only on devices that contain a cellular radio.
The foreground-only and background location services both use the standard location Core Location service
to retrieve location data. The only difference is that the foreground-only location services stop delivering
updates if the app is ever suspended, which is likely to happen if the app does not support other background
services or tasks. Foreground-only location services are intended for apps that only need location data while
they are in the foreground.
You enable location support from the Background modes section of the Capabilities tab in your Xcode project.
(You can also enable this support by including the UIBackgroundModes key with the location value in
your app’s Info.plist file.) Enabling this mode does not prevent the system from suspending the app, but
it does tell the system that it should wake up the app whenever there is new location data to deliver. Thus,
this key effectively lets the app run in the background to process location updates whenever they occur.
Important: You are encouraged to use the standard services sparingly or use the significant location change
service instead. Location services require the active use of an iOS device’s onboard radio hardware. Running
this hardware continuously can consume a significant amount of power. If your app does not need to
provide precise and continuous location information to the user, it is best to minimize the use of location
services.
For information about how to use each of the different location services in your app, see Location and Maps
Programming Guide .
Playing and Recording Background Audio
An app that plays or records audio continuously (even while the app is running in the background) can register
to perform those tasks in the background. You enable audio support from the Background modes section of
the Capabilities tab in your Xcode project. (You can also enable this support by including the
UIBackgroundModes key with the audio value in your app’s Info.plist file.) Apps that play audio content
in the background must play audible content and not silence.
Typical examples of background audio apps include:
●
Music player apps
●
Audio recording apps
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Implementing Long-Running Tasks
●
Apps that support audio or video playback over AirPlay
●
VoIP apps
When the UIBackgroundModes key contains the audio value, the system’s media frameworks automatically
prevent the corresponding app from being suspended when it moves to the background. As long as it is
playing audio or video content or recording audio content, the app continues to run in the background.
However, if recording or playback stops, the system suspends the app.
You can use any of the system audio frameworks to work with background audio content, and the process for
using those frameworks is unchanged. (For video playback over AirPlay, you can use the Media Player or AV
Foundation framework to present your video.) Because your app is not suspended while playing media files,
callbacks operate normally while your app is in the background. In your callbacks, though, you should do only
the work necessary to provide data for playback. For example, a streaming audio app would need to download
the music stream data from its server and push the current audio samples out for playback. Apps should not
perform any extraneous tasks that are unrelated to playback.
Because more than one app may support audio, the system determines which app is allowed to play or record
audio at any given time. The foreground app always has priority for audio operations. It is possible for more
than one background app to be allowed to play audio and such determinations are based on the configuration
of each app’s audio session objects. You should always configure your app’s audio session object appropriately
and work carefully with the system frameworks to handle interruptions and other types of audio-related
notifications. For information on how to configure audio session objects for background execution, see Audio
Session Programming Guide .
Implementing a VoIP App
A Voice over Internet Protocol (VoIP) app allows the user to make phone calls using an Internet connection
instead of the device’s cellular service. Such an app needs to maintain a persistent network connection to its
associated service so that it can receive incoming calls and other relevant data. Rather than keep VoIP apps
awake all the time, the system allows them to be suspended and provides facilities for monitoring their sockets
for them. When incoming traffic is detected, the system wakes up the VoIP app and returns control of its sockets
to it.
To configure a VoIP app, you must do the following:
1.
Enable support for Voice over IP from the Background modes section of the Capabilities tab in your Xcode
project. (You can also enable this support by including the UIBackgroundModes key with the voip value
in your app’s Info.plist file.)
2.
Configure one of the app’s sockets for VoIP usage.
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Implementing Long-Running Tasks
3.
Before moving to the background, call the setKeepAliveTimeout:handler: method to install a handler
to be executed periodically. Your app can use this handler to maintain its service connection.
4.
Configure your audio session to handle transitions to and from active use.
Including the voip value in the UIBackgroundModes key lets the system know that it should allow the app
to run in the background as needed to manage its network sockets. An app with this key is also relaunched in
the background immediately after system boot to ensure that the VoIP services are always available.
Most VoIP apps also need to be configured as background audio apps to deliver audio while in the background.
Therefore, you should include both the audio and voip values to the UIBackgroundModes key. If you do
not do this, your app cannot play or record audio while it is in the background. For more information about
the UIBackgroundModes key, see Information Property List Key Reference .
For specific information about the steps you must take to implement a VoIP app, see Tips for Developing a
VoIP App (page 91).
Fetching Small Amounts of Content Opportunistically
Apps that need to check for new content periodically can ask the system to wake them up so that they can
initiate a fetch operation for that content. To support this mode, enable the Background fetch option from the
Background modes section of the Capabilities tab in your Xcode project. (You can also enable this support by
including the UIBackgroundModes key with the fetch value in your app’s Info.plist file.) Enabling this
mode is not a guarantee that the system will give your app any time to perform background fetches. The
system must balance your app’s need to fetch content with the needs of other apps and the system itself. After
assessing that information, the system gives time to apps when there are good opportunities to do so.
When a good opportunity arises, the system wakes or launches your app into the background and calls the
app delegate’s application:performFetchWithCompletionHandler: method. Use that method to
check for new content and initiate a download operation if content is available. As soon as you finish
downloading the new content, you must execute the provided completion handler block, passing a result that
indicates whether content was available. Executing this block tells the system that it can move your app back
to the suspended state and evaluate its power usage. Apps that download small amounts of content quickly,
and accurately reflect when they had content available to download, are more likely to receive execution time
in the future than apps that take a long time to download their content or that claim content was available
but then do not download anything.
When downloading any content, it is recommended that you use the NSURLSession class to initiate and
manage your downloads. For information about how to use this class to manage upload and download tasks,
see URL Loading System Programming Guide .
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Implementing Long-Running Tasks
Using Push Notifications to Initiate a Download
If your server sends push notifications to a user’s device when new content is available for your app, you can
ask the system to run your app in the background so that it can begin downloading the new content right
away. The intent of this background mode is to minimize the amount of time that elapses between when a
user sees a push notification and when your app is able to able to display the associated content. Apps are
typically woken up at roughly the same time that the user sees the notification but that still gives you more
time than you might have otherwise.
To support this background mode, enable the Remote notifications option from the Background modes section
of the Capabilities tab in your Xcode project. (You can also enable this support by including the
UIBackgroundModes key with the remote-notification value in your app’s Info.plist file.)
For a push notification to trigger a download operation, the notification’s payload must include the
content-available key with its value set to 1. When that key is present, the system wakes the app in the
background (or launches it into the background) and calls the app delegate’s
application:didReceiveRemoteNotification:fetchCompletionHandler: method. Your
implementation of that method should download the relevant content and integrate it into your app.
When downloading any content, it is recommended that you use the NSURLSession class to initiate and
manage your downloads. For information about how to use this class to manage upload and download tasks,
see URL Loading System Programming Guide .
Downloading Newsstand Content in the Background
A Newsstand app that downloads new magazine or newspaper issues can register to perform those downloads
in the background. You enable support for newsstand downloads from the Background modes section of the
Capabilities tab in your Xcode project. (You can also enable this support by including the UIBackgroundModes
key with the newsstand-content value in your app’s Info.plist file.) When this key is present, the system
launches your app, if it is not already running, so that it can initiate the downloading of the new issue.
When you use the Newsstand Kit framework to initiate a download, the system handles the download process
for your app. The system continues to download the file even if your app is suspended or terminated. When
the download operation is complete, the system transfers the file to your app sandbox and notifies your app.
If the app is not running, this notification wakes it up and gives it a chance to process the newly downloaded
file. If there are errors during the download process, your app is similarly woken up to handle them.
For information about how to download content using the Newsstand Kit framework, see NewsstandKit
Framework Reference .
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Implementing Long-Running Tasks
Communicating with an External Accessory
Apps that work with external accessories can ask to be woken up if the accessory delivers an update when the
app is suspended. This support is important for some types of accessories that deliver data at regular intervals,
such as heart-rate monitors. You enable support for external accessory communication from the Background
modes section of the Capabilities tab in your Xcode project. (You can also enable this support by including
the UIBackgroundModes key with the external-accessory value in your app’s Info.plist file.) When
you enable this mode, the external accessory framework does not close active sessions with accessories. (In
iOS 4 and earlier, these sessions are closed automatically when the app is suspended.) When new data arrives
from the accessory, the framework wakes your app so that it can process that data. The system also wakes the
app to process accessory connection and disconnection notifications.
Any app that supports the background processing of accessory updates must follow a few basic guidelines:
●
Apps must provide an interface that allows the user to start and stop the delivery of accessory update
events. That interface should then open or close the accessory session as appropriate.
●
Upon being woken up, the app has around 10 seconds to process the data. Ideally, it should process the
data as fast as possible and allow itself to be suspended again. However, if more time is needed, the app
can use the beginBackgroundTaskWithExpirationHandler: method to request additional time; it
should do so only when absolutely necessary, though.
Communicating with a Bluetooth Accessory
Apps that work with Bluetooth peripherals can ask to be woken up if the peripheral delivers an update when
the app is suspended. This support is important for Bluetooth-LE accessories that deliver data at regular intervals,
such as a Bluetooth heart rate belt. You enable support for using bluetooth accessories from the Background
modes section of the Capabilities tab in your Xcode project. (You can also enable this support by including
the UIBackgroundModes key with the bluetooth-central value in your app’s Info.plist file.) When
you enable this mode, the Core Bluetooth framework keeps open any active sessions for the corresponding
peripheral. In addition, new data arriving from the peripheral causes the system to wake up the app so that it
can process the data. The system also wakes up the app to process accessory connection and disconnection
notifications.
In iOS 6, an app can also operate in peripheral mode with Bluetooth accessories. To act as a Bluetooth accessory,
you must enable support for that mode from the Background modes section of the Capabilities tab in your
Xcode project. (You can also enable this support by including the UIBackgroundModes key with the
bluetooth-peripheral value in your app’s Info.plist file.) Enabling this mode lets the Core Bluetooth
framework wake the app up briefly in the background so that it can handle accessory-related requests. Apps
woken up for these events should process them and return as quickly as possible so that the app can be
suspended again.
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Background Execution
Getting the User’s Attention While in the Background
Any app that supports the background processing of Bluetooth data must be session-based and follow a few
basic guidelines:
●
Apps must provide an interface that allows the user to start and stop the delivery of Bluetooth events.
That interface should then open or close the session as appropriate.
●
Upon being woken up, the app has around 10 seconds to process the data. Ideally, it should process the
data as fast as possible and allow itself to be suspended again. However, if more time is needed, the app
can use the beginBackgroundTaskWithExpirationHandler: method to request additional time; it
should do so only when absolutely necessary, though.
Getting the User’s Attention While in the Background
Notifications are a way for an app that is suspended, is in the background, or is not running to get the user’s
attention. Apps can use local notifications to display alerts, play sounds, badge the app’s icon, or a combination
of the three. For example, an alarm clock app might use local notifications to play an alarm sound and display
an alert to disable the alarm. When a notification is delivered to the user, the user must decide if the information
warrants bringing the app back to the foreground. (If the app is already running in the foreground, local
notifications are delivered quietly to the app and not to the user.)
To schedule the delivery of a local notification, create an instance of the UILocalNotification class, configure
the notification parameters, and schedule it using the methods of the UIApplication class. The local
notification object contains information about the type of notification to deliver (sound, alert, or badge) and
the time (when applicable) at which to deliver it. The methods of the UIApplication class provide options
for delivering notifications immediately or at the scheduled time.
Listing 3-2 shows an example that schedules a single alarm using a date and time that is set by the user. This
example configures only one alarm at a time and cancels the previous alarm before scheduling a new one.
(Your own apps can have no more than 128 local notifications active at any given time, any of which can be
configured to repeat at a specified interval.) The alarm itself consists of an alert box and a sound file that is
played if the app is not running or is in the background when the alarm fires. If the app is active and therefore
running in the foreground, the app delegate’s application:didReceiveLocalNotification: method
is called instead.
Listing 3-2
Scheduling an alarm notification
- (void)scheduleAlarmForDate:(NSDate*)theDate
{
UIApplication* app = [UIApplication sharedApplication];
NSArray*
oldNotifications = [app scheduledLocalNotifications];
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Understanding When Your App Gets Launched into the Background
// Clear out the old notification before scheduling a new one.
if ([oldNotifications count] > 0)
[app cancelAllLocalNotifications];
// Create a new notification.
UILocalNotification* alarm = [[UILocalNotification alloc] init];
if (alarm)
{
alarm.fireDate = theDate;
alarm.timeZone = [NSTimeZone defaultTimeZone];
alarm.repeatInterval = 0;
alarm.soundName = @"alarmsound.caf";
alarm.alertBody = @"Time to wake up!";
[app scheduleLocalNotification:alarm];
}
}
Sound files used with local notifications have the same requirements as those used for push notifications.
Custom sound files must be located inside your app’s main bundle and support one of the following formats:
Linear PCM, MA4, µ-Law, or a-Law. You can also specify the UILocalNotificationDefaultSoundName
constant to play the default alert sound for the device. When the notification is sent and the sound is played,
the system also triggers a vibration on devices that support it.
You can cancel scheduled notifications or get a list of notifications using the methods of the UIApplication
class. For more information about these methods, see UIApplication Class Reference . For additional information
about configuring local notifications, see Local and Remote Notification Programming Guide .
Understanding When Your App Gets Launched into the Background
Apps that support background execution may be relaunched by the system to handle incoming events. If an
app is terminated for any reason other than the user force quitting it, the system launches the app when one
of the following events happens:
●
For location apps:
●
The system receives a location update that meets the app’s configured criteria for delivery.
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Background Execution
Being a Responsible Background App
●
The device entered or exited a registered region. (Regions can be geographic regions or iBeacon
regions.)
●
For audio apps, the audio framework needs the app to process some data. (Audio apps include those that
play audio or use the microphone.)
●
For Bluetooth apps:
●
●
An app acting in the central role receives data from a connected peripheral.
●
An app acting in the peripheral role receives commands from a connected central.
For background download apps:
●
A push notification arrives for an app and the payload of the notification contains the
content-available key with a value of 1.
●
The system wakes the app at opportunistic moments to begin downloading new content.
●
For apps downloading content in the background using the NSURLSession class, all tasks associated
with that session object either completed successfully or received an error.
●
A download initiated by a Newsstand app finishes.
In most cases, the system does not relaunch apps after they are force quit by the user. One exception is location
apps, which in iOS 8 and later are relaunched after being force quit by the user. In other cases, though, the
user must launch the app explicitly or reboot the device before the app can be launched automatically into
the background by the system.
Being a Responsible Background App
The foreground app always has precedence over background apps when it comes to the use of system resources
and hardware. Apps running in the background need to be prepared for this discrepancy and adjust their
behavior when running in the background. Specifically, apps moving to the background should follow these
guidelines:
●
Do not make any OpenGL ES calls from your code. You must not create an EAGLContext object or issue
any OpenGL ES drawing commands of any kind while running in the background. Using these calls causes
your app to be killed immediately. Apps must also ensure that any previously submitted commands have
completed before moving to the background. For information about how to handle OpenGL ES when
moving to and from the background, see Implementing a Multitasking-aware OpenGL ES Application in
OpenGL ES Programming Guide for iOS .
●
Cancel any Bonjour-related services before being suspended. When your app moves to the background,
and before it is suspended, it should unregister from Bonjour and close listening sockets associated with
any network services. A suspended app cannot respond to incoming service requests anyway. Closing out
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Being a Responsible Background App
those services prevents them from appearing to be available when they actually are not. If you do not
close out Bonjour services yourself, the system closes out those services automatically when your app is
suspended.
●
Be prepared to handle connection failures in your network-based sockets. The system may tear down
socket connections while your app is suspended for any number of reasons. As long as your socket-based
code is prepared for other types of network failures, such as a lost signal or network transition, this should
not lead to any unusual problems. When your app resumes, if it encounters a failure upon using a socket,
simply reestablish the connection.
●
Save your app state before moving to the background. During low-memory conditions, background
apps may be purged from memory to free up space. Suspended apps are purged first, and no notice is
given to the app before it is purged. As a result, apps should take advantage of the state preservation
mechanism in iOS 6 and later to save their interface state to disk. For information about how to support
this feature, see Preserving Your App’s Visual Appearance Across Launches (page 67).
●
Remove strong references to unneeded objects when moving to the background. If your app maintains
a large in-memory cache of objects (especially images), remove all strong references to those caches when
moving to the background. For more information, see Reduce Your Memory Footprint (page 62).
●
Stop using shared system resources before being suspended. Apps that interact with shared system
resources such as the Address Book or calendar databases should stop using those resources before being
suspended. Priority for such resources always goes to the foreground app. When your app is suspended,
if it is found to be using a shared resource, the app is killed.
●
Avoid updating your windows and views. Because your app’s windows and views are not visible when
your app is in the background, you should avoid updating them. The exception is in cases where you need
to update the contents of a window prior to having a snapshot of your app taken.
●
Respond to connect and disconnect notifications for external accessories. For apps that communicate
with external accessories, the system automatically sends a disconnection notification when the app moves
to the background. The app must register for this notification and use it to close out the current accessory
session. When the app moves back to the foreground, a matching connection notification is sent, giving
the app a chance to reconnect. For more information on handling accessory connection and disconnection
notifications, see External Accessory Programming Topics .
●
Clean up resources for active alerts when moving to the background. In order to preserve context when
switching between apps, the system does not automatically dismiss action sheets (UIActionSheet) or
alert views (UIAlertView) when your app moves to the background. It is up to you to provide the
appropriate cleanup behavior prior to moving to the background. For example, you might want to cancel
the action sheet or alert view programmatically or save enough contextual information to restore the view
later (in cases where your app is terminated).
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Background Execution
Opting Out of Background Execution
●
Remove sensitive information from views before moving to the background. When an app transitions
to the background, the system takes a snapshot of the app’s main window, which it then presents briefly
when transitioning your app back to the foreground. Before returning from your
applicationDidEnterBackground: method, you should hide or obscure passwords and other sensitive
personal information that might be captured as part of the snapshot.
●
Do minimal work while running in the background. The execution time given to background apps is
more constrained than the amount of time given to the foreground app. Apps that spend too much time
executing in the background can be throttled back by the system or terminated.
If you are implementing a background audio app, or any other type of app that is allowed to run in the
background, your app responds to incoming messages in the usual way. In other words, the system may notify
your app of low-memory warnings when they occur. And in situations where the system needs to terminate
apps to free even more memory, the app calls its delegate’s applicationWillTerminate: method to
perform any final tasks before exiting.
Opting Out of Background Execution
If you do not want your app to run in the background at all, you can explicitly opt out of background by adding
the UIApplicationExitsOnSuspend key (with the value YES) to your app’s Info.plist file. When an app
opts out, it cycles between the not-running, inactive, and active states and never enters the background or
suspended states. When the user presses the Home button to quit the app, the applicationWillTerminate:
method of the app delegate is called and the app has approximately 5 seconds to clean up and exit before it
is terminated and moved back to the not-running state.
Opting out of background execution is strongly discouraged but may be the preferred option under certain
conditions. Specifically, if coding for background execution adds significant complexity to your app, terminating
the app might be a simpler solution. Also, if your app consumes a large amount of memory and cannot easily
release any of it, the system might kill your app quickly anyway to make room for other apps. Thus, opting to
terminate, instead of switching to the background, might yield the same results and save you development
time and effort.
For more information about the keys you can include in your app’s Info.plist file, see Information Property
List Key Reference .
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Strategies for Handling App State Transitions
For each of the possible runtime states of an app, the system has different expectations while your app is in
that state. When state transitions occur, the system notifies the app object, which in turn notifies its app
delegate. You can use the state transition methods of the UIApplicationDelegate protocol to detect these
state changes and respond appropriately. For example, when transitioning from the foreground to the
background, you might write out any unsaved data and stop any ongoing tasks. The following sections offer
tips and guidance for how to implement your state transition code.
What to Do at Launch Time
When your app is launched (either into the foreground or background), use your app delegate’s
application:willFinishLaunchingWithOptions: and
application:didFinishLaunchingWithOptions: methods to do the following:
●
Check the contents of the launch options dictionary for information about why the app was launched,
and respond appropriately.
●
Initialize your app’s critical data structures.
●
Prepare your app’s window and views for display:
●
Apps that use OpenGL ES for drawing must not use these methods to prepare their drawing
environment. Instead, defer any OpenGL ES drawing calls to the applicationDidBecomeActive:
method.
●
Show your app window from your application:willFinishLaunchingWithOptions: method.
UIKit delays making the window visible until after the
application:didFinishLaunchingWithOptions: method returns.
At launch time, the system automatically loads your app’s main storyboard file and loads the initial view
controller. For apps that support state restoration, the state restoration machinery restores your interface to
its previous state between calls to the application:willFinishLaunchingWithOptions: and
application:didFinishLaunchingWithOptions: methods. Use the
application:willFinishLaunchingWithOptions: method to show your app window and to determine
if state restoration should happen at all. Use the application:didFinishLaunchingWithOptions:
method to make any final adjustments to your app’s user interface.
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What to Do at Launch Time
Your application:willFinishLaunchingWithOptions: and
application:didFinishLaunchingWithOptions: methods should always be as lightweight as possible
to reduce your app’s launch time. Apps are expected to launch, initialize themselves, and start handling events
in less than 5 seconds. If an app does not finish its launch cycle in a timely manner, the system kills it for being
unresponsive. Thus, any tasks that might slow down your launch (such as accessing the network) should be
scheduled performed on a secondary thread.
The Launch Cycle
When your app is launched, it moves from the not running state to the active or background state, transitioning
briefly through the inactive state. As part of the launch cycle, the system creates a process and main thread
for your app and calls your app’s main function on that main thread. The default main function that comes
with your Xcode project promptly hands control over to the UIKit framework, which does most of the work in
initializing your app and preparing it to run.
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Strategies for Handling App State Transitions
What to Do at Launch Time
Figure 4-1 shows the sequence of events that occurs when an app is launched into the foreground, including
the app delegate methods that are called.
Figure 4-1
Launching an app into the foreground
Launch Time
Your Code
Running
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Strategies for Handling App State Transitions
What to Do at Launch Time
When your app is launched into the background—usually to handle some type of background event—the
launch cycle changes slightly to the one shown in Figure 4-2. The main difference is that instead of your app
being made active, it enters the background state to handle the event and may be suspended at some point
after that. When launching into the background, the system still loads your app’s user interface files but it does
not display the app’s window.
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Strategies for Handling App State Transitions
What to Do at Launch Time
Figure 4-2
Launching an app into the background
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Strategies for Handling App State Transitions
What to Do at Launch Time
To determine whether your app is launching into the foreground or background, check the applicationState
property of the shared UIApplication object in your application:willFinishLaunchingWithOptions:
or application:didFinishLaunchingWithOptions: delegate method. When the app is launched into
the foreground, this property contains the value UIApplicationStateInactive. When the app is launched
into the background, the property contains the value UIApplicationStateBackground instead. You can
use this difference to adjust the launch-time behavior of your delegate methods accordingly.
Note: When an app is launched so that it can open a URL, the sequence of startup events is slightly
different from those shown in Figure 4-1 (page 49) and Figure 4-2 (page 51). For information about
the startup sequences that occur when opening a URL, see Handling URL Requests (page 99).
Launching in Landscape Mode
Apps that uses only landscape orientations for their interface must explicitly ask the system to launch the app
in that orientation. Normally, apps launch in portrait mode and rotate their interface to match the device
orientation as needed. For apps that support both portrait and landscape orientations, always configure your
views for portrait mode and then let your view controllers handle any rotations. If, however, your app supports
landscape but not portrait orientations, perform the following tasks to make it launch in landscape mode
initially:
●
Add the UIInterfaceOrientation key to your app’s Info.plist file and set the value of this key to
either UIInterfaceOrientationLandscapeLeft or UIInterfaceOrientationLandscapeRight.
●
Lay out your views in landscape mode and make sure that their layout or autosizing options are set
correctly.
●
Override your view controller’s shouldAutorotateToInterfaceOrientation: method and return
YES for the left or right landscape orientations and NO for portrait orientations.
Important: Apps should always use view controllers to manage their window-based content.
The UIInterfaceOrientation key in the Info.plist file tells iOS that it should configure the orientation
of the app status bar (if one is displayed) as well as the orientation of views managed by any view controllers
at launch time. View controllers respect this key and set their view’s initial orientation to match. Using this key
is equivalent to calling the setStatusBarOrientation:animated: method of UIApplication early in
the execution of your applicationDidFinishLaunching: method.
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What to Do When Your App Is Interrupted Temporarily
Installing App-Specific Data Files at First Launch
You can use your app’s first launch cycle to set up any data or configuration files required to run. App-specific
data files should be created in the Library/Application Support/<bundleID>/ directory of your app
sandbox, where <bundleID> is your app’s bundle identifier. You can further subdivide this directory to organize
your data files as needed. You can also create files in other directories, such as to your app’s iCloud container
directory or to the local Documents directory, depending on your needs.
If your app’s bundle contains data files that you plan to modify, copy those files out of the app bundle and
modify the copies. You must not modify any files inside your app bundle. Because iOS apps are code signed,
modifying files inside your app bundle invalidates your app’s signature and will prevent your app from launching
in the future. Copying those files to the Application Support directory (or another writable directory in
your sandbox) and modifying them there is the only way to use such files safely.
For more information about where to put app-related data files, see File System Programming Guide .
What to Do When Your App Is Interrupted Temporarily
Alert-based interruptions result in a temporary loss of control by your app. Your app continues to run in the
foreground, but it does not receive touch events from the system. (It does continue to receive notifications
and other types of events, such as accelerometer events, though.) In response to this change, your app should
do the following in its applicationWillResignActive: method:
●
Save data and any relevant state information.
●
Stop timers and other periodic tasks.
●
Stop any running metadata queries.
●
Do not initiate any new tasks.
●
Pause movie playback (except when playing back over AirPlay).
●
Enter into a pause state if your app is a game.
●
Throttle back OpenGL ES frame rates.
●
Suspend any dispatch queues or operation queues executing non-critical code. (You can continue processing
network requests and other time-sensitive background tasks while inactive.)
When your app is moved back to the active state, its applicationDidBecomeActive: method should
reverse any of the steps taken in the applicationWillResignActive: method. Thus, upon reactivation,
your app should restart timers, resume dispatch queues, and throttle up OpenGL ES frame rates again. However,
games should not resume automatically; they should remain paused until the user chooses to resume them.
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What to Do When Your App Is Interrupted Temporarily
When the user presses the Sleep/Wake button, apps with files protected by the NSFileProtectionComplete
protection option must close any references to those files. For devices configured with an appropriate password,
pressing the Sleep/Wake button locks the screen and forces the system to throw away the decryption keys for
files with complete protection enabled. While the screen is locked, any attempts to access the corresponding
files will fail. So if you have such files, you should close any references to them in your
applicationWillResignActive: method and open new references in your
applicationDidBecomeActive: method.
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Important: Always save user data at appropriate checkpoints in your app. Although you can use app state
transitions to force objects to write unsaved changes to disk, never wait for an app state transition to save
data. For example, a view controller that manages user data should save its data when it is dismissed.
Responding to Temporary Interruptions
When an alert-based interruption occurs, such as an incoming phone call, the app moves temporarily to the
inactive state so that the system can prompt the user about how to proceed. The app remains in this state
until the user dismisses the alert. At this point, the app either returns to the active state or moves to the
background state. Figure 4-3 shows the flow of events through your app when an alert-based interruption
occurs.
Figure 4-3
Handling alert-based interruptions
Notifications that display a banner do not deactivate your app in the way that alert-based notifications do.
Instead, the banner is laid along the top edge of your app window and your app continues receive touch
events as before. However, if the user pulls down the banner to reveal the notification center, your app moves
to the inactive state just as if an alert-based interruption had occurred. Your app remains in the inactive state
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What to Do When Your App Enters the Foreground
until the user dismisses the notification center or launches another app. At this point, your app moves to the
appropriate active or background state. The user can use the Settings app to configure which notifications
display a banner and which display an alert.
Pressing the Sleep/Wake button is another type of interruption that causes your app to be deactivated
temporarily. When the user presses this button, the system disables touch events, moves the app to the
background, sets the value of the app’s applicationState property to UIApplicationStateBackground,
and locks the screen. A locked screen has additional consequences for apps that use data protection to encrypt
files. Those consequences are described in What to Do When Your App Is Interrupted Temporarily (page 53).
What to Do When Your App Enters the Foreground
Returning to the foreground is your app’s chance to restart the tasks that it stopped when it moved to the
background. The steps that occur when moving to the foreground are shown in Figure 4-4. The
applicationWillEnterForeground: method should undo anything that was done in your
applicationDidEnterBackground: method, and the applicationDidBecomeActive: method should
continue to perform the same activation tasks that it would at launch time.
Figure 4-4
Transitioning from the background to the foreground
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What to Do When Your App Enters the Foreground
Note: The UIApplicationWillEnterForegroundNotification notification is also available
for tracking when your app reenters the foreground. Objects in your app can use the default
notification center to register for this notification.
Be Prepared to Process Queued Notifications
An app in the suspended state must be ready to handle any queued notifications when it returns to a foreground
or background execution state. A suspended app does not execute any code and therefore cannot process
notifications related to orientation changes, time changes, preferences changes, and many others that would
affect the app’s appearance or state. To make sure these changes are not lost, the system queues many relevant
notifications and delivers them to the app as soon as it starts executing code again (either in the foreground
or background). To prevent your app from becoming overloaded with notifications when it resumes, the system
coalesces events and delivers a single notification (of each relevant type) that reflects the net change since
your app was suspended.
Table 4-1 lists the notifications that can be coalesced and delivered to your app. Most of these notifications
are delivered directly to the registered observers. Some, like those related to device orientation changes, are
typically intercepted by a system framework and delivered to your app in another way.
Table 4-1
Notifications delivered to waking apps
Event
Notifications
An accessory is connected or
disconnected.
EAAccessoryDidConnectNotification
The device orientation changes.
UIDeviceOrientationDidChangeNotification
EAAccessoryDidDisconnectNotification
In addition to this notification, view controllers update
their interface orientations automatically.
There is a significant time change.
UIApplicationSignificantTimeChangeNotification
The battery level or battery state changes.
UIDeviceBatteryLevelDidChangeNotification
UIDeviceBatteryStateDidChangeNotification
The proximity state changes.
UIDeviceProximityStateDidChangeNotification
The status of protected files changes.
UIApplicationProtectedDataWillBecomeUnavailable
UIApplicationProtectedDataDidBecomeAvailable
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Event
Notifications
An external display is connected or
disconnected.
UIScreenDidConnectNotification
The screen mode of a display changes.
UIScreenModeDidChangeNotification
Preferences that your app exposes
through the Settings app changed.
NSUserDefaultsDidChangeNotification
The current language or locale settings
changed.
NSCurrentLocaleDidChangeNotification
The status of the user’s iCloud account
changed.
NSUbiquityIdentityDidChangeNotification
UIScreenDidDisconnectNotification
Queued notifications are delivered on your app’s main run loop and are typically delivered before any touch
events or other user input. Most apps should be able to handle these events quickly enough that they would
not cause any noticeable lag when resumed. However, if your app appears sluggish when it returns from the
background state, use Instruments to determine whether your notification handler code is causing the delay.
An app returning to the foreground also receives view-update notifications for any views that were marked
dirty since the last update. An app running in the background can still call the setNeedsDisplay or
setNeedsDisplayInRect: methods to request an update for its views. However, because the views are not
visible, the system coalesces the requests and updates the views only after the app returns to the foreground.
Handle iCloud Changes
If the status of iCloud changes for any reason, the system delivers a
NSUbiquityIdentityDidChangeNotification notification to your app. The state of iCloud changes when
the user logs into or out of an iCloud account or enables or disables the syncing of documents and data. This
notification is your app’s cue to update caches and any iCloud-related user interface elements to accommodate
the change. For example, when the user logs out of iCloud, you should remove references to all iCloud–based
files or data.
If your app has already prompted the user about whether to store files in iCloud, do not prompt again when
the status of iCloud changes. After prompting the user the first time, store the user’s choice in your app’s local
preferences. You might then want to expose that preference using a Settings bundle or as an option in your
app. But do not repeat the prompt again unless that preference is not currently in the user defaults database.
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Handle Locale Changes
If a user changes the current locale while your app is suspended, you can use the
NSCurrentLocaleDidChangeNotification notification to force updates to any views containing
locale-sensitive information, such as dates, times, and numbers when your app returns to the foreground. Of
course, the best way to avoid locale-related issues is to write your code in ways that make it easy to update
views. For example:
●
Use the autoupdatingCurrentLocale class method when retrieving NSLocale objects. This method
returns a locale object that updates itself automatically in response to changes, so you never need to
recreate it. However, when the locale changes, you still need to refresh views that contain content derived
from the current locale.
●
Re-create any cached date and number formatter objects whenever the current locale information changes.
For more information about internationalizing your code to handle locale changes, see Internationalization
and Localization Guide .
Handle Changes to Your App’s Settings
If your app has settings that are managed by the Settings app, it should observe the
NSUserDefaultsDidChangeNotification notification. Because the user can modify settings while your
app is suspended or in the background, you can use this notification to respond to any important changes in
those settings. In some cases, responding to this notification can help close a potential security hole. For
example, an email program should respond to changes in the user’s account information. Failure to monitor
these changes could cause privacy or security issues. Specifically, the current user might be able to send email
using the old account information, even if the account no longer belongs to that person.
Upon receiving the NSUserDefaultsDidChangeNotification notification, your app should reload any
relevant settings and, if necessary, reset its user interface appropriately. In cases where passwords or other
security-related information has changed, you should also hide any previously displayed information and force
the user to enter the new password.
What to Do When Your App Enters the Background
When moving from foreground to background execution, use the applicationDidEnterBackground:
method of your app delegate to do the following:
●
Prepare to have your app’s picture taken. When your applicationDidEnterBackground: method
returns, the system takes a picture of your app’s user interface and uses the resulting image for transition
animations. If any views in your interface contain sensitive information, you should hide or modify those
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What to Do When Your App Enters the Background
views before the applicationDidEnterBackground: method returns. If you add new views to your
view hierarchy as part of this process, you must force those views to draw themselves, as described in
Prepare for the App Snapshot (page 62).
●
Save any relevant app state information. Prior to entering the background, your app should already
have saved all critical user data. Use the transition to the background to save any last minute changes to
your app’s state.
●
Free up memory as needed. Release any cached data that you do not need and do any simple cleanup
that might reduce your app’s memory footprint. Apps with large memory footprints are the first to be
terminated by the system, so release image resources, data caches, and any other objects that you no
longer need. For more information, see Reduce Your Memory Footprint (page 62).
Your app delegate’s applicationDidEnterBackground: method has approximately 5 seconds to finish
any tasks and return. In practice, this method should return as quickly as possible. If the method does not
return before time runs out, your app is killed and purged from memory. If you still need more time to perform
tasks, call the beginBackgroundTaskWithExpirationHandler: method to request background execution
time and then start any long-running tasks in a secondary thread. Regardless of whether you start any
background tasks, the applicationDidEnterBackground: method must still exit within 5 seconds.
Note: The system sends the UIApplicationDidEnterBackgroundNotification notification
in addition to calling the applicationDidEnterBackground: method. You can use that notification
to distribute cleanup tasks to other objects of your app.
Depending on the features of your app, there are other things your app should do when moving to the
background. For example, any active Bonjour services should be suspended and the app should stop calling
OpenGL ES functions. For a list of things your app should do when moving to the background, see Being a
Responsible Background App (page 44).
The Background Transition Cycle
When the user presses the Home button, presses the Sleep/Wake button, or the system launches another app,
the foreground app transitions to the inactive state and then to the background state. These transitions result
in calls to the app delegate’s applicationWillResignActive: and applicationDidEnterBackground:
methods, as shown in Figure 4-5. After returning from the applicationDidEnterBackground: method,
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What to Do When Your App Enters the Background
most apps move to the suspended state shortly afterward. Apps that request specific background tasks (such
as playing music) or that request a little extra execution time from the system may continue to run for a while
longer.
Figure 4-5
Moving from the foreground to the background
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What to Do When Your App Enters the Background
Prepare for the App Snapshot
Shortly after an app delegate’s applicationDidEnterBackground: method returns, the system takes a
snapshot of the app’s windows. Similarly, when an app is woken up to perform background tasks, the system
may take a new snapshot to reflect any relevant changes. For example, when an app is woken to process
downloaded items, the system takes a new snapshot so that can reflect any changes caused by the incorporation
of the items. The system uses these snapshot images in the multitasking UI to show the state of your app.
If you make changes to your views upon entering the background, you can call the
snapshotViewAfterScreenUpdates: method of your main view to force those changes to be rendered.
Calling the setNeedsDisplay method on a view is ineffective for snapshots because the snapshot is taken before
the next drawing cycle, thus preventing any changes from being rendered. Calling the
snapshotViewAfterScreenUpdates: method with a value of YES forces an immediate update to the
underlying buffers that the snapshot machinery uses.
Reduce Your Memory Footprint
Every app should free up as much memory as is practical upon entering the background. The system tries to
keep as many apps in memory at the same time as it can, but when memory runs low it terminates suspended
apps to reclaim that memory. Apps that consume large amounts of memory while in the background are the
first apps to be terminated.
Practically speaking, your app should remove strong references to objects as soon as they are no longer needed.
Removing strong references gives the compiler the ability to release the objects right away so that the
corresponding memory can be reclaimed. However, if you want to cache some objects to improve performance,
you can wait until the app transitions to the background before removing references to them.
Some examples of objects that you should remove strong references to as soon as possible include:
●
Image objects you created. (Some methods of UIImage return images whose underlying image data is
purged automatically by the system. For more information, see the discussion in the overview of UIImage
Class Reference . )
●
Large media or data files that you can load again from disk
●
Any other objects that your app does not need and can recreate easily later
To help reduce your app’s memory footprint, the system automatically purges some data allocated on behalf
of your app when your app moves to the background.
●
The system purges the backing store for all Core Animation layers. This effort does not remove your app’s
layer objects from memory, nor does it change the current layer properties. It simply prevents the contents
of those layers from appearing onscreen, which given that the app is in the background should not happen
anyway.
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●
It removes any system references to cached images.
●
It removes strong references to some other system-managed data caches.
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Strategies for Implementing Specific App Features
Different apps have different needs but some behaviors are common to many types of app. The following
sections provide guidance about how to implement specific types of features in your app.
Privacy Strategies
Protecting a user’s privacy is an important consideration in the design of an app. Privacy protection includes
protecting the user’s data, including the user’s identity and personal information. The system frameworks
already provide privacy controls for managing data such as contacts but your app should take steps to protect
the data that you use locally.
Protecting Data Using On-Disk Encryption
Data protection uses built-in hardware to store files in an encrypted format on disk and to decrypt them on
demand. While the user’s device is locked, protected files are inaccessible, even to the app that created them.
The user must unlock the device (by entering the appropriate passcode) before an app can access one of its
protected files.
Data protection is available on most iOS devices and is subject to the following requirements:
●
The file system on the user’s device must support data protection. Most devices support this behavior.
●
The user must have an active passcode lock set for the device.
To protect a file, you add an attribute to the file indicating the desired level of protection. Add this attribute
using either the NSData class or the NSFileManager class. When writing new files, you can use the
writeToFile:options:error: method of NSData with the appropriate protection value as one of the
write options. For existing files, you can use the setAttributes:ofItemAtPath:error: method of
NSFileManager to set or change the value of the NSFileProtectionKey. When using these methods,
specify one of the following protection levels for the file:
●
No protection—The file is encrypted but is not protected by the passcode and is available when the device
is locked. Specify the NSDataWritingFileProtectionNone option (NSData) or the
NSFileProtectionNone attribute (NSFileManager).
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Privacy Strategies
●
Complete—The file is encrypted and inaccessible while the device is locked. Specify the
NSDataWritingFileProtectionComplete option (NSData) or the NSFileProtectionComplete
attribute (NSFileManager).
●
Complete unless already open—The file is encrypted. A closed file is inaccessible while the device is locked.
After the user unlocks the device, your app can open the file and use it. If the user locks the device while
the file is open, though, your app can continue to access it. Specify the
NSDataWritingFileProtectionCompleteUnlessOpen option (NSData) or the
NSFileProtectionCompleteUnlessOpen attribute (NSFileManager).
●
Complete until first login—The file is encrypted and inaccessible until after the device has booted and the
user has unlocked it once. Specify the
NSDataWritingFileProtectionCompleteUntilFirstUserAuthentication option (NSData) or
the NSFileProtectionCompleteUntilFirstUserAuthentication attribute (NSFileManager).
If you protect a file, your app must be prepared to lose access to that file. When complete file protection is
enabled, your app loses the ability to read and write the file’s contents when the user locks the device. You
can track changes to the state of protected files using one of the following techniques:
●
The app delegate can implement the applicationProtectedDataWillBecomeUnavailable: and
applicationProtectedDataDidBecomeAvailable: methods.
●
Any object can register for the UIApplicationProtectedDataWillBecomeUnavailable and
UIApplicationProtectedDataDidBecomeAvailable notifications.
●
Any object can check the value of the protectedDataAvailable property of the shared UIApplication
object to determine whether files are currently accessible.
For new files, it is recommended that you enable data protection before writing any data to them. If you are
using the writeToFile:options:error: method to write the contents of an NSData object to disk, this
happens automatically. For existing files, adding data protection replaces an unprotected file with a new
protected version.
Identifying Unique Users of Your App
You should identify a user of your app only when doing so offers a clear benefit to that user. In cases where
you only need to differentiate one user of your app from another, iOS provides identifiers that can help you
do that. However, if you need a higher level of security, you might need to do more work on your own. For
example, an app that provides financial services would likely want to prompt the user for login credentials to
ensure that the user is authorized to access a specific account.
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Supporting Multiple Versions of iOS
Important: When identifying a user, always be transparent about what you intend to do with any information
you obtain. It is not acceptable to identify a user so that you can track them surreptitiously.
Here are some common scenarios that might require you to identify a user, along with solutions for how to
implement them.
●
You want to link a user to a specific account on your server. Include a login screen that requires the
user to enter their account information securely. Always protect the account information you gather from
the user by storing it in an encrypted form.
●
You want to differentiate instances of your app running on different devices. Use the
identifierForVendor property of the UIDevice class to obtain an ID that differentiates a user on one
device from users on other devices. This technique does now allow you to identify specific users. A single
user can have multiple devices, each with a different ID value.
●
You want to identify a user for the purposes of advertising. Use the advertisingIdentifier property
of the ASIdentifierManager class to obtain an ID for the user.
Because users are allowed to run apps on all of their iOS devices, Apple does not provide a way to identify the
same user on multiple devices. If you need to identify a specific user, you must provide your own solution
using universally unique IDs (UUIDs), a login account, or some other type of identification system.
Supporting Multiple Versions of iOS
An app that supports the latest version of iOS plus one or more earlier versions must use runtime checks to
prevent the use of newer APIs on older versions of iOS. Runtime checks prevent your app from crashing when
it tries to use a feature that is not available on the current operating system.
There are several types of checks that you can make:
●
To determine whether a class exists, see if its Class object is nil. The linker returns nil for any unknown
class objects, making it possible to use a conditional check similar to the following:
if ([UIPrintInteractionController class]) {
// Create an instance of the class and use it.
}
else {
// The print interaction controller is not available so use an
alternative technique.
}
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Preserving Your App’s Visual Appearance Across Launches
●
To determine whether a method is available on an existing class, use the instancesRespondToSelector:
class method or the respondsToSelector: instance method.
●
To determine whether a C-based function is available, perform a Boolean comparison of the function name
to NULL. If the symbol is not NULL, you can call the function. For example:
if (UIGraphicsBeginPDFPage != NULL) {
UIGraphicsBeginPDFPage();
}
For more information and examples of how to write code that supports multiple deployment targets, see SDK
Compatibility Guide .
Preserving Your App’s Visual Appearance Across Launches
Even if your app supports background execution, it cannot run forever. At some point, the system might need
to terminate your app to free up memory for the current foreground app. However, the user should never
have to care if an app is already running or was terminated. From the user’s perspective, quitting an app should
just seem like a temporary interruption. When the user returns to an app, that app should always return the
user to the last point of use, so that the user can continue with whatever task was in progress. This behavior
provides a better experience for the user and with the state restoration support built in to UIKit is relatively
easy to achieve.
The state preservation system in UIKit provides a simple but flexible infrastructure for preserving and restoring
the state of your app’s view controllers and views. The job of the infrastructure is to drive the preservation and
restoration processes at the appropriate times. To do that, UIKit needs help from your app. Only you understand
the content of your app, and so only you can write the code needed to save and restore that content. And
when you update your app’s UI, only you know how to map older preserved content to the newer objects in
your interface.
There are three places where you have to think about state preservation in your app:
●
Your app delegate object, which manages the app’s top-level state
●
Your app’s view controller objects, which manage the overall state for your app’s user interface
●
Your app’s custom views, which might have some custom data that needs to be preserved
UIKit allows you to choose which parts of your user interface you want to preserve. And if you already have
custom code for handling state preservation, you can continue to use that code and migrate portions to the
UIKit state preservation system as needed.
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Enabling State Preservation and Restoration in Your App
State preservation and restoration is not an automatic feature and apps must opt-in to use it. Apps indicate
their support for the feature by implementing the following methods in their app delegate:
application:shouldSaveApplicationState:
application:shouldRestoreApplicationState:
Normally, your implementations of these methods just return YES to indicate that state preservation and
restoration can occur. However, apps that want to preserve and restore their state conditionally can return NO
in situations where the operations should not occur. For example, after releasing an update to your app, you
might want to return NO from your application:shouldRestoreApplicationState: method if your
app is unable to usefully restore the state from a previous version.
The Preservation and Restoration Process
State preservation and restoration is an opt-in feature and works with the help of your app. Your app identifies
objects that should be preserved and UIKit does the work of preserving and restoring those objects at
appropriate times. Because UIKit handles so much of the process, it helps to understand what it does behind
the scenes so that you know how your custom code fits into the overall scheme.
When thinking about state preservation and restoration, it helps to separate the two processes first. UIKit
preserves your app’s state at appropriate times, such as when your app moves from the foreground to the
background. When UIKit determines new state information is needed, it looks at your app’s views and view
controllers to see which ones should be preserved. For each of those objects, UIKit writes preservation-related
data to an encrypted on-disk file. The next time your app launches from scratch, UIKit looks for that file and,
if it is present, uses it to try and restore your app’s state. Because the file is encrypted, state preservation and
restoration only happens when the device is unlocked.
During the restoration process, UIKit uses the preserved data to reconstitute your interface but the creation
of actual objects is handled by your code. Because your app might load objects from a storyboard file
automatically, only your code knows which objects need to be created and which might already exist and can
simply be returned. After creating each object, UIKit initializes them with the preserved state information.
During the preservation and restoration process, your app has a handful of responsibilities.
●
●
During preservation, your app is responsible for:
●
Telling UIKit that it supports state preservation.
●
Telling UIKit which view controllers and views should be preserved.
●
Encoding relevant data for any preserved objects.
During restoration, your app is responsible for:
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●
Telling UIKit that it supports state restoration.
●
Providing (or creating) the objects that are requested by UIKit.
●
Decoding the state of your preserved objects and using it to return the object to its previous state.
Of your app’s responsibilities, the most significant are telling UIKit which objects to preserve and providing
those objects during subsequent launches. Those two behaviors are where you should spend most of your
time when designing your app’s preservation and restoration code. They are also where you have the most
control over the actual process. To understand why that is the case, it helps to look at an example.
Figure 5-1 shows the view controller hierarchy of a tab bar interface after the user has interacted with several
of the tabs. As you can see, some of the view controllers are loaded automatically as part of the app’s main
storyboard file but some of the view controllers were presented or pushed onto the view controllers in different
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tabs. Without state restoration, only the view controllers from the main storyboard file would be restored
during subsequent launches. By adding support for state restoration to your app, you can preserve all of the
view controllers.
Figure 5-1
A sample view controller hierarchy
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UIKit preserves only those objects that have an assigned restoration identifier. A restoration identifier is a
string that identifies the view or view controller to UIKit and your app. The value of this string is significant
only to your code but the presence of this string tells UIKit that it needs to preserve the tagged object. During
the preservation process, UIKit walks your app’s view controller hierarchy and preserves all objects that have
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a restoration identifier. If a view controller does not have a restoration identifier, that view controller and all
of its views and child view controllers are not preserved. Figure 5-2 shows an updated version of the previous
view hierarchy, now with restoration identifies applied to most (but not all) of the view controllers.
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Figure 5-2
Adding restoration identifies to view controllers
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Depending on your app, it might or might not make sense to preserve every view controller. If a view controller
presents transitory information, you might not want to return to that same point on restore, opting instead
to return the user to a more stable point in your interface.
For each view controller you choose to preserve, you also need to decide how you want to restore it later. UIKit
offers two ways to recreate objects. You can let your app delegate recreate it or you can assign a restoration
class to the view controller and let that class recreate it. A restoration class implements the
UIViewControllerRestoration protocol and is responsible for finding or creating a designated object at
restore time. Here are some tips for when to use each one:
●
If the view controller is always loaded from your app’s main storyboard file at launch time, do not
assign a restoration class. Instead, let your app delegate find the object or take advantage of UIKit’s
support for implicitly finding restored objects.
●
For view controllers that are not loaded from your main storyboard file at launch time, assign a
restoration class. The simplest option is to make each view controller its own restoration class.
During the preservation process, UIKit identifies the objects to save and writes each affected object’s state to
disk. Each view controller object is given a chance to write out any data it wants to save. For example, a tab
view controller saves the identity of the selected tab. UIKit also saves information such as the view controller’s
restoration class to disk. And if any of the view controller’s views has a restoration identifier, UIKit asks them
to save their state information too.
The next time the app is launched, UIKit loads the app’s main storyboard or nib file as usual, calls the app
delegate’s application:willFinishLaunchingWithOptions: method, and then tries to restore the
app’s previous state. The first thing it does is ask your app to provide the set of view controller objects that
match the ones that were preserved. If a given view controller had an assigned restoration class, that class is
asked to provide the object; otherwise, the app delegate is asked to provide it.
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Flow of the Preservation Process
Figure 5-3 shows the high-level events that happen during state preservation and shows how the objects of
your app are affected. Before preservation even occurs, UIKit asks your app delegate if it should occur by calling
the application:shouldSaveApplicationState: method. If that method returns YES, UIKit begins
gathering and encoding your app’s views and view controllers. When it is finished, it writes the encoded data
to disk.
Figure 5-3
High-level flow interface preservation
App Delegate
UIKit
View /View Controller Objects
View Controller Only
The next time your app launches, the system automatically looks for a preserved state file, and if present, uses
it to restore your interface. Because this state information is only relevant between the previous and current
launch cycles of your app, the file is typically discarded after your app finishes launching. The file is also discarded
any time there is an error restoring your app. For example, if your app crashes during the restoration process,
the system automatically throws away the state information during the next launch cycle to avoid another
crash.
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Flow of the Restoration Process
Figure 5-4 shows the high-level events that happen during state restoration and shows how the objects of
your app are affected. After the standard initialization and UI loading is complete, UIKit asks your app delegate
if state restoration should occur at all by calling the application:shouldRestoreApplicationState:
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method. This is your app delegate’s opportunity to examine the preserved data and determine if state restoration
is possible. If it is, UIKit uses the app delegate and restoration classes to obtain references to your app’s view
controllers. Each object is then provided with the data it needs to restore itself to its previous state.
Figure 5-4
High-level flow for restoring your user interface
UIKit
App Delegate
Restoration Classes
View /View Controller Object
App Delegate
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Although UIKit helps restore the individual view controllers, it does not automatically restore the relationships
between those view controllers. Instead, each view controller is responsible for encoding enough state
information to return itself to its previous state. For example, a navigation controller encodes information
about the order of the view controllers on its navigation stack. It then uses this information later to return
those view controllers to their previous positions on the stack. Other view controllers that have embedded
child view controllers are similarly responsible for encoding any information they need to restore their children
later.
Note: Not all view controllers need to encode their child view controllers. For example, tab bar
controllers do not encode information about their child view controllers. Instead, it is assumed that
your app follows the usual pattern of creating the appropriate child view controllers prior to creating
the tab bar controller itself.
Because you are responsible for recreating your app’s view controllers, you have some flexibility to change
your interface during the restoration process. For example, you could reorder the tabs in a tab bar controller
and still use the preserved data to return each tab to its previous state. Of course, if you make dramatic changes
to your view controller hierarchy, such as during an app update, you might not be able to use the preserved
data.
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What Happens When You Exclude Groups of View Controllers?
When the restoration identifier of a view controller is nil, that view controller and any child view controllers
it manages are not preserved automatically. For example, in Figure 5-5, because a navigation controller did
not have a restoration identifier, it and all of its child view controllers and views are omitted from the preserved
data.
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Figure 5-5
Excluding view controllers from the automatic preservation process
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Even if you decide not to preserve view controllers, that does not mean all of those view controllers disappear
from the view hierarchy altogether. At launch time, your app might still create the view controllers as part of
its default setup. For example, if any view controllers are loaded automatically from your app’s storyboard file,
they would still appear, albeit in their default configuration, as shown in Figure 5-6.
Figure 5-6
Loading the default set of view controllers
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Something else to realize is that even if a view controller is not preserved automatically, you can still encode
a reference to that view controller and preserve it manually. In Figure 5-5 (page 80), the three child view
controllers of the first navigation controller have restoration identifiers, even though there parent navigation
controller does not. If your app delegate (or any preserved object) encodes a reference to those view controllers,
their state is preserved. Even though their order in the navigation controller is not saved, you could still use
those references to recreate the view controllers and install them in the navigation controller during subsequent
launch cycles.
Checklist for Implementing State Preservation and Restoration
Supporting state preservation and restoration requires modifying your app delegate and view controller objects
to encode and decode the state information. If your app has any custom views that also have preservable state
information, you need to modify those objects too.
When adding state preservation and restoration to your code, use the following list to remind you of the code
you need to write.
●
(Required) Implement the application:shouldSaveApplicationState: and
application:shouldRestoreApplicationState: methods in your app delegate; see Enabling State
Preservation and Restoration in Your App (page 83).
●
(Required) Assign restoration identifiers to each view controller you want to preserve by assigning a non
empty string to their restorationIdentifier property; see Marking Your View Controllers for
Preservation (page 84).
If you want to save the state of specific views too, assign non empty strings to their
restorationIdentifier properties; see Preserving the State of Your Views (page 87).
●
(Required) Show your app’s window from the application:willFinishLaunchingWithOptions:
method of your app delegate. The state restoration machinery needs the window so that it can restore
scroll positions and other relevant bits of your app’s interface.
●
Assign restoration classes to the appropriate view controllers. (If you do not do this, your app delegate is
asked to provide the corresponding view controller at restore time.) See Restoring Your View Controllers
at Launch Time (page 84).
●
(Recommended) Encode and decode the state of your views and view controllers using the
encodeRestorableStateWithCoder: and decodeRestorableStateWithCoder: methods of those
objects; see Encoding and Decoding Your View Controller’s State (page 86).
●
Encode and decode any version information or additional state information for your app using the
application:willEncodeRestorableStateWithCoder: and
application:didDecodeRestorableStateWithCoder: methods of your app delegate; see Preserving
Your App’s High-Level State (page 90).
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●
Objects that act as data sources for table views and collection views should implement the
UIDataSourceModelAssociation protocol. Although not required, this protocol helps preserve the
selected and visible items in those types of views. See Implementing Preservation-Friendly Data
Sources (page 89).
Enabling State Preservation and Restoration in Your App
State preservation and restoration is not an automatic feature and apps must opt-in to use it. Apps indicate
their support for the feature by implementing the following methods in their app delegate:
application:shouldSaveApplicationState:
application:shouldRestoreApplicationState:
Normally, your implementations of these methods just return YES to indicate that state preservation and
restoration can occur. However, apps that want to preserve and restore their state conditionally can return NO
in situations where the operations should not occur. For example, after releasing an update to your app, you
might want to return NO from your application:shouldRestoreApplicationState: method if your
app is unable to usefully restore the state from a previous version.
Preserving the State of Your View Controllers
Preserving the state of your app’s view controllers should be your main goal. View controllers define the
structure of your user interface. They manage the views needed to present that interface and they coordinate
the getting and setting of the data that backs those views. To preserve the state of a single view controller,
you must do the following:
●
(Required) Assign a restoration identifier to the view controller; see Marking Your View Controllers for
Preservation (page 84).
●
(Required) Provide code to create or locate new view controller objects at launch time; see Restoring Your
View Controllers at Launch Time (page 84).
●
(Optional) Implement the encodeRestorableStateWithCoder: and
decodeRestorableStateWithCoder: methods to encode and restore any state information that cannot
be recreated during a subsequent launch; see Encoding and Decoding Your View Controller’s State (page
86).
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Marking Your View Controllers for Preservation
UIKit preserves only those view controllers whose restorationIdentifier property contains a valid string
object. For view controllers that you know you want to preserve, set the value of this property when you
initialize the view controller object. If you load the view controller from a storyboard or nib file, you can set
the restoration identifier there.
Choosing an appropriate value for restoration identifiers is important. During the restoration process, your
code uses the restoration identifier to determine which view controller to retrieve or create. If every view
controller object is based on a different class, you can use the class name for the restoration identifier. However,
if your view controller hierarchy contains multiple instances of the same class, you might need to choose
different names based on each view usage.
When it asks you to provide a view controller, UIKit provides you with the restoration path of the view controller
object. A restoration path is the sequence of restoration identifiers starting at the root view controller and
walking down the view controller hierarchy to the current object. For example, imagine you have a tab bar
controller whose restoration identifier is TabBarControllerID, and the first tab contains a navigation
controller whose identifier is NavControllerID and whose root view controller’s identifier is
MyViewController. The full restoration path for the root view controller would be
TabBarControllerID/NavControllerID/MyViewController.
The restoration path for every object must be unique. If a view controller has two child view controllers, each
child must have a different restoration identifier. However, two view controllers with different parent objects
may use the same restoration identifier because the rest of the restoration path provides the needed uniqueness.
Some UIKit view controllers, such as navigation controllers, automatically disambiguate their child view
controllers, allowing you to use the same restoration identifiers for each child. For more information about the
behavior of a given view controller, see the corresponding class reference.
At restore time, you use the provided restoration path to determine which view controller to return to UIKit.
For more information on how you use restoration identifiers and restoration paths to restore view controllers,
see Restoring Your View Controllers at Launch Time (page 84).
Restoring Your View Controllers at Launch Time
During the restoration process, UIKit asks your app to create (or locate) the view controller objects that comprise
your preserved user interface. UIKit adheres to the following process when trying to locate view controllers:
1.
If the view controller had a restoration class, UIKit asks that class to provide the view controller. UIKit
calls the viewControllerWithRestorationIdentifierPath:coder: method of the associated
restoration class to retrieve the view controller. If that method returns nil, it is assumed that the app does
not want to recreate the view controller and UIKit stops looking for it.
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2.
If the view controller did not have a restoration class, UIKit asks the app delegate to provide the view
controller. UIKit calls the application:viewControllerWithRestorationIdentifierPath:coder:
method of your app delegate to look for view controllers without a restoration class. If that method returns
nil, UIKit tries to find the view controller implicitly.
3.
If a view controller with the correct restoration path already exists, UIKit uses that object. If your app
creates view controllers at launch time (either programmatically or by loading them from a resource file)
and assigns restoration identifiers to them, UIKit finds them implicitly through their restoration paths.
4.
If the view controller was originally loaded from a storyboard file, UIKit uses the saved storyboard
information to locate and create it. UIKit saves information about a view controller’s storyboard inside
the restoration archive. At restore time, it uses that information to locate the same storyboard file and
instantiate the corresponding view controller if the view controller was not found by any other means.
It is worth noting that if you specify a restoration class for a view controller, UIKit does not try to find your view
controller implicitly. If the viewControllerWithRestorationIdentifierPath:coder: method of your
restoration class returns nil, UIKit stops trying to locate your view controller. This gives you control over
whether you really want to create the view controller. If you do not specify a restoration class, UIKit does
everything it can to find the view controller for you, creating it as necessary from your app’s storyboard files.
If you choose to use a restoration class, the implementation of your
viewControllerWithRestorationIdentifierPath:coder: method should create a new instance of
the class, perform some minimal initialization, and return the resulting object. Listing 5-1 shows an example
of how you might use this method to load a view controller from a storyboard. Because the view controller
was originally loaded from a storyboard, this method uses the
UIStateRestorationViewControllerStoryboardKey key to get the storyboard from the archive. Note
that this method does not try to configure the view controller’s data fields. That step occurs later when the
view controller’s state is decoded.
Listing 5-1
Creating a new view controller during restoration
+ (UIViewController*) viewControllerWithRestorationIdentifierPath:(NSArray
*)identifierComponents
coder:(NSCoder *)coder {
MyViewController* vc;
UIStoryboard* sb = [coder
decodeObjectForKey:UIStateRestorationViewControllerStoryboardKey];
if (sb) {
vc = (PushViewController*)[sb
instantiateViewControllerWithIdentifier:@"MyViewController"];
vc.restorationIdentifier = [identifierComponents lastObject];
vc.restorationClass = [MyViewController class];
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}
return vc;
}
Reassigning the restoration identifier and restoration class, as in the preceding example, is a good habit to
adopt when creating new view controllers. The simplest way to restore the restoration identifier is to grab the
last item in the identifierComponents array and assign it to your view controller.
For objects that were already loaded from your app’s main storyboard file at launch time, do not create a new
instance of each object. Instead, implement the
application:viewControllerWithRestorationIdentifierPath:coder: method of your app delegate
and use it to return the appropriate objects or let UIKit find those objects implicitly.
Encoding and Decoding Your View Controller’s State
For each object slated for preservation, UIKit calls the object’s encodeRestorableStateWithCoder: method
to give it a chance to save its state. During the decode process, a matching call to the
decodeRestorableStateWithCoder: method is made to decode that state and apply it to the object. The
implementation of these methods is optional, but recommended, for your view controllers. You can use them
to save and restore the following types of information:
●
References to any data being displayed (not the data itself )
●
For a container view controller, references to its child view controllers
●
Information about the current selection
●
For view controllers with a user-configurable view, information about the current configuration of that
view.
In your encode and decode methods, you can encode any values supported by the coder, including other
objects. For all objects except views and view controllers, the object must adopt the NSCoding protocol and
use the methods of that protocol to write its state. For views and view controllers, the coder does not use the
methods of the NSCoding protocol to save the object’s state. Instead, the coder saves the restoration identifier
of the object and adds it to the list of preservable objects, which results in that object’s
encodeRestorableStateWithCoder: method being called.
The encodeRestorableStateWithCoder: and decodeRestorableStateWithCoder: methods of your
view controllers should always call super at some point in their implementation. Calling super gives the
parent class a chance to save and restore any additional information. Listing 5-2 shows a sample implementation
of these methods that save a numerical value used to identify the specified view controller.
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Listing 5-2
Encoding and decoding a view controller’s state.
- (void)encodeRestorableStateWithCoder:(NSCoder *)coder {
[super encodeRestorableStateWithCoder:coder];
[coder encodeInt:self.number forKey:MyViewControllerNumber];
}
- (void)decodeRestorableStateWithCoder:(NSCoder *)coder {
[super decodeRestorableStateWithCoder:coder];
self.number = [coder decodeIntForKey:MyViewControllerNumber];
}
Coder objects are not shared during the encode and decode process. Each object with preservable state receives
its own coder that it can use to read or write data. The use of unique coders means that you do not have to
worry about key namespace collisions among your own objects. However, you must still avoid using some
special key names that UIKit provides. Specifically, each coder contains the
UIApplicationStateRestorationBundleVersionKey and
UIApplicationStateRestorationUserInterfaceIdiomKey keys, which provide information about the
bundle version and current user interface idiom. Coders associated with view controllers may also contain the
UIStateRestorationViewControllerStoryboardKey key, which identifies the storyboard from which
that view controller originated.
For more information about implementing your encode and decode methods for your view controllers, see
UIViewController Class Reference .
Preserving the State of Your Views
If a view has state information worth preserving, you can save that state with the rest of your app’s view
controllers. Because they are usually configured by their owning view controller, most views do not need to
save state information. The only time you need to save a view’s state is when the view itself can be altered by
the user in a way that is independent of its data or the owning view controller. For example, scroll views save
the current scroll position, which is information that is not interesting to the view controller but which does
affect how the view presents itself.
To designate that a view’s state should be saved, you do the following:
●
Assign a valid string to the view’s restorationIdentifier property.
●
Use the view from a view controller that also has a valid restoration identifier.
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●
For table views and collection views, assign a data source that adopts the
UIDataSourceModelAssociation protocol.
As with view controllers, assigning a restoration identifier to a view tells the system that the view object has
state that your app wants to save. The restoration identifier can also be used to locate the view later.
Like view controllers, views define methods for encoding and decoding their custom state. If you create a view
with state worth saving, you can use these methods to read and write any relevant data.
UIKit Views with Preservable State
In order to save the state of any view, including both custom and standard system views, you must assign a
restoration identifier to the view. Views without a restoration identifier are not added to the list of preservable
objects by UIKit.
The following UIKit views have state information that can be preserved:
●
UICollectionView
●
UIImageView
●
UIScrollView
●
UITableView
●
UITextField
●
UITextView
●
UIWebView
Other frameworks may also have views with preservable state. For information about whether a view saves
state information and what state it saves, see the reference for the corresponding class.
Preserving the State of a Custom View
If you are implementing a custom view that has restorable state, implement the
encodeRestorableStateWithCoder: and decodeRestorableStateWithCoder: methods and use them
to encode and decode that state. Use those methods to save only the data that cannot be easily reconfigured
by other means. For example, use these methods to save data that is modified by user interactions with the
view. Do not use these methods to save the data being presented by the view or any data that the owning
view controller can configure easily.
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Listing 5-3 shows an example of how to preserve and restore the selection for a custom view that contains
editable text. In the example, the range is accessible using the selectionRange and setSelectionRange:
methods, which are custom methods the view uses to manage the selection. Encoding the data only requires
writing it to the provided coder object. Restoring the data requires reading it and applying it to the view.
Listing 5-3
Preserving the selection of a custom text view
// Preserve the text selection
- (void) encodeRestorableStateWithCoder:(NSCoder *)coder {
[super encodeRestorableStateWithCoder:coder];
NSRange range = [self selectionRange];
[coder encodeInt:range.length forKey:kMyTextViewSelectionRangeLength];
[coder encodeInt:range.location forKey:kMyTextViewSelectionRangeLocation];
}
// Restore the text selection.
- (void) decodeRestorableStateWithCoder:(NSCoder *)coder {
[super decodeRestorableStateWithCoder:coder];
if ([coder containsValueForKey:kMyTextViewSelectionRangeLength] &&
[coder containsValueForKey:kMyTextViewSelectionRangeLocation]) {
NSRange range;
range.length = [coder decodeIntForKey:kMyTextViewSelectionRangeLength];
range.location = [coder decodeIntForKey:kMyTextViewSelectionRangeLocation];
if (range.length > 0)
[self setSelectionRange:range];
}
}
Implementing Preservation-Friendly Data Sources
Because the data displayed by a table or collection view can change, both classes save information about the
current selection and visible cells only if their data source implements the UIDataSourceModelAssociation
protocol. This protocol provides a way for a table or collection view to identify the content it contains without
relying on the index path of that content. Thus, regardless of where the data source places an item during the
next launch cycle, the view still has all the information it needs to locate that item.
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In order to implement the UIDataSourceModelAssociation protocol successfully, your data source object
must be able to identify items between subsequent launches of the app. This means that any identification
scheme you devise must be invariant for a given piece of data. This is essential because the data source must
be able to retrieve the same piece of data for the same identifier each time it is requested. Implementing the
protocol itself is a matter of mapping from a data item to its unique ID and back again.
Apps that use Core Data can implement the protocol by taking advantage of object identifiers. Each object in
a Core Data store has a unique object identifier that can be converted into a URI and used to locate the object
later. If your app does not use Core Data, you need to devise your own form of unique identifiers if you want
to support state preservation for your views.
Note: Remember that implementing the UIDataSourceModelAssociation protocol is only
necessary to preserve attributes such as the current selection in a table or collection view. This
protocol is not used to preserve the actual data managed by your data source. It is your app’s
responsibility to ensure that its data is saved at appropriate times.
Preserving Your App’s High-Level State
In addition to the data preserved by your app’s view controllers and views, UIKit provides hooks for you to
save any miscellaneous data needed by your app. Specifically, the UIApplicationDelegate protocol includes
the following methods for you to override:
●
application:willEncodeRestorableStateWithCoder:
●
application:didDecodeRestorableStateWithCoder:
If your app contains state that does not live in a view controller, but that needs to be preserved, you can use
the preceding methods to save and restore it. The application:willEncodeRestorableStateWithCoder:
method is called at the very beginning of the preservation process so that you can write out any high-level
app state, such as the current version of your user interface. The
application:didDecodeRestorableStateWithCoder: method is called at the end of the restoration
state so that you can decode any data and perform any final cleanup that your app requires.
Tips for Saving and Restoring State Information
As you add support for state preservation and restoration to your app, consider the following guidelines:
●
Encode version information along with the rest of your app’s state. During the preservation process,
it is recommended that you encode a version string or number that identifies the current revision of your
app’s user interface. You can encode this state in the
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application:willEncodeRestorableStateWithCoder: method of your app delegate. When your
app delegate’s application:shouldRestoreApplicationState: method is called, you can retrieve
this information from the provided coder and use it to determine if state preservation is possible.
●
Do not include objects from your data model in your app’s state. Apps should continue to save their
data separately in iCloud or to local files on disk. Never use the state restoration mechanism to save that
data. Preserved interface data may be deleted if problems occur during a restore operation. Therefore,
any preservation-related data you write to disk should be considered purgeable.
●
The state preservation system expects you to use view controllers in the ways they were designed to
be used. The view controller hierarchy is created through a combination of view controller containment
and by presenting one view controller from another. If your app displays the view of a view controller by
another means—for example, by adding it to another view without creating a containment relationship
between the corresponding view controllers—the preservation system will not be able to find your view
controller to preserve it.
●
Remember that you might not want to preserve all view controllers. In some cases, it might not make
sense to preserve a view controller. For example, if the user left your app while it was displaying a view
controller to change the user’s password, you might want to cancel the operation and restore the app to
the previous screen. In such a case, you would not preserve the view controller that asks for the new
password information.
●
Avoid swapping view controller classes during the restoration process. The state preservation system
encodes the class of the view controllers it preserves. During restoration, if your app returns an object
whose class does not match (or is not a subclass of ) the original object, the system does not ask the view
controller to decode any state information. Thus, swapping out the old view controller for a completely
different one does not restore the full state of the object.
●
The system automatically deletes an app’s preserved state when the user force quits the app. Deleting
the preserved state information when the app is killed is a safety precaution. (As a safety precaution, the
system also deletes preserved state if the app crashes twice during launch.) If you want to test your app’s
ability to restore its state, you should not use the multitasking bar to kill the app during debugging. Instead,
use Xcode to kill the app or kill the app programmatically by installing a temporary command or gesture
to call exit on demand.
Tips for Developing a VoIP App
A Voice over Internet Protocol (VoIP) app allows the user to make phone calls using an Internet connection
instead of the device’s cellular service. Such an app needs to maintain a persistent network connection to its
associated service so that it can receive incoming calls and other relevant data. Rather than keep VoIP apps
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awake all the time, the system allows them to be suspended and provides facilities for monitoring their sockets
for them. When incoming traffic is detected, the system wakes up the VoIP app and returns control of its sockets
to it.
There are several requirements for implementing a VoIP app:
1.
Enable the Voice over IP background mode for your app. (Because VoIP apps involve audio content, it is
recommended that you also enable the Audio and AirPlay background mode.) You enable background
modes in the Capabilities tab of your Xcode project.
2.
Configure one of the app’s sockets for VoIP usage.
3.
Before moving to the background, call the setKeepAliveTimeout:handler: method to install a handler
to be executed periodically. Your app can use this handler to maintain its service connection.
4.
Configure your audio session to handle transitions to and from active use.
5.
To ensure a better user experience on iPhone, use the Core Telephony framework to adjust your behavior
in relation to cell-based phone calls; see Core Telephony Framework Reference .
6.
To ensure good performance for your VoIP app, use the System Configuration framework to detect network
changes and allow your app to sleep as much as possible.
Enabling the VoIP background mode lets the system know that it should allow the app to run in the background
as needed to manage its network sockets. This key also permits your app to play background audio (although
enabling the Audio and AirPlay mode is still encouraged). An app that supports this mode is also relaunched
in the background immediately after system boot to ensure that the VoIP services are always available.
Configuring Sockets for VoIP Usage
In order for your app to maintain a persistent connection while it is in the background, you must tag your app’s
main communication socket specifically for VoIP usage. Tagging this socket tells the system that it should take
over management of the socket when your app is suspended. The handoff itself is totally transparent to your
app. And when new data arrives on the socket, the system wakes up the app and returns control of the socket
so that the app can process the incoming data.
You need to tag only the socket you use for communicating with your VoIP service. This is the socket you use
to receive incoming calls or other data relevant to maintaining your VoIP service connection. Upon receipt of
incoming data, the handler for this socket needs to decide what to do. For an incoming call, you likely want
to post a local notification to alert the user to the call. For other noncritical data, though, you might just process
the data quietly and allow the system to put your app back into the suspended state.
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In iOS, most sockets are managed using streams or other high-level constructs. To configure a socket for VoIP
usage, the only thing you have to do beyond the normal configuration is add a special key that tags the
interface as being associated with a VoIP service. Table 5-1 lists the stream interfaces and the configuration
for each.
Table 5-1
Configuring stream interfaces for VoIP usage
Interface
Configuration
NSInputStream and
NSOutputStream
For Cocoa streams, use the setProperty:forKey: method to add the
NSStreamNetworkServiceType property to the stream. The value of
this property should be set to NSStreamNetworkServiceTypeVoIP.
NSURLRequest
When using the URL loading system, use the setNetworkServiceType:
method of your NSMutableURLRequest object to set the network service
type of the request. The service type should be set to
NSURLNetworkServiceTypeVoIP.
CFReadStreamRef and
CFWriteStreamRef
For Core Foundation streams, use the CFReadStreamSetProperty or
CFWriteStreamSetProperty function to add the
kCFStreamNetworkServiceType property to the stream. The value for
this property should be set to kCFStreamNetworkServiceTypeVoIP.
Note: When configuring your sockets, you need to configure only your main signaling channel with
the appropriate service type key. You do not need to include this key when configuring your voice
channels.
Because VoIP apps need to stay running in order to receive incoming calls, the system automatically relaunches
the app if it exits with a nonzero exit code. (This type of exit could happen when there is memory pressure
and your app is terminated as a result.) However, terminating the app also releases all of its sockets, including
the one used to maintain the VoIP service connection. Therefore, when the app is launched, it always needs
to create its sockets from scratch.
For more information about configuring Cocoa stream objects, see Stream Programming Guide . For information
about using URL requests, see URL Loading System Programming Guide . And for information about configuring
streams using the CFNetwork interfaces, see CFNetwork Programming Guide .
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Installing a Keep-Alive Handler
To prevent the loss of its connection, a VoIP app typically needs to wake up periodically and check in with its
server. To facilitate this behavior, iOS lets you install a special handler using the
setKeepAliveTimeout:handler: method of UIApplication. You typically install this handler in the
applicationDidEnterBackground: method of your app delegate. Once installed, the system calls your
handler at least once before the timeout interval expires, waking up your app as needed to do so.
Your keep-alive handler executes in the background and should return as quickly as possible. Handlers are
given a maximum of 10 seconds to perform any needed tasks and return. If a handler has not returned after
10 seconds, or has not requested extra execution time before that interval expires, the system suspends the
app.
When installing your handler, specify the largest timeout value that is practical for your app’s needs. The
minimum allowable interval for running your handler is 600 seconds, and attempting to install a handler with
a smaller timeout value will fail. Although the system promises to call your handler block before the timeout
value expires, it does not guarantee the exact call time. To improve battery life, the system typically groups
the execution of your handler with other periodic system tasks, thereby processing all tasks in one quick burst.
As a result, your handler code must be prepared to run earlier than the actual timeout period you specified.
Configuring Your App’s Audio Session
As with any background audio app, the audio session for a VoIP app must be configured properly to ensure
the app works smoothly with other audio-based apps. Because audio playback and recording for a VoIP app
are not used all the time, it is especially important that you create and configure your app’s audio session
object only when it is needed. For example, you would create the audio session to notify the user of an incoming
call or while the user was actually on a call. As soon as the call ends, you would then remove strong references
to the audio session and give other audio apps the opportunity to play their audio.
For information about how to configure and manage an audio session for a VoIP app, see Audio Session
Programming Guide .
Using the Reachability Interfaces to Improve the User Experience
Because VoIP apps rely heavily on the network, they should use the reachability interfaces of the System
Configuration framework to track network availability and adjust their behavior accordingly. The reachability
interfaces allow an app to be notified whenever network conditions change. For example, a VoIP app could
close its network connections when the network becomes unavailable and recreate them when it becomes
available again. The app could also use those kinds of changes to keep the user apprised about the state of
the VoIP connection.
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To use the reachability interfaces, you must register a callback function with the framework and use it to track
changes. To register a callback function:
1.
Create a SCNetworkReachabilityRef structure for your target remote host.
2.
Assign a callback function to your structure (using the SCNetworkReachabilitySetCallback function)
that processes changes in your target’s reachability status.
3.
Add that target to an active run loop of your app (such as the main run loop) using the
SCNetworkReachabilityScheduleWithRunLoop function.
Adjusting your app’s behavior based on the availability of the network can also help improve the battery life
of the underlying device. Letting the system track the network changes means that your app can let itself go
to sleep more often.
For more information about the reachability interfaces, see System Configuration Framework Reference .
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Inter-App Communication
Apps communicate only indirectly with other apps on a device. You can use AirDrop to share files and data
with other apps. You can also define a custom URL scheme so that apps can send information to your app
using URLs.
Note: You can also send files between apps using a UIDocumentInteractionController object
or a document picker. For information about adding support for a document interaction controller,
see Document Interaction Programming Topics for iOS . For information about using a document
picker to open files, see Document Picker Programming Guide .
Supporting AirDrop
AirDrop lets you share photos, documents, URLs, and other types of data with nearby devices. AirDrop takes
advantage of peer-to-peer networking to find nearby devices and connect to them.
Sending Files and Data to Another App
To send files and data using AirDrop, use a UIActivityViewController object to display an activity sheet
from your user interface using. When creating this view controller, you specify the data objects that you want
to share. The view controller displays only those activities that support the specified data. For AirDrop, you
can specify images, strings, URLs, and several other types of data. You can also pass custom objects that adopt
the UIActivityItemSource protocol.
To display an activity view controller, you can use code similar to that shown in Listing 6-1. The activity view
controller automatically uses the type of the specified object to determine what activities to display in the
activity sheet. You do not have to specify the AirDrop activity explicitly. However, you can prevent the sheet
from displaying specific types using the view controller’s excludedActivityTypes property. When displaying
an activity view controller on iPad, you must use a popover.
Listing 6-1
Displaying an activity sheet on iPhone
- (void)displayActivityControllerWithDataObject:(id)obj {
UIActivityViewController* vc = [[UIActivityViewController alloc]
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Supporting AirDrop
initWithActivityItems:@[obj]
applicationActivities:nil];
[self presentViewController:vc animated:YES completion:nil];
}
For more information about using the activity view controller, see UIActivityViewController Class Reference . For
a complete list of activities and the data types they support, see UIActivity Class Reference .
Receiving Files and Data Sent to Your App
To receive files sent to your app using AirDrop, do the following:
●
In Xcode, declare support for the document types your app is capable of opening.
●
In your app delegate, implement the application:openURL:sourceApplication:annotation:
method. Use that method to receive the data that was sent by the other app.
●
Be prepared to look for files in your app’s Documents/Inbox directory and move them out of that directory
as needed.
The Info tab of your Xcode project contains a Document Types section for specifying the document types your
app supports. At a minimum, you must specify a name for your document type and one or more UTIs that
represent the data type. For example, to declare support for PNG files, you would include public.png as the
UTI string. iOS uses the specified UTIs to determine if your app is eligible to open a given document.
After transferring an eligible document to your app’s container, iOS launches your app (if needed) and calls
the application:openURL:sourceApplication:annotation: method of its app delegate. If your app
is in the foreground, you should use this method to open the file and display it to the user. If your app is in
the background, you might decide only to note that the file is there so that you can open it later. Because files
transferred via AirDrop are encrypted using data protection, you cannot open files unless the device is currently
unlocked.
Files transferred to your app using AirDrop are placed in your app’s Documents/Inbox directory. Your app
has permission to read and delete files in this directory but it does not have permission to write to files. If you
plan to modify the file, you must move it out of the Inbox directory before doing so. It is recommended that
you delete files from the Inbox directory when you no longer need them.
For more information about supporting document types in your app, see Document-Based App Programming
Guide for iOS .
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Inter-App Communication
Using URL Schemes to Communicate with Apps
Using URL Schemes to Communicate with Apps
A URL scheme lets you communicate with other apps through a protocol that you define. To communicate
with an app that implements such a scheme, you must create an appropriately formatted URL and ask the
system to open it. To implement support for a custom scheme, you must declare support for the scheme and
handle incoming URLs that use the scheme.
Note: Apple provides built-in support for the http, mailto, tel, and sms URL schemes among
others. It also supports http–based URLs targeted at the Maps, YouTube, and iPod apps. The handlers
for these schemes are fixed and cannot be changed. If your URL type includes a scheme that is
identical to one defined by Apple, the Apple-provided app is launched instead of your app. For
information about the schemes supported by apple, see Apple URL Scheme Reference .
Sending a URL to Another App
When you want to send data to an app that implements a custom URL scheme, create an appropriately
formatted URL and call the openURL: method of the app object. The openURL: method launches the app
with the registered scheme and passes your URL to it. At that point, control passes to the new app.
The following code fragment illustrates how one app can request the services of another app (“todolist” in this
example is a hypothetical custom scheme registered by an app):
NSURL *myURL = [NSURL
URLWithString:@"todolist://www.acme.com?Quarterly%20Report#200806231300"];
[[UIApplication sharedApplication] openURL:myURL];
If your app defines a custom URL scheme, it should implement a handler for that scheme as described in
Implementing Custom URL Schemes (page 98). For more information about the system-supported URL schemes,
including information about how to format the URLs, see Apple URL Scheme Reference .
Implementing Custom URL Schemes
If your app can receive specially formatted URLs, you should register the corresponding URL schemes with the
system. Apps often use custom URL schemes to vend services to other apps. For example, the Maps app
supports URLs for displaying specific map locations.
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Using URL Schemes to Communicate with Apps
Registering Custom URL Schemes
To register a URL type for your app, include the CFBundleURLTypes key in your app’s Info.plist file. The
CFBundleURLTypes key contains an array of dictionaries, each of which defines a URL scheme the app
supports. Table 6-1 describes the keys and values to include in each dictionary.
Table 6-1
Keys and values of the CFBundleURLTypes property
Key
Value
CFBundleURLName
A string containing the abstract name of the URL scheme. To ensure
uniqueness, it is recommended that you specify a reverse-DNS style of
identifier, for example, com.acme.myscheme.
The string you specify is also used as a key in your app’s
InfoPlist.strings file. The value of the key is the human-readable
scheme name.
CFBundleURLSchemes
An array of strings containing the URL scheme names—for example, http,
mailto, tel, and sms.
Note: If more than one third-party app registers to handle the same URL scheme, there is currently
no process for determining which app will be given that scheme.
Handling URL Requests
An app that has its own custom URL scheme must be able to handle URLs passed to it. All URLs are passed to
your app delegate, either at launch time or while your app is running or in the background. To handle incoming
URLs, your delegate should implement the following methods:
●
Use the application:willFinishLaunchingWithOptions: and
application:didFinishLaunchingWithOptions: methods to retrieve information about the URL
and decide whether you want to open it. If either method returns NO, your app’s URL handling code is not
called.
●
Use the application:openURL:sourceApplication:annotation: method to open the file.
If your app is not running when a URL request arrives, it is launched and moved to the foreground so that it
can open the URL. The implementation of your application:willFinishLaunchingWithOptions: or
application:didFinishLaunchingWithOptions: method should retrieve the URL from its options
dictionary and determine whether the app can open it. If it can, return YES and let your
application:openURL:sourceApplication:annotation: (or application:handleOpenURL:)
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method handle the actual opening of the URL. (If you implement both methods, both must return YES before
the URL can be opened.) Figure 6-1 shows the modified launch sequence for an app that is asked to open a
URL.
Figure 6-1
Launching an app to open a URL
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If your app is running but is in the background or suspended when a URL request arrives, it is moved to the
foreground to open the URL. Shortly thereafter, the system calls the delegate’s
application:openURL:sourceApplication:annotation: to check the URL and open it. Figure 6-2
shows the modified process for moving an app to the foreground to open a URL.
Figure 6-2
Waking a background app to open a URL
Note: Apps that support custom URL schemes can specify different launch images to be displayed
when launching the app to handle a URL. For more information about how to specify these launch
images, see Displaying a Custom Launch Image When a URL is Opened (page 103).
All URLs are passed to your app in an NSURL object. It is up to you to define the format of the URL, but the
NSURL class conforms to the RFC 1808 specification and therefore supports most URL formatting conventions.
Specifically, the class includes methods that return the various parts of a URL as defined by RFC 1808, including
the user, password, query, fragment, and parameter strings. The “protocol” for your custom scheme can use
these URL parts for conveying various kinds of information.
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In the implementation of application:openURL:sourceApplication:annotation: shown in Listing
6-2, the passed-in URL object conveys app-specific information in its query and fragment parts. The delegate
extracts this information—in this case, the name of a to-do task and the date the task is due—and with it
creates a model object of the app. This example assumes that the user is using a Gregorian calendar. If your
app supports non-Gregorian calendars, you need to design your URL scheme accordingly and be prepared to
handle those other calendar types in your code.
Listing 6-2
Handling a URL request based on a custom scheme
- (BOOL)application:(UIApplication *)application openURL:(NSURL *)url
sourceApplication:(NSString *)sourceApplication annotation:(id)annotation
{
if ([[url scheme] isEqualToString:@"todolist"]) {
ToDoItem *item = [[ToDoItem alloc] init];
NSString *taskName = [url query];
if (!taskName || ![self isValidTaskString:taskName]) { // must have a task
name
return NO;
}
taskName = [taskName
stringByReplacingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
item.toDoTask = taskName;
NSString *dateString = [url fragment];
if (!dateString || [dateString isEqualToString:@"today"]) {
item.dateDue = [NSDate date];
} else {
if (![self isValidDateString:dateString]) {
return NO;
}
// format: yyyymmddhhmm (24-hour clock)
NSString *curStr = [dateString substringWithRange:NSMakeRange(0, 4)];
NSInteger yeardigit = [curStr integerValue];
curStr = [dateString substringWithRange:NSMakeRange(4, 2)];
NSInteger monthdigit = [curStr integerValue];
curStr = [dateString substringWithRange:NSMakeRange(6, 2)];
NSInteger daydigit = [curStr integerValue];
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curStr = [dateString substringWithRange:NSMakeRange(8, 2)];
NSInteger hourdigit = [curStr integerValue];
curStr = [dateString substringWithRange:NSMakeRange(10, 2)];
NSInteger minutedigit = [curStr integerValue];
NSDateComponents *dateComps = [[NSDateComponents alloc] init];
[dateComps setYear:yeardigit];
[dateComps setMonth:monthdigit];
[dateComps setDay:daydigit];
[dateComps setHour:hourdigit];
[dateComps setMinute:minutedigit];
NSCalendar *calendar = [s[NSCalendar alloc]
initWithCalendarIdentifier:NSGregorianCalendar];
NSDate *itemDate = [calendar dateFromComponents:dateComps];
if (!itemDate) {
return NO;
}
item.dateDue = itemDate;
}
[(NSMutableArray *)self.list addObject:item];
return YES;
}
return NO;
}
Be sure to validate the input you get from URLs passed to your app; see Validating Input and Interprocess
Communication in Secure Coding Guide to find out how to avoid problems related to URL handling. To learn
about URL schemes defined by Apple, see Apple URL Scheme Reference .
Displaying a Custom Launch Image When a URL is Opened
Apps that support custom URL schemes can provide a custom launch image for each scheme. When the system
launches your app to handle a URL and no relevant snapshot is available, it displays the launch image you
specify. To specify a launch image, provide a PNG image whose name uses the following naming conventions:
<basename> -<url_scheme> <other_modifiers> .png
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In this naming convention, basename represents the base image name specified by the UILaunchImageFile
key in your app’s Info.plist file. If you do not specify a custom base name, use the string Default. The
<url_scheme> portion of the name is your URL scheme name. To specify a generic launch image for the myapp
URL scheme, you would include an image file with the name [email protected] in the app’s bundle.
(The @2x modifier signifies that the image is intended for Retina displays. If your app also supports standard
resolution displays, you would also provide a Default-myapp.png image.)
For information about the other modifiers you can include in launch image names, see the description of the
UILaunchImageFile name key in Information Property List Key Reference .
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Performance Tips
At each step in the development of your app, consider the implications of your design choices on the overall
performance of your app. Power usage and memory consumption are extremely important considerations for
iOS apps, and there are many other considerations as well. The following sections describe the factors you
should consider throughout the development process.
Reduce Your App’s Power Consumption
Power consumption on mobile devices is always an issue. The power management system in iOS conserves
power by shutting down any hardware features that are not currently being used. You can help improve battery
life by optimizing your use of the following features:
●
The CPU
●
Wi-Fi, Bluetooth, and baseband (EDGE, 3G) radios
●
The Core Location framework
●
The accelerometers
●
The disk
The goal of your optimizations should be to do the most work you can in the most efficient way possible. You
should always optimize your app’s algorithms using Instruments. But even the most optimized algorithm can
still have a negative impact on a device’s battery life. You should therefore consider the following guidelines
when writing your code:
●
Avoid doing work that requires polling. Polling prevents the CPU from going to sleep. Instead of polling,
use the NSRunLoop or NSTimer classes to schedule work as needed.
●
Leave the idleTimerDisabled property of the shared UIApplication object set to NO whenever
possible. The idle timer turns off the device’s screen after a specified period of inactivity. If your app does
not need the screen to stay on, let the system turn it off. If your app experiences side effects as a result of
the screen being turned off, you should modify your code to eliminate the side effects rather than disable
the idle timer unnecessarily.
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Reduce Your App’s Power Consumption
●
Coalesce work whenever possible to maximize idle time. It generally takes less power to perform a set of
calculations all at once than it does to perform them in small chunks over an extended period of time.
Doing small bits of work periodically requires waking up the CPU more often and getting it into a state
where it can perform your tasks.
●
Avoid accessing the disk too frequently. For example, if your app saves state information to the disk, do
so only when that state information changes, and coalesce changes whenever possible to avoid writing
small changes at frequent intervals.
●
Do not draw to the screen faster than is needed. Drawing is an expensive operation when it comes to
power. Do not rely on the hardware to throttle your frame rates. Draw only as many frames as your app
actually needs.
●
If you use the UIAccelerometer class to receive regular accelerometer events, disable the delivery of
those events when you do not need them. Similarly, set the frequency of event delivery to the smallest
value that is suitable for your needs. For more information, see Event Handling Guide for iOS .
The more data you transmit to the network, the more power must be used to run the radios. In fact, accessing
the network is the most power-intensive operation you can perform. You can minimize that time by following
these guidelines:
●
Connect to external network servers only when needed, and do not poll those servers.
●
When you must connect to the network, transmit the smallest amount of data needed to do the job. Use
compact data formats, and do not include excess content that simply is ignored.
●
Transmit data in bursts rather than spreading out transmission packets over time. The system turns off
the Wi-Fi and cell radios when it detects a lack of activity. When it transmits data over a longer period of
time, your app uses much more power than when it transmits the same amount of data in a shorter amount
of time.
When using the NSURLSession class to enqueue multiple upload or download tasks, enqueue those
items together rather than waiting for one to finish before starting the next one. The system manages
automatically executes queued tasks when it is most efficient to do so.
●
Connect to the network using the Wi-Fi radios whenever possible. Wi-Fi uses less power and is preferred
over cellular radios.
●
If you use the Core Location framework to gather location data, disable location updates as soon as you
can and set the distance filter and accuracy levels to appropriate values. Core Location uses the available
GPS, cell, and Wi-Fi networks to determine the user’s location. Although Core Location works hard to
minimize the use of these radios, setting the accuracy and filter values gives Core Location the option to
turn off hardware altogether in situations where it is not needed. For more information, see Location and
Maps Programming Guide .
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Use Memory Efficiently
The Instruments app includes several instruments for gathering power-related information. You can use these
instruments to gather general information about power consumption and to gather specific measurements
for hardware such as the Wi-Fi and Bluetooth radios, GPS receiver, display, and CPU. You can also enable Energy
Diagnostics Logging on a device to gather information. For information about using Instruments to gather
power-related data, see Instruments User Guide . For information about how to enable Energy Diagnostics
Logging on a device, see Instruments Help .
Use Memory Efficiently
Apps are encouraged to use as little memory as possible so that the system may keep more apps in memory
or dedicate more memory to foreground apps that truly need it. There is a direct correlation between the
amount of free memory available to the system and the relative performance of your app. Less free memory
means that the system is more likely to have trouble fulfilling future memory requests.
To ensure there is always enough free memory available, you should minimize your app’s memory usage and
be responsive when the system asks you to free up memory.
Observe Low-Memory Warnings
When the system dispatches a low-memory warning to your app, respond immediately . Low-memory warnings
are your opportunity to remove references to objects that you do not need. Responding to these warnings is
crucial because apps that fail to do so are more likely to be terminated. The system delivers memory warnings
to your app using the following APIs:
●
The applicationDidReceiveMemoryWarning: method of your app delegate.
●
The didReceiveMemoryWarning method of your UIViewController classes.
●
The UIApplicationDidReceiveMemoryWarningNotificationnotification.
●
Dispatch sources of type DISPATCH_SOURCE_TYPE_MEMORYPRESSURE. This technique is the only one
that you can use to distinguish the severity of the memory pressure.
Upon receiving any of these warnings, your handler method should respond by immediately freeing up any
unneeded memory. Use the warnings to clear out caches and release images. If you have large data structures
that are not being used, write those structures to disk and release the in-memory copies of the data.
If your data model includes known purgeable resources, you can have a corresponding manager object register
for the UIApplicationDidReceiveMemoryWarningNotification notification and remove strong references
to its purgeable resources directly. Handling this notification directly avoids the need to route all memory
warning calls through the app delegate.
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Use Memory Efficiently
Note: You can test your app’s behavior under low-memory conditions using the Simulate Memory
Warning command in iOS Simulator.
Reduce Your App’s Memory Footprint
Starting off with a low footprint gives you more room for expanding your app later. Table 7-1 lists some tips
on how to reduce your app’s overall memory footprint.
Table 7-1
Tips for reducing your app’s memory footprint
Tip
Actions to take
Eliminate memory
leaks.
Because memory is a critical resource in iOS, your app should never have
memory leaks. Use the Instruments app to track down leaks in your code,
both in Simulator and on actual devices. For more information on using
Instruments, see Instruments User Guide .
Make resource files as
small as possible.
Files reside on disk but must be loaded into memory before they can be used.
Compress all image files to make them as small as possible. (To compress PNG
images—the preferred image format for iOS apps—use the pngcrush tool.)
You can make property list files smaller by writing them out in a binary format
using the NSPropertyListSerialization class.
Use Core Data or
SQLite for large data
sets.
If your app manipulates large amounts of structured data, store it in a Core
Data persistent store or in a SQLite database instead of in a flat file. Both Core
Data and SQLite provides efficient ways to manage large data sets without
requiring the entire set to be in memory all at once.
Load resources lazily.
You should never load a resource file until it is actually needed. Prefetching
resource files may seem like a way to save time, but this practice actually slows
down your app right away. In addition, if you end up not using the resource,
loading it wastes memory for no good purpose.
Allocate Memory Wisely
Table 7-2 lists tips for improving memory usage in your app.
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Performance Tips
Tune Your Networking Code
Table 7-2
Tips for allocating memory
Tip
Actions to take
Impose size limits
on resources.
Avoid loading a large resource file when a smaller one will do. Instead of using
a high-resolution image, use one that is appropriately sized for iOS-based devices.
If you must use large resource files, find ways to load only the portion of the file
that you need at any given time. For example, rather than load the entire file
into memory, use the mmap and munmap functions to map portions of the file
into and out of memory. For more information about mapping files into memory,
see File-System Performance Guidelines .
Avoid unbounded
problem sets.
Unbounded problem sets might require an arbitrarily large amount of data to
compute. If the set requires more memory than is available, your app may be
unable to complete the calculations. Your apps should avoid such sets whenever
possible and work on problems with known memory limits.
For detailed information about ARC and memory management, see Transitioning to ARC Release Notes .
Tune Your Networking Code
The networking stack in iOS includes several interfaces for communicating over the radio hardware of iOS
devices. The main programming interface is the CFNetwork framework, which builds on top of BSD sockets
and opaque types in the Core Foundation framework to communicate with network entities. You can also use
the NSStream classes in the Foundation framework and the low-level BSD sockets found in the Core OS layer
of the system.
For information about how to use the CFNetwork framework for network communication, see CFNetwork
Programming Guide and CFNetwork Framework Reference . For information about using the NSStream class,
see Foundation Framework Reference .
Tips for Efficient Networking
Implementing code to receive or transmit data across the network is one of the most power-intensive operations
on a device. Minimizing the amount of time spent transmitting or receiving data helps improve battery life.
To that end, you should consider the following tips when writing your network-related code:
●
For protocols you control, define your data formats to be as compact as possible.
●
Avoid using chatty protocols.
●
Transmit data packets in bursts whenever you can.
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Performance Tips
Tune Your Networking Code
Cellular and Wi-Fi radios are designed to power down when there is no activity. Depending on the radio,
though, doing so can take several seconds. If your app transmits small bursts of data every few seconds, the
radios may stay powered up and continue to consume power, even when they are not actually doing anything.
Rather than transmit small amounts of data more often, it is better to transmit a larger amount of data once
or at relatively large intervals.
When communicating over the network, packets can be lost at any time. Therefore, when writing your
networking code, you should be sure to make it as robust as possible when it comes to failure handling. It is
perfectly reasonable to implement handlers that respond to changes in network conditions, but do not be
surprised if those handlers are not called consistently. For example, the Bonjour networking callbacks may not
always be called immediately in response to the disappearance of a network service. The Bonjour system
service immediately invokes browsing callbacks when it receives a notification that a service is going away,
but network services can disappear without notification. This situation might occur if the device providing the
network service unexpectedly loses network connectivity or the notification is lost in transit.
Using Wi-Fi
If your app accesses the network using the Wi-Fi radios, you must notify the system of that fact by including
the UIRequiresPersistentWiFi key in the app’s Info.plist file. The inclusion of this key lets the system
know that it should display the network selection dialog if it detects any active Wi-Fi hot spots. It also lets the
system know that it should not attempt to shut down the Wi-Fi hardware while your app is running.
To prevent the Wi-Fi hardware from using too much power, iOS has a built-in timer that turns off the hardware
completely after 30 minutes if no running app has requested its use through the UIRequiresPersistentWiFi
key. If the user launches an app that includes the key, iOS effectively disables the timer for the duration of the
app’s life cycle. As soon as that app quits or is suspended, however, the system reenables the timer.
Note: Note that even when UIRequiresPersistentWiFi has a value of true, it has no effect
when the device is idle (that is, screen-locked). The app is considered inactive, and although it may
function on some levels, it has no Wi-Fi connection.
For more information on the UIRequiresPersistentWiFi key and the keys of the Info.plist file, see
The Information Property List File (page 14).
The Airplane Mode Alert
If your app launches while the device is in airplane mode, the system may display an alert to notify the user
of that fact. The system displays this alert only when all of the following conditions are met:
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Performance Tips
Improve Your File Management
●
Your app’s information property list (Info.plist) file contains the UIRequiresPersistentWiFi key
and the value of that key is set to true.
●
Your app launches while the device is currently in airplane mode.
●
Wi-Fi on the device has not been manually reenabled after the switch to airplane mode.
Improve Your File Management
Minimize the amount of data you write to the disk. File operations are relatively slow and involve writing to
the flash drive, which has a limited lifespan. Some specific tips to help you minimize file-related operations
include:
●
Write only the portions of the file that changed, and aggregate changes when you can. Avoid writing out
the entire file just to change a few bytes.
●
When defining your file format, group frequently modified content together to minimize the overall
number of blocks that need to be written to disk each time.
●
If your data consists of structured content that is randomly accessed, store it in a Core Data persistent
store or a SQLite database, especially if the amount of data you are manipulating could grow to more than
a few megabytes.
Avoid writing cache files to disk. The only exception to this rule is when your app quits and you need to write
state information that can be used to put your app back into the same state when it is next launched.
Make App Backups More Efficient
Backups occur wirelessly via iCloud or when the user syncs the device with iTunes. During backups, files are
transferred from the device to the user’s computer or iCloud account. The location of files in your app sandbox
determines whether or not those files are backed up and restored. If your application creates many large files
that change regularly and puts them in a location that is backed up, backups could be slowed down as a result.
As you write your file-management code, you need to be mindful of this fact.
App Backup Best Practices
You do not have to prepare your app in any way for backup and restore operations. Devices with an active
iCloud account have their app data backed up to iCloud at appropriate times. For devices that are plugged
into a computer, iTunes performs an incremental backup of the app’s data files. However, iCloud and iTunes
do not back up the contents of the following directories:
●
<Application_Home> /AppName .app
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Performance Tips
Make App Backups More Efficient
●
<Application_Data> /Library/Caches
●
<Application_Data> /tmp
To prevent the syncing process from taking a long time, be selective about where you place files inside your
app’s home directory. Apps that store large files can slow down the process of backing up to iTunes or iCloud.
These apps can also consume a large amount of a user's available storage, which may encourage the user to
delete the app or disable backup of that app's data to iCloud. With this in mind, you should store app data
according to the following guidelines:
●
Critical data should be stored in the <Application_Data> /Documents directory. Critical data is any data
that cannot be recreated by your app, such as user documents and other user-generated content.
●
Support files include files your application downloads or generates and that your application can recreate
as needed. The location for storing your application’s support files depends on the current iOS version.
●
In iOS 5.1 and later, store support files in the <Application_Data> /Library/Application Support
directory and add the NSURLIsExcludedFromBackupKey attribute to the corresponding NSURL
object using the setResourceValue:forKey:error: method. (If you are using Core Foundation,
add the kCFURLIsExcludedFromBackupKey key to your CFURLRef object using the
CFURLSetResourcePropertyForKey function.) Applying this attribute prevents the files from being
backed up to iTunes or iCloud. If you have a large number of support files, you may store them in a
custom subdirectory and apply the extended attribute to just the directory.
●
In iOS 5.0 and earlier, store support files in the <Application_Data> /Library/Caches directory to
prevent them from being backed up. If you are targeting iOS 5.0.1, see How do I prevent files from
being backed up to iCloud and iTunes? for information about how to exclude files from backups.
●
Cached data should be stored in the <Application_Data> /Library/Caches directory. Examples of files
you should put in the Caches directory include (but are not limited to) database cache files and
downloadable content, such as that used by magazine, newspaper, and map apps. Your app should be
able to gracefully handle situations where cached data is deleted by the system to free up disk space.
●
Temporary data should be stored in the <Application_Data> /tmp directory. Temporary data comprises
any data that you do not need to persist for an extended period of time. Remember to delete those files
when you are done with them so that they do not continue to consume space on the user's device.
Although iTunes backs up the app bundle itself, it does not do this during every sync operation. Apps purchased
directly from a device are backed up when that device is next synced with iTunes. Apps are not backed up
during subsequent sync operations, though, unless the app bundle itself has changed (because the app was
updated, for example).
For additional guidance about how you should use the directories in your app, see File System Programming
Guide .
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Performance Tips
Move Work off the Main Thread
Files Saved During App Updates
When a user downloads an app update, iTunes installs the update in a new app directory. It then moves the
user’s data files from the old installation over to the new app directory before deleting the old installation.
Files in the following directories are guaranteed to be preserved during the update process:
●
<Application_Data> /Documents
●
<Application_Data> /Library
Although files in other user directories may also be moved over, you should not rely on them being present
after an update.
Move Work off the Main Thread
Be sure to limit the type of work you do on the main thread of your app. The main thread is where your app
handles touch events and other user input. To ensure that your app is always responsive to the user, you should
never use the main thread to perform long-running or potentially unbounded tasks, such as tasks that access
the network. Instead, you should always move those tasks onto background threads. The preferred way to do
so is to use Grand Central Dispatch (GCD) or NSOperation objects to perform tasks asynchronously.
Moving tasks into the background leaves your main thread free to continue processing user input, which is
especially important when your app is starting up or quitting. During these times, your app is expected to
respond to events in a timely manner. If your app’s main thread is blocked at launch time, the system could
kill the app before it even finishes launching. If the main thread is blocked at quitting time, the system could
similarly kill the app before it has a chance to write out crucial user data.
For more information about using GCD, operation objects, and threads, see Concurrency Programming Guide .
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113
Document Revision History
This table describes the changes to App Programming Guide for iOS .
Date
Notes
2014-09-17
Reorganized the content and removed outdated information.
Changed the name of the book from iOS App Programming Guide .
Removed information about legacy techniques for specifying app icons
and launch images. Relevant information moved to the corresponding
key descriptions in Information Property List Key Reference .
Added the Understanding When Your App Gets Launched into the
Background (page 43) section to provide guidance about what
technologies cause an app to be launched automatically.
Fixed numerous bugs in the existing content.
2013-10-23
Added links to the Japanese smartphone privacy initiatives.
2013-09-18
Added information about new background execution modes and about
app icon sizes in iOS 7.
2013-04-23
Added a section about privacy best practices.
2013-01-28
Added explicit information about how to support iPhone 5.
2012-09-19
Contains information about new features in iOS 6.
2012-03-07
Added information about the NSURL and CFURL keys used to prevent a
file from being backed up.
2012-01-09
Updated the section that describes the behavior of apps in the
background.
2011-10-12
Added information about features introduced in iOS 5.0.
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114
Document Revision History
Date
Notes
Reorganized book and added more design-level information.
Added high-level information about iCloud and how it impacts the design
of applications.
2011-02-24
Added information about using AirPlay in the background.
2010-12-13
Made minor editorial changes.
2010-11-15
Incorporated additional iPad-related design guidelines into this document.
Updated the information about how keychain data is preserved and
restored.
2010-08-20
Fixed several typographical errors and updated the code sample on
initiating background tasks.
2010-06-30
Updated the guidance related to specifying application icons and launch
images.
Changed the title from iPhone Application Programming Guide .
2010-06-14
Reorganized the book so that it focuses on the design of the core parts
of your application.
Added information about how to support multitasking in iOS 4 and later.
For more information, see Core App Objects (page 15).
Updated the section describing how to determine what hardware is
available.
Added information about how to support devices with high-resolution
screens.
Incorporated iPad-related information.
2010-02-24
Made minor corrections.
2010-01-20
Updated the “Multimedia Support” chapter with improved descriptions
of audio formats and codecs.
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115
Document Revision History
Date
Notes
2009-10-19
Moved the iPhone specific Info.plist keys to Information Property List
Key Reference .
Updated the “Multimedia Support” chapter for iOS 3.1.
2009-06-17
Added information about using the compass interfaces.
Moved information about OpenGL support to OpenGL ES Programming
Guide for iOS .
Updated the list of supported Info.plist keys.
2009-03-12
Updated for iOS 3.0
Added code examples to "Copy and Paste Operations" in the Event
Handling chapter.
Added a section on keychain data to the Files and Networking chapter.
Added information about how to display map and email interfaces.
Made various small corrections.
2009-01-06
Fixed several typos and clarified the creation process for child pages in
the Settings application.
2008-11-12
Added guidance about floating-point math considerations
Updated information related to what is backed up by iTunes.
2008-10-15
Reorganized the contents of the book.
Moved the high-level iOS information to iOS Technology Overview .
Moved information about the standard system URL schemes to Apple URL
Scheme Reference .
Moved information about the development tools and how to configure
devices to Tools Workflow Guide for iOS .
Created the Core Application chapter, which now introduces the
application architecture and covers much of the guidance for creating
iPhone applications.
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116
Document Revision History
Date
Notes
Added a Text and Web chapter to cover the use of text and web classes
and the manipulation of the onscreen keyboard.
Created a separate chapter for Files and Networking and moved existing
information into it.
Changed the title from iPhone OS Programming Guide .
2008-07-08
New document that describes iOS and the development process for iPhone
applications.
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117
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Copyright © 2014 Apple Inc.
All rights reserved.
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No licenses, express or implied, are granted with
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applications only for Apple-branded products.
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Instruments, iPad, iPhone, iPod, iTunes, Mac,
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