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Chapter 1
Hello Purr
This chapter gets you started building apps. It presents the
key elements of App Inventor—the Component Designer
and the Blocks Editor—and leads you through the basic
steps of creating your first app, HelloPurr. When you’re
finished, you’ll be ready to build apps on your own.
A typical first program with a new computer system prints the
message “Hello World” to show that everything is connected
correctly. This tradition goes back to the 1970s and Brian
Kernighan’s work on the C programming language at Bell Labs
(Brian is now a visiting scholar at Google working on the App
Inventor team!). With App Inventor, even the simplest apps do
more than just show messages: they play sounds and react
when you touch the phone. So we’re going to get started right
away with something more exciting; your first app (as shown
in Figure 1-1) will be “HelloPurr,” a picture of a cat that meows
when you touch it and purrs when you shake it.
What You’ll Learn
Figure 1-1. The
HelloPurr app
The chapter covers the following topics:
• Building apps by selecting components and then telling them what to do and
when to do it.
• Using the Component Designer to select components. Some components are
visible on the phone screen and some aren’t.
• Adding media (sounds and images) to apps by uploading them from your
• Working in the Blocks Editor to assemble blocks that define the components’
• Testing apps with App Inventor’s live testing. This lets you see how apps will look
and behave on the phone step by step, even as you’re building them.
• Packaging the apps you build and downloading them to a phone.
2 Chapter 1: Hello Purr
The App Inventor Environment
You can set up App Inventor using the instructions at http://appinventor.googlelabs
.com/learn/setup/. App Inventor runs primarily through the browser, but you need to
download some software to your computer’s desktop and change some settings on
your phone. Typically you can get set up in just a few minutes, though sometimes
there are issues with setting up device drivers for particular Android phones. If you
have any phone issues, we suggest you get started using the Android emulator that
comes packaged with the App Inventor download.
The App Inventor programming environment has three key parts, all shown in
Figure 1-2:
• The Component Designer, shown on the left side of Figure 1-2, runs in your
browser window. You use it to select components for your app and specify their
• The Blocks Editor runs in a window separate from the Component Designer—it
is often easiest to arrange this to the right of the Component Designer on your
screen while you are working on your app. You use the Blocks Editor to create
behaviors for the components.
• A phone allows you to actually run and test your app as you are developing it. If
you don’t have an Android phone handy, you can test the apps you build using
the Android emulator (shown in the bottom right of Figure 1-2) that comes integrated with the system.
Figure 1-2. The Component Designer, Blocks Editor, and Android emulator
Designing the Components 3 You start App Inventor by browsing to If this is
the first time you’ve used App Inventor, you’ll see the Projects page, which will be
mostly blank because you haven’t created any projects yet. To create a project, click
New at the top left of the page, enter the project name “HelloPurr” (one word with no
spaces), and click OK.
The first window that opens is the Component Designer. When it appears, click Open
Blocks Editor in the menu at the top right. The Blocks Editor comes up in a separate
window, aided by a tool called Java Web Start. (You don’t have to worry about all the
Java messages—App Inventor is using Java, which should already be installed on
your computer, to help launch the Blocks Editor.) This process usually takes about 30
If everything is OK, the Blocks Editor will appear and you’ll see two buttons near the
top right of the screen, as shown in Figure 1-3.
Figure 1-3. Plug a phone into your computer or click “New emulator”; then, click “Connect to Device”
If you have an Android phone and a USB cable, plug the phone into the computer
and select “Connect to Device.” If instead you want to test the apps you build using
an emulator, click “New emulator” and wait about 30 seconds while the Android
emulator loads. When it is fully operational, click “Connect to Device” so that App
Inventor will run your app in the emulator.
If all is well, you should see a window for the Component Designer, a window for the
Blocks Editor, and the emulator window if you chose that option (your screen should
look something like Figure 1-2, shown previously, but with the windows mostly
empty). If you’re having problems here, review the setup instructions at http://app
Designing the Components
The first tool you’ll use is the Component Designer (or just Designer). Components are
the elements you combine to create apps, like ingredients in a recipe. Some components are very simple, like a Label component, which shows text on the screen, or a
Button component, which you tap to initiate an action. Other components are more
elaborate: a drawing Canvas that can hold still images or animations; an accelerometer, a motion sensor that works like a Wii controller and detects when you move or
shake the phone; or components that make or send text messages, play music and
video, get information from websites, and so on.
4 Chapter 1: Hello Purr
When you open the Designer, it will appear as shown in Figure 1-4.
Figure 1-4. The App Inventor Component Designer
The Designer is divided into several areas:
• Toward the center is a white area called the Viewer. This is where you place components and arrange them to map out what you want your app to look like. The
Viewer shows only a rough indication of how the app will look, so, for example, a
line of text might break at a different place in your app than what you see in the
Viewer. To see how your app will really appear, you’ll need to either download
the app to your phone (we’ll go through how to do this a bit later, in the section
“Packaging the App for Downloading”) or view it in the emulator that comes
with App Inventor.
• To the left of the Viewer is the Palette, which is a list of components you can select
from. The Palette is divided into sections; at this point, only the Basic components
are visible, but you can see components in other sections of the Palette by clicking
the headers labeled Media, Animation, and so on.
Designing the Components 5 • To the right of the Viewer is the Components list, which lists the components
in your project. Any component that you drag into the Viewer will show up in
this list. Currently, the project has only one component listed: Screen1, which
represents the phone screen itself.
• Under the Components list is an area that shows the Media (pictures and sound)
in the project. This project doesn’t have any media yet, but you’ll be adding
some soon.
At the far right is a section that shows the Properties of components; when you click
a component in the Viewer, you’ll see its Properties listed here. Properties are details
about each component that you can change. (For example, when clicking on a Label
component, you might see properties related to color, text, font, and so on.) Right
now, it shows the properties of the screen (called Screen1), which include a background color, a background image, and a title.
For the HelloPurr app, you’ll need two visible components (you can think of these
as components you can actually see in the app): the Label component reading “Pet
the Kitty” and a Button component with an image of a cat in it. You’ll also need a
non-visible Sound component that knows how to play sounds, such as “meow,” and
an Accelerometer component for detecting when the phone is being shaken. Don’t
worry—we’ll walk you through each component step by step.
Making a Label
The first component to add is a Label:
1. Go to the Palette, click Label (which appears about five spots down in the list of
components), and drag it to the Viewer. You’ll see a rectangular shape appear on
the Viewer, with the words “Text for Label1.”
2. Look at the Properties box on the right side of the Designer. It shows the properties of the label. There’s a property called Text about halfway down, with a box
for the label’s text. Change the text to “Pet the Kitty” and press Return. You’ll see
the text change in the Viewer.
3. Change the BackgroundColor of the label by clicking the box, which currently
reads None, to select a color from the list that appears. Select Blue. Also change
the TextColor of the label to Yellow. Finally, change the FontSize to 20.
The Designer should now appear as shown in Figure 1-5.
6 Chapter 1: Hello Purr
Figure 1-5. The app now has a label
Be sure you have your phone connected and the Blocks Editor open. You should see
the label appear on the phone as you add it in the Designer. In App Inventor, you
build the application on the phone as you pick the components in the Designer. That
way, you can see right away how your application will look. This is called live testing,
and it also applies to the behaviors you create for the components in the Blocks
Editor, as you’ll see shortly.
Adding the Button
The kitty for HelloPurr is implemented as a Button component—you create a normal
button, and then change the button image to the kitty. To make the basic button
first, go to the Palette in the Designer and click Button (at the top of the list of components). Drag it onto the Viewer, placing it below the label. You’ll see a rectangular
button appear on the Viewer. After about 10 seconds, the button should appear on
the phone. Go ahead and tap the phone button—do you think anything will happen?
It won’t, because your app hasn’t told the button to do anything yet. This is the first
important point to understand about App Inventor: for every component you add
in the Designer, you have to move over to the Blocks Editor and create the code to
make something happen with that component (we’ll do that after we finish adding
the components we need in the Designer).
Now we’ve got a button that we’ll use to trigger the sound effect when someone
clicks it, but we really want it to look like the picture of the kitty, not a plain old rectangle. To make the button look like the kitty:
Designing the Components 7 1. First, you need to download a picture of the kitty and save it on your computer
desktop. You can download it from the site for this book at http://examples​ The picture is the file called kitty.png. (.png is a
standard image format similar to .jpg and .gif; all of these file types will work in
App Inventor, as will most standard sound files like .mpg or .mp3.) You can also
download the sound file we need, meow.mp3.
2. The Properties box should show the properties of the button. If it doesn’t, click
the image of the button in the Viewer to expose the button’s properties on the
right. In the Properties box, click the area under Image (which currently reads
None). A box appears with a button marked Add.
3. Click Add and you’ll see “Upload file.” Click Choose File, browse to select the kitty.png
file you downloaded to your computer earlier, and click OK.
4. You’ll see a yellow message at the top of the screen: “Uploading kitty.png to
the AppInventor server.” After about 30 seconds, the message and the upload
box will disappear, and kitty.png should be listed as the image property for the
button. You’ll also see this listed in the Media area of the Designer window, just
below the Components list. And if you look at the phone, you’ll see the kitty
picture displayed—the button now looks like a kitty.
5. You may have also noticed that the kitty picture on your phone has the words
“Text for button 1” displayed on it. You probably don’t want that in your app, so
go ahead and change the Text property of Button1 to something like “Pet the
Kitty,” or just delete the text altogether.
Now the Designer should appear as shown in Figure 1-6.
Figure 1-6. The app with a label and a button with an image on it
8 Chapter 1: Hello Purr
Adding the Meow Sound
In your app, the kitty will meow when you tap the button. For this, you’ll need to
add the meow sound and program the button behavior to play that sound when the
button is clicked:
1. If you haven’t downloaded the meow.mp3 file to your computer’s desktop, do so
now at
2. Go to the Palette at the left of the Designer window and click the header marked
Media to expand the Media section. Drag out a Sound component and place it in
the Viewer. Wherever you drop it, it will appear in the area at the bottom of the
Viewer marked “Non-visible components.” Non-visible components are objects
that do things for the app but don’t appear in the visual user interface of the app.
3. Click Sound1 to show its properties. Set its Source to meow.mp3. You’ll need to
follow the same steps to upload this file from your computer as you did for the
kitty picture. When you’re done, you should see both kitty.png and meow.mp3
listed in the Media section of the Designer.
You should now have the components depicted in Table 1-1.
Table 1-1. The components you’ve added to the HelloPurr app
Component type
Palette group
Name of component
Press to make the kitty meow.
Shows the text “Pet the Kitty.”
Play the meow sound.
Adding Behaviors to the Components
You’ve just added Button, Label, and Sound components as the building blocks for
your first app. Now let’s make the kitty meow when you tap the button. You do this
with the Blocks Editor. If your Blocks Editor isn’t yet open, click “Open the Blocks
Editor” in the top right of the Component Designer.
Look at the Blocks Editor window. This is where you tell the components what to do
and when to do it. You’re going to tell the kitty button to play a sound when the user
taps it. If components are ingredients in a recipe, you can think of blocks as the cooking instructions.
Adding Behaviors to the Components 9 Making the Kitty Meow
At the top left of the window, you’ll see buttons labeled “Built-In” and “My Blocks.”
Click My Blocks, and you’ll see a column that includes a drawer for each component
you created in the Designer: Button1, Label1, Screen1, and Sound1. When you click
a drawer, you get a bunch of options (blocks) for that component you created. (Don’t
worry about the Built-In column for now—we’ll get to that in Chapter 2.) Click the
drawer for Button1. The drawer opens, showing a selection of blocks that you can
use to tell the button what to do, starting with Button1.Click at the top, as shown in
Figure 1-7.
Figure 1-7. Clicking Button1 shows the component’s blocks
Click the block labeled Button1.Click and drag it into the workspace. When you’re
looking for the block, you’ll notice that the word “when” is smaller than Button1​
.Click. Blocks including the word “when” are called event handlers; they specify what
components should do when some particular event happens. In this case, the event
we’re interested in happens when the app user clicks on the kitty (which is really a
button), as shown in Figure 1-8. Next, we’ll add some blocks to program what will
happen in response to that event.
10 Chapter 1: Hello Purr
Figure 1-8. You’ll specify a response to the user clicking within the Button.Click block
Click Sound1 in My Blocks to open the drawer for the sound component, and drag
out the call Sound1.Play block. (Remember, earlier we set the property for Sound1
to the meow sound file you downloaded to your computer.) You may notice at this
point that the call Sound1.Play block is shaped so it can fit into a gap marked
“do” in the Button1.Click block. App Inventor is set up so that only certain blocks fit
together; this way, you always know you’re connecting blocks that actually work
together. In this case, blocks with the word “call” make components do things. The
two blocks should snap together to form a unit, as shown in Figure 1-9, and you’ll
hear a snapping sound when they connect.
Figure 1-9. Now when someone clicks the button, the meow sound will play
Unlike traditional programming code (which often looks like a jumbled mess of
gobbledygook “words”), blocks in App Inventor spell out the behaviors you’re trying
to create. In this case, we’re essentially saying, “Hey, App Inventor, when someone
clicks on the kitty button, play the meow sound.”
Adding Behaviors to the Components 11 Test your app. Let’s check to make sure everything is working
properly—it’s important to test your app each time you add something new. Tap the button on the phone (or click it using the emulator). You should hear the kitty meow. Congratulations, your first app
is running!
Adding a Purr
Now we’re going to make the kitty purr and meow when you tap the button. We’ll
simulate the purr by making the phone vibrate. That may sound hard, but in fact,
it’s easy to do because the Sound component we used to play the meow sound can
make the phone vibrate as well. App Inventor helps you tap into this kind of core
phone functionality without having to deal with how the phone actually vibrates.
You don’t need to do anything different in the Designer; you can just add a second
behavior to the button click in the Blocks Editor:
1. Go to the Blocks Editor and click Sound1 in My Blocks to open the drawer.
2. Select call Sound1.Vibrate and drag it under the call Sound1.Play block in the
Button1.Click slot. The block should click into place, as shown in Figure 1-10. If
it doesn’t, try dragging it so that the little dip on the top of call Sound1.Vibrate
touches the little bump on the bottom of call Sound1.Play.
Figure 1-10. Playing the sound and vibrating on the Click event
3. You’ve likely noticed that the call Sound1.Vibrate block includes the text “millisecs” at the top right. An open slot in a block means you can plug something into
it to specify more about how the behavior should work. In this case, you must
tell the Vibrate block how long it should vibrate. You need to input this time
in thousandths of a second (milliseconds), which is pretty common for many
programming languages. So, to make the phone vibrate for half a second, put in
12 Chapter 1: Hello Purr
a value of 500 milliseconds. To put in a value of 500, you need to grab a number
block. Click in an empty spot on the Designer screen, and then click the green
Math button in the menu that pops up, as shown in Figure 1-11. You should see
a drop-down list, with 123 as the first item; 123 indicates a block that represents
a number.
Figure 1-11. Opening the Math drawer
4. Click the 123 at the top of the list and you’ll see a green block with the number
123, as shown in Figure 1-12.
Figure 1-12. Choosing a number block (123 is the default value)
5. Change the 123 to 500 by clicking it and typing a new value, as shown in
Figure 1-13.
Figure 1-13. Changing the value to 500
6. Plug the 500 number block into the socket at the right of call Sound1.Vibrate,
as shown in Figure 1-14.
Adding Behaviors to the Components 13 Figure 1-14. Plugging the 500 into the millisecs slot
Test your app. Try it! Tap the button on the phone, and you’ll feel
the purr for half a second.
Shaking the Phone
Now let’s add a final element that taps into another cool feature of Android phones:
make the kitty meow when you shake the phone. To do this, you’ll use a component
called AccelerometerSensor that can sense when you shake or move the phone
1. In the Designer, expand the Sensors area in the Palette components list and drag
out an AccelerometerSensor. Don’t worry about where you drag it—as with
any non-visible component, no matter where you place it in the Viewer, it will
move to the “Non-visible components” section at the bottom of the Viewer.
2. You’ll want to treat someone shaking the phone as a different, separate event
from the button click. That means you need a new event handler. Go to the
Blocks Editor. There should be a new drawer for AccelerometerSensor1 under
My Blocks. Open it and drag out the AccelerometerSensor1.Shaking block—it
should be the second block in the list.
3. Just as you did with the sound and the button click, drag out a call Sound1.Play
block and fit it into the gap in AccelerometerSensor1.Shaking. Try it out by shaking the phone.
Figure 1-15 shows the blocks for the completed HelloPurr app.
14 Chapter 1: Hello Purr
Figure 1-15. The blocks for HelloPurr
Packaging the App for Downloading
App Inventor is a cloud computing tool, meaning your app is stored on Google’s
online servers as you work. So if you close App Inventor, your app will be there when
you return; you don’t have to save anything on your computer as you would with a
Word file or a music track. This also allows you to easily test the app while connected
to your phone (what we call live testing), without having to download anything to
your phone, either. The only problem is that if you disconnect your phone from App
Inventor, the app running on the phone will stop, and you won’t find an icon for it
anywhere because it was never truly installed.
You can package up and install the completed app so that it works on any phone,
even when it’s not connected to the computer. First, make sure your phone allows
apps to be downloaded from places other than the Android Market. Typically, you
do this by going to Settings→Applications on your phone and checking the box
next to “Unknown sources.” Then, go to back into the Designer in App Inventor, click
“Package for Phone,” and select “Download to Connected Phone.” You should see the
messages “Saving” and then “Packaging,” a process that takes up to a minute. After
the “Packaging” message disappears, continue to wait for another 10–15 seconds while
the finished app is downloaded to the phone. You’ll get a download confirmation
when everything is complete.
Once you’ve downloaded it, look at the apps available on your phone, and you’ll now
see HelloPurr, the app we just built. You run it just like any other app. (Make sure that
you run your new app, not the App Inventor Phone application.) You can now unplug
or even reboot the phone and kill all applications, and your new packaged application
will still be there.
It’s important to understand that this means your packaged app is now separate
from the project on App Inventor. You can do more work on the project in App
Inventor by connecting the phone with the USB cable as before. But that won’t
Sharing the App 15 change the packaged app that is now installed on your phone. If you make further
changes to your app in App Inventor, you’ll want to package the result and download
the new version to replace the old one on the phone.
Go ahead and package your HelloPurr app so you have it on your phone. Once
you’ve done this, you can share it with your family and friends, too!
Sharing the App
You can share your app in a couple of ways. To share the executable app, first click
“Package for Phone” and choose “Download to this Computer.” This will create a file
with a .apk extension on your computer. You need to upload this file so that it is
accessible on the Web. Once the app is on the Web, other people can install it on
their phones by opening the phone’s browser and downloading it. Just let them
know they need to allow “unknown sources” in their phone’s Application settings in
order to install apps that aren’t from the Android Market.
You can also share the source code (blocks) of your app with another App Inventor
developer. To do this, click My Projects, check the app you want to share (in this case,
HelloPurr), and select More Actions→Download Source. The file created on your
computer will have a .zip extension. You can email this file to someone, and she can
open App Inventor, choose More Actions→Upload Source, and select the .zip file.
This will give the user her own complete copy of your app, which she can then edit
and customize without affecting your version.
The process of sharing apps will soon be easier and more fun—work is currently
underway on a community sharing site.
Now that you’ve built a complete app and had the chance to play with it (and maybe
download it to share with other people), you might have noticed a couple of things.
Take a look at the following items and consider how you’d address them in your app.
As you’ll likely soon discover, you’ll often build an app, find ways to improve and
change it, and then go back into it to program those new ideas. Don’t worry, that’s a
good thing—it means you’re on your way to becoming a full-fledged app developer!
• As you shake the phone, the meows will sound strange, as if they are echoing.
That’s because the accelerometer sensor is triggering the shaking event many
times a second, so the meows are overlapping. If you look at the Sound component in the Designer, you’ll see a property called Minimum interval. That
determines how close together successive sounds can start. It’s currently set
at a half-second (500 milliseconds), which is less than the duration of a single
meow. By playing with the minimum interval, you can change how much the
meows overlap.
16 Chapter 1: Hello Purr
• If you run the packaged app and walk around with the phone in your pocket,
your phone will meow every time you move suddenly—something you might
find embarrassing. Android apps are typically designed to keep running even
when you’re not looking at them; your app continues to communicate with the
accelerometer and the meow just keeps going. To really quit the app, bring up
HelloPurr and press the phone’s menu button. You’ll be offered an option to stop
the application.
Here are some of the concepts we’ve covered in this chapter:
• You build apps by selecting components in the Designer and then telling them
what to do and when to do it in the Blocks Editor.
• Some components are visible and some aren’t. The visible ones appear in the
user interface of the app. The non-visible ones do things like play sounds.
• You define components’ behavior by assembling blocks in the Blocks Editor. You
first drag out an event handler like Button1.Click, and then place command
blocks like Sound.Play within it. Any blocks within Button1.Click will be performed when the user clicks the button.
• Some commands need extra information to make them work. An example is
Vibrate, which needs to know how many milliseconds to vibrate. These values
are called arguments.
• Numbers are represented as number blocks. You can plug these into commands
that take numbers as arguments.
• App Inventor has sensor components. The AccelerometerSensor can detect
when the phone is moved.
• You can package the apps you build and download them to the phone, where
they run independently of App Inventor.
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