CHAPTER 13 Amazon at the Bookstore

CHAPTER 13 Amazon at the Bookstore
Amazon at the Bookstore
Figure 13-1.
Suppose that you’re browsing books at your
favorite bookstore and want to know how much a
book costs on With the Amazon at
the Bookstore app, you can scan a book or enter an
ISBN, and the app will tell you the current lowest
price of the book at You can also
search for books on a particular topic.
Amazon at the Bookstore demonstrates how
you can use App Inventor to create apps that talk
to web services (aka, application programming
interfaces, or APIs). This app will get data from a web service created by one of the
authors of this book. By the end of this chapter, you’ll be able to create your own
custom app for talking to Amazon. The application has a simple user interface with
which the user can enter keywords or a book’s ISBN (international standard book
number—a 10- or 13-digit code that uniquely identifies a book) and then lists the
title, ISBN, and lowest price for a new copy at Amazon. It also uses the BarcodeScanner
component so that the user can scan a book to trigger a search instead of entering
text (technically, the scanner just inputs the book’s ISBN for you).
220 Chapter 13: Amazon at the Bookstore
What You’ll Learn
In this app (shown in Figure 13-1), you’ll learn:
• How to use a barcode scanner within an
• How to access a web information source
(Amazon’s API) through the TinyWebDB
• How to process complex data returned
from that web information source. In
particular, you’ll learn how to process a list
of books in which each book is itself a list
of three items (title, price, and ISBN).
Figure 13-2. Amazon at the
Bookstore running in the emulator
Chapter 13, Amazon at the Bookstore
What is an API? 221
You’ll also be introduced to source code that you can use to create your own web
service API with the Python programming language and Google’s App Engine.
What is an API?
Before you start designing your components and programming the app, let’s take a
closer look at what an application programmer interface (API) is and how it works. An
API is like a website, but instead of communicating with humans, it communicates
with other computer programs. APIs are often called “server” programs because they
typically serve information to “client” programs that actually interface with humans—
such as an App Inventor app. If you’ve ever used a Facebook app on your phone,
you’re using a client program that communicates with the Facebook API server app.
In this chapter, you’ll create an Android client app that communicates with an
Amazon API. Your app will request book and ISBN information from the Amazon API,
and the API will return up-to-date listings to your app. The app will then present the
book data to the user.
The Amazon API you’ll use is specially configured for use with App Inventor. We
won’t get into the gory details here, but it’s useful to know that as a result of this
configuration, you can use the TinyWebDB component to communicate with Amazon.
The good news is you already know how to do that! You’ll call TinyWebDB.GetValue to
request information and then process the information returned in the
TinyWebDB.GotValue event handler, just as you do when you use a web database. (You
can go back to the MakeQuiz and TakeQuiz apps in Chapter 10 to refresh your
memory, if needed.)
Before creating the app, you’ll need to understand the Amazon API’s protocol,
which specifies the format for your request and the format of the data returned. Just
as different human cultures have different protocols (when you meet someone, do
you shake hands, bow, or nod your head?), computers talking to one another have
protocols, as well. The Amazon API you’ll be using here provides a web interface for
exploring how the API works before you start using it. Although the API is designed to
talk to other computers, this web interface makes it possible for you to see just how
that communication will happen. Following these steps, you can try out what
particular GetValue calls will return via the website, and know that the API interface
will behave exactly the same when you ask it for data via the TinyWebDB component in
App Inventor. Let’s get underway:
Open a browser and go to You’ll see the
website shown in Figure 13-2.
What is an API?
222 Chapter 13: Amazon at the Bookstore
Figure 13-3. The web interface for the App Inventor Amazon API
On this web page, you can try the one function you can call with this API:
getvalue. Enter a term (e.g., “baseball”) in the Tag field and then click “Get value.”
The web page will display a listing of the top five books returned from Amazon,
as shown in Figure 13-3.
Figure 13-4. Making a call to the Amazon API to search for books related to the
tag (or keyword) “baseball”
The value returned is a list of books, each one enclosed in brackets [like this]
and providing the title, cost, and ISBN.
If you look closely, you’ll see that each book is in fact represented as a sublist
of another main list. The main list (about baseball) is enclosed in brackets, and
each sublist (or book) is enclosed in its own set of brackets within the main
brackets. So, the return value from this API is actually a list of lists, with each
sublist providing the information for one book. Let’s look at this a bit more
closely. Each left bracket ([) in the data denotes the beginning of a list. The first
left bracket of the result denotes the beginning of the outer list (the list of
books). To its immediate right is the beginning of the first sublist, the first book,
as demonstrated here:
Chapter 13, Amazon at the Bookstore
Getting Started 223
[“The Baseball Codes: Beanballs, Sign Stealing, and Bench-Clearing Brawls:
The Unwritten Rules of America’s Pastime,” ‘$12.98,’ ‘0375424695’]
The sublist has three parts: a title, the lowest current price for the book at
Amazon, and the book’s ISBN. When you get this information into your App
Inventor app, you’ll be able to access each part by using select list item, with
index 1 for the title, index 2 for the price, and index 3 for the ISBN. (To refresh
your memory on working with an index and lists, revisit the MakeQuiz app in
Chapter 10.)
3. Instead of searching by keyword, you can search for a book by entering an
ISBN. To perform such a search, you enter a tag in the form “isbn:
00000000000,” where the list of 0s represent an actual ISBN number (see
Figure 13-4). The double brackets ([[) in the result [[‘“App Inventor,”’ ‘$21.93,’
‘1449397484’]] denote that a list of lists is still returned, even though there is
only one book. It might seem a bit strange now, but this will be important when
we access the information for our app.
Figure 13-5. Querying the Amazon API by ISBN instead of keyword
Getting Started
Connect to the App Inventor website and start a new project. Then name it
“AmazonBooks”, and set the screen’s title to “Amazon at the Bookstore”. Then, connect
your device or emulator for live testing.
Designing the Components
The user interface for the Amazon book app is relatively simple: give it a Textbox for
entering keywords or ISBNs, two buttons for starting the two types of searches
(keyword or ISBN), and a third button for letting the user scan a book (we’ll get to that
in a bit). Then, add a heading label and another label for listing the results that the
Amazon API returns, and finally two non-visible components: TinyWebDB and a
BarcodeScanner. Check your results against Figure 13-5.
Getting Started
224 Chapter 13: Amazon at the Bookstore
Figure 13-6. The Amazon at the Bookstore user interface shown in the Designer
Table 13-1 lists all the components you’ll need to build the UI shown in Figure 13-5.
Table 13-1. Component list for the Amazon at the Bookstore app
Component type
Palette group
What you’ll name it
User Interface
The user enters keywords or ISBN
HorizontalArrangement Layout
HorizontalArrangement1 Arrange the buttons in a line.
User Interface
Click to search by keyword.
User Interface
Click to search by ISBN.
User Interface
Click to scan an ISBN from a book.
User Interface
The header “Search Results.”
User Interface
Where you’ll display the results.
Talk to
Scan barcodes.
Set the properties of the components in the following way:
1. Set the Hint of the SearchTextBox to “Enter keywords or ISBN”.
Chapter 13, Amazon at the Bookstore
Programming the App’s Behavior 225
2. Set the properties of the buttons and labels so that they appear as shown in
Figure 13-5.
3. Set the ServiceURL property of the TinyWebDB component to http://
Programming the App’s Behavior
For this app, you’ll specify the following behaviors in the Blocks Editor:
Searching by keyword
The user enters some terms and clicks the KeywordSearchButton to invoke an
Amazon search. You’ll call TinyWebDB1.GetValue to make it happen.
Searching by ISBN
The user enters an ISBN and clicks the ISBNButton. You’ll package the prefix
“isbn:” with the number entered and run the Amazon search.
Barcode scanning
The user clicks a button and the scanner is launched. When the user scans an
ISBN from a book, your app will launch a search by ISBN.
Processing the list of books
At first, your app will display the data returned from Amazon in a rudimentary
way. Later, you’ll modify the blocks so that the app extracts the title, price, and
ISBN from each book returned and displays them in an organized way.
When the user clicks the KeywordSearchButton, you want to grab the text from the
SearchTextBox and send it as the tag in your request to the Amazon API. You’ll use the
TinyWebDB.GetValue block to request the Amazon search. When the results come back
from Amazon, the TinyWebDB.GotValue event handler will be triggered. For now, just
display the result that is returned directly into the ResultsLabel, as shown in
Figure 13-6. Later, after you see that the data is indeed being retrieved, you can display
the data in a more sophisticated fashion.
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226 Chapter 13: Amazon at the Bookstore
Figure 13-7. Send the search request to the API and put results in the ResultsLabel
How the blocks work
When the user clicks the KeywordSearchButton, the TinyWebDB1.GetValue request is
made. The tag sent with the request is the information the user entered in the
SearchTextBox. If you completed the MakeQuiz app (Chapter 10), you know that
TinyWebDB1.GetValue requests are not answered immediately. Instead, when the data
arrives from the API, TinyWebDB1.GotValue is triggered. In GotValue, the blocks check
the value returned to see if it’s a list (it won’t be if the Amazon API is offline or there is
no data for the keywords). If it is a list, the data is placed into the ResultsLabel.
Test your app Enter a term in the search box and click Search
By Keyword. You should get a listing similar to what is shown
in Figure 13-7. (It’s not terribly nice-looking, but we’ll deal with
that shortly.)
Chapter 13, Amazon at the Bookstore
Programming the App’s Behavior 227
Figure 13-8. Keyword search result for “dogs”
The code for searching by ISBN is similar, but in this case the Amazon API expects the
tag to be in the form “isbn:xxxxxxxxxxxxx” (this is the protocol the API expects for
searching by ISBN). You don’t want to force the user to know this protocol; the user
should just be able to enter the ISBN in the text box and click Search by ISBN, and the
app should add the “isbn:” prefix behind the scenes with make text. Figure 13-8 shows
the blocks to do that.
Figure 13-9. The app prefixes “isbn:” to the search so it will look up a particular book
How the blocks work
The join block concatenates the “isbn:” prefix with the information the user has input
in the SearchTextBox and sends the result as the tag to TinyWebDB1.GetValue.
Programming the App’s Behavior
228 Chapter 13: Amazon at the Bookstore
Just as with keyword search, the API sends back a list result for an ISBN search—in
this case, a list of just the one item whose ISBN matches the user’s input exactly.
Because the TinyWebDB.GotValue event handler is already set up to process a list of
books (even a list with only one item), you won’t have to change your event handler
to make this work.
Test your app Enter an ISBN (e.g., 9781449397487) in the
SearchTextBox and click the ISBNButton. Does the book
information appear?
When you call a web service (API) with TinyWebDB1.GetValue, there can be a delay
before the data arrives and TinyWebDB1.GotValue is triggered. It is generally a good
idea to let users know the request is being processed to reassure them that the app
hasn’t hung. For this app, you can place a message in the ResultsLabel each time you
make the call to TinyWebDB1.GetValue, as shown in Figure 13-9.
Figure 13-10. Adding a message to let the user know what is happening
How the blocks work
For both the keyword and ISBN searches, a “Searching Amazon...” message is placed in
ResultsLabel when the data is requested. Note that when GotValue is triggered, this
message is overwritten with the actual results from Amazon.
Chapter 13, Amazon at the Bookstore
Programming the App’s Behavior 229
Let’s face it: typing on a cell phone isn’t always the easiest thing, and you tend to
make a mistake here and there. It would certainly be easier (and result in fewer
mistakes) if a user could just launch your app and scan the barcode of the book. This
is another great built-in Android phone feature that you can tap into easily with App
The function BarcodeScanner.DoScan starts up the scanner. You’ll want to call this
when the ScanButton is clicked. The event handler BarcodeScanner.AfterScan is
triggered as soon as something has been scanned. It has one argument, result,
which contains the information that was scanned. In this case, you want to initiate an
ISBN search using that result, as shown in Figure 13-10.
Figure 13-11. Blocks for initiating an ISBN search after a user scans
How the blocks work
When the user clicks the ScanButton, DoScan launches the scanner. When something
has been scanned, AfterScan is triggered. The argument result holds the result of the
scan—in this case, a book’s ISBN. The user is notified that a request has been made,
the result (the scanned ISBN number) is placed in the SearchTextBox, and
TinyWebDB1.GetValue is called to initiate the search. Again, the TinyWebDB1.GotValue
event handler will process the book information returned.
Programming the App’s Behavior
230 Chapter 13: Amazon at the Bookstore
Test your app Click the ScanButton and scan the barcode of a
book. Does the app display the book information?
A client app like the one you’re creating can do whatever it wants with the data it
receives—you could compare the price information with that of other online stores,
or use the title information to search for similar books from another library. Almost
always, you’ll want to get the API information loaded into variables that you can then
process further. In the TinyWebDB.GotValue event handler you have so far, you just
place all the information returned from Amazon into the ResultsLabel. Instead, let’s
process the data by 1) putting the title, price, and ISBN of each book returned into
separate variables, and 2) displaying those items in an orderly fashion. If you’ve
completed some of the earlier chapters, you’re probably getting the hang of defining
variables and using them in your display, so try building out the variables you think
you’ll need and the blocks to display each search result on its own separate line. Then,
compare what you’ve done with Figure 13-11.
Figure 13-12. Extracting the title, cost, and ISBN of each book, and then displaying
them on separate lines
Chapter 13, Amazon at the Bookstore
Programming the App’s Behavior 231
How the blocks work
Four variables—resultList, title, cost, and isbn—are defined to hold each piece of
data as it is returned from the API. The result from the API, valueFromWebDB, is placed
into the variable resultList. This app could have processed the argument
valueFromWebDB directly, but in general, you’ll put it in a variable in case you want to
process the data outside the event handler. (Event arguments like valueFromWebDB
hold their value only within the event handler.)
A for each loop is used to iterate through each item of the result. Recall that the
data returned from Amazon is a list of lists, with each sublist representing the
information for a book. So, the placeholder of the for each is renamed bookitem, and
it holds the current book information (a list) on each iteration.
Now we have to deal with the fact that the variable bookitem is a list—the first item
is the title; the second, the price; and the third, the ISBN. Thus, we use select list
item blocks to extract these items and place them into their respective variables
(title, price, and isbn).
After the data has been organized into variables, you can process it however you’d
like. This app just uses the variables as part of a join block that displays the title, price,
and ISBN on separate lines.
Test your app Try another search and check out how the book
information is displayed. It should look similar to
Figure 13-12.
Programming the App’s Behavior
232 Chapter 13: Amazon at the Bookstore
Figure 13-13. The search listing displayed in a more sophisticated fashion
The Complete App: Amazon at the Bookstore
Figure 13-13 shows the final block configuration for Amazon at the Bookstore.
Chapter 13, Amazon at the Bookstore
The Complete App: Amazon at the Bookstore 233
The Complete App: Amazon at the Bookstore
234 Chapter 13: Amazon at the Bookstore
Customizing the API
The API you connected to,, was created with the
programming language Python and Google’s App Engine. App Engine lets you create
and deploy websites and services (APIs) that live on Google’s servers. You only pay for
App Engine if your site or API becomes popular and attracts lots of hits.
The API service used in this tutorial provides only partial access to the full Amazon
API and returns a maximum of five books for any search. If you’d like to provide more
flexibility—for example, have it search for items other than books—you can
download the source code for the service from
and customize it. Such customization does require knowledge of Python
programming, so beware! But, if you’ve been completing the App Inventor apps in
this book, you might just be ready for the challenge. To get started learning Python,
check out the interactive version of the book How to Think Like a Computer Scientist:
Learning with Python and then check out the section on App Inventor API building in
Chapter 24 of this book.
After you get the app working, you might want to explore some of the following
• As is, the app hangs if the search doesn’t return any books (for instance, when
the user enters an invalid ISBN). Modify the blocks so that the app reports
when there are no results.
• Modify the app so that it only displays books under $10.
• Modify the app so that after you scan a book, its lowest Amazon price is spoken
aloud (use the TextToSpeech component discussed in the No Text While
Driving app in Chapter 4).
• Download the API code and modify it so that it
returns more information. For example, you might have it return the Amazon
URL of each book, display the URL along with each listed book, and let the user
click the URL to open that page. As mentioned earlier, modifying the API
requires Python programming and some knowledge of Google’s App Engine.
For more information, check out Chapter 24.
Here are some of the concepts we covered with this app:
Chapter 13, Amazon at the Bookstore
Summary 235
• You can access the Web from an app by using TinyWebDB and specially
constructed APIs. You set the ServiceURL of the TinyWebDB component to the
API URL and then call TinyWebDB.GetValue to request the information. The
data isn’t immediately returned but can instead be accessed within the
TinyWebDB.GotValue event handler.
• The BarcodeScanner.DoScan function launches the scan. When the user scans a
barcode, the BarcodeScanner.AfterScan event is triggered and the scanned data
is placed in the argument result.
• In App Inventor, complex data is represented with lists and lists of lists. If you
know the format of the data returned from an API, you can use for each and
select list item to extract the separate pieces of information into variables,
and then perform whatever processing or set up the display however you’d like
using those variables.
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