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Monday 16 March 2015

Building Your First App



Building Your First App


Welcome to Android application development!
This class teaches you how to build your first Android app. You’ll learn how to create an Android project and run a debuggable version of the app. You'll also learn some fundamentals of Android app design, including how to build a simple user interface and handle user input.

Set Up Your Environment

Before you start this class, be sure you have your development environment set up. You need to:
  1. Download Android Studio.
  2. Download the latest SDK tools and platforms using the SDK Manager.
Note: Although most of this training class expects that you're using Android Studio, some procedures include alternative instructions for using the SDK tools from the command line instead.
This class uses a tutorial format to create a small Android app that teaches you some fundamental concepts about Android development, so it's important that you follow each step.

Creating an Android Project


An Android project contains all the files that comprise the source code for your Android app.
This lesson shows how to create a new project either using Android Studio or using the SDK tools from a command line.
Note: You should already have the Android SDK installed, and if you're using Android Studio, you should also have Android Studio installed. If you don't have these, follow the guide toInstalling the Android SDK before you start this lesson.

Create a Project with Android Studio

  1. In Android Studio, create a new project:
    • If you don't have a project opened, in the Welcome screen, click New Project.
    • If you have a project opened, from the File menu, select New Project.

    2. Figure 1. Configuring a new project in Android Studio.
  2. Under Configure your new project, fill in the fields as shown in figure 1 and click Next.
    It will probably be easier to follow these lessons if you use the same values as shown.
    • Application Name is the app name that appears to users. For this project, use "My First App."
    • Company domain provides a qualifier that will be appended to the package name; Android Studio will remember this qualifier for each new project you create.
    • Package name is the fully qualified name for the project (following the same rules as those for naming packages in the Java programming language). Your package name must be unique across all packages installed on the Android system. You can Edit this value independently from the application name or the company domain.
    • Project location is the directory on your system that holds the project files.
  3. Under Select the form factors your app will run on, check the box for Phone and Tablet.
  4. For Minimum SDK, select API 8: Android 2.2 (Froyo).
    The Minimum Required SDK is the earliest version of Android that your app supports, indicated using the API level. To support as many devices as possible, you should set this to the lowest version available that allows your app to provide its core feature set. If any feature of your app is possible only on newer versions of Android and it's not critical to the app's core feature set, you can enable the feature only when running on the versions that support it (as discussed in Supporting Different Platform Versions).
  5. Leave all of the other options (TV, Wear, and Glass) unchecked and click Next.

    6. Activities

    An activity is one of the distinguishing features of the Android framework. Activities provide the user with access to your app, and there may be many activities. An application will usually have a main activity for when the user launches the application, another activity for when she selects some content to view, for example, and other activities for when she performs other tasks within the app. See Activities for more information.
  6. Under Add an activity to <template>, select Blank Activity and click Next.
  7. Under Choose options for your new file, change the Activity Name to MyActivity. The Layout Name changes to activity_my, and the Title to MyActivity. The Menu Resource Name ismenu_my.
  8. Click the Finish button to create the project.
Your Android project is now a basic "Hello World" app that contains some default files. Take a moment to review the most important of these:
app/src/main/res/layout/activity_my.xml
This is the XML layout file for the activity you added when you created the project with Android Studio. Following the New Project workflow, Android Studio presents this file with both a text view and a preview of the screen UI. The file includes some default settings and a TextView element that displays the message, "Hello world!"
app/src/main/java/com.mycompany.myfirstapp/MyActivity.java
A tab for this file appears in Android Studio when the New Project workflow finishes. When you select the file you see the class definition for the activity you created. When you build and run the app, the Activityclass starts the activity and loads the layout file that says "Hello World!"
app/src/main/AndroidManifest.xml
The manifest file describes the fundamental characteristics of the app and defines each of its components. You'll revisit this file as you follow these lessons and add more components to your app.
app/build.gradle
Android Studio uses Gradle to compile and build your app. There is a build.gradle file for each module of your project, as well as a build.gradle file for the entire project. Usually, you're only interested in thebuild.gradle file for the module, in this case the app or application module. This is where your app's build dependencies are set, including the defaultConfig settings:
  • compiledSdkVersion is the platform version against which you will compile your app. By default, this is set to the latest version of Android available in your SDK. (It should be Android 4.1 or greater; if you don't have such a version available, you must install one using the SDK Manager.) You can still build your app to support older versions, but setting this to the latest version allows you to enable new features and optimize your app for a great user experience on the latest devices.
  • applicationId is the fully qualified package name for your application that you specified during the New Project workflow.
  • minSdkVersion is the Minimum SDK version you specified during the New Project workflow. This is the earliest version of the Android SDK that your app supports.
  • targetSdkVersion indicates the highest version of Android with which you have tested your application. As new versions of Android become available, you should test your app on the new version and update this value to match the latest API level and thereby take advantage of new platform features. For more information, read Supporting Different Platform Versions.
See Building Your Project with Gradle for more information about Gradle.
Note also the /res subdirectories that contain the resources for your application:
drawable<density>/
Directories for drawable objects (such as bitmaps) that are designed for various densities, such as medium-density (mdpi) and high-density (hdpi) screens. Other drawable directories contain assets designed for other screen densities. Here you'll find the ic_launcher.png that appears when you run the default app.
layout/
Directory for files that define your app's user interface like activity_my.xml, discussed above, which describes a basic layout for the MyActivity class.
menu/
Directory for files that define your app's menu items.
values/
Directory for other XML files that contain a collection of resources, such as string and color definitions. The strings.xml file defines the "Hello world!" string that displays when you run the default app.
To run the app, continue to the next lesson.

Create a Project with Command Line Tools

If you're not using the Android Studio IDE, you can instead create your project using the SDK tools from a command line:
  1. Change directories into the Android SDK’s sdk/ path.
  2. Execute:
    tools/android list targets
    This prints a list of the available Android platforms that you’ve downloaded for your SDK. Find the platform against which you want to compile your app. Make a note of the target ID. We recommend that you select the highest version possible. You can still build your app to support older versions, but setting the build target to the latest version allows you to optimize your app for the latest devices.
    If you don't see any targets listed, you need to install some using the Android SDK Manager tool. See Adding SDK Packages.
  3. Execute:
    android create project --target <target-id> --name MyFirstApp \
--path <path-to-workspace>/MyFirstApp --activity MyActivity \
--package com.example.myfirstapp
    Replace <target-id> with an ID from the list of targets (from the previous step) and replace <path-to-workspace> with the location in which you want to save your Android projects.
Tip: Add the platform-tools/ as well as the tools/ directory to your PATH environment variable.
Your Android project is now a basic "Hello World" app that contains some default files. To run the app, continue to the

Running Your App

PREVIOUSNEXT
If you followed the previous lesson to create an Android project, it includes a default set of "Hello World" source files that allow you to immediately run the app.
How you run your app depends on two things: whether you have a real device running Android and whether you're using Android Studio. This lesson shows you how to install and run your app on a real device and on the Android emulator, and in both cases with either Android Studio or the command line tools.

Run on a Real Device

If you have a device running Android, here's how to install and run your app.

Set up your device

  1. Plug in your device to your development machine with a USB cable. If you're developing on Windows, you might need to install the appropriate USB driver for your device. For help installing drivers, see the OEM USB Drivers document.
  2. Enable USB debugging on your device.
    • On most devices running Android 3.2 or older, you can find the option under Settings > Applications > Development.
    • On Android 4.0 and newer, it's in Settings > Developer options.
      Note: On Android 4.2 and newer, Developer options is hidden by default. To make it available, go toSettings > About phone and tap Build number seven times. Return to the previous screen to findDeveloper options.

Run the app from Android Studio

  1. Select one of your project's files and click Run  from the toolbar.
  2. In the Choose Device window that appears, select the Choose a running device radio button, select your device, and click OK .
Android Studio installs the app on your connected device and starts it.

Run the app from a command line

Open a command-line and navigate to the root of your project directory. Use Gradle to build your project in debug mode, invoke the assembleDebug build task using the Gradle wrapper script (gradlew assembleRelease).
This creates your debug .apk file inside the module build/ directory, named MyFirstApp-debug.apk.
On Windows platforms, type this command:
> gradlew.bat assembleDebug
On Mac OS and Linux platforms, type these commands:
$ chmod +x gradlew
$ ./gradlew assembleDebug
After you build the project, the output APK for the app module is located in app/build/outputs/apk/
Note: The first command (chmod) adds the execution permission to the Gradle wrapper script and is only necessary the first time you build this project from the command line.
Make sure the Android SDK platform-tools/ directory is included in your PATH environment variable, then execute:
adb install app/build/outputs/MyFirstApp-debug.apk
On your device, locate MyFirstApp and open it.
That's how you build and run your Android app on a device! To start developing, continue to the next lesson.

Run on the Emulator

Whether you're using Android Studio or the command line, to run your app on the emulator you need to first create an Android Virtual Device (AVD). An AVD is a device configuration for the Android emulator that allows you to model a specific device.

Create an AVD

  1. Launch the Android Virtual Device Manager:
    • In Android Studio, select Tools > Android > AVD Manager, or click the AVD Manager icon  in the toolbar.
    • Or, from the command line, change directories to sdk/ and execute:
      tools/android avd
      Note: The AVD Manager that appears when launched from the command line is different from the version in Android Studio, so the following instructions may not all apply.
    Figure 1. The AVD Manager main screen shows your current virtual devices.
  2. On the AVD Manager main screen (figure 1), click Create Virtual Device.
  3. In the Select Hardware window, select a device configuration, such as Nexus 6, then click Next.
  4. Select the desired system version for the AVD and click Next.
  5. Verify the configuration settings, then click Finish.
For more information about using AVDs, see Managing AVDs with AVD Manager.

Run the app from Android Studio

  1. In Android Studio, select your project and click Run  from the toolbar.
  2. In the Choose Device window, click the Launch emulator radio button.
  3. From the Android virtual device pull-down menu, select the emulator you created, and click OK.
It can take a few minutes for the emulator to load itself. You may have to unlock the screen. When you do, My First App appears on the emulator screen.

Run your app from the command line

  1. Build the project from the command line. The output APK for the app module is located inapp/build/outputs/apk/.
  2. Make sure the Android SDK platform-tools/ directory is included in your PATH environment variable.
  3. Execute this command:
    adb install app/build/outputs/MyFirstApp-debug.apk
  4. On the emulator, locate MyFirstApp and open it.
That's how you build and run your Android app on the emulator! To start developing, continue to the next lesson.

Building a Simple User Interface

PREVIOUSNEXT
In this lesson, you create a layout in XML that includes a text field and a button. In the next lesson, your app responds when the button is pressed by sending the content of the text field to another activity.
The graphical user interface for an Android app is built using a hierarchy of View and ViewGroup objects. View objects are usually UI widgets such as buttons or text fields. ViewGroupobjects are invisible view containers that define how the child views are laid out, such as in a grid or a vertical list.
Android provides an XML vocabulary that corresponds to the subclasses of View and ViewGroup so you can define your UI in XML using a hierarchy of UI elements.
Layouts are subclasses of the ViewGroup. In this exercise, you'll work with a LinearLayout.

Alternative Layouts

Declaring your UI layout in XML rather than runtime code is useful for several reasons, but it's especially important so you can create different layouts for different screen sizes. For example, you can create two versions of a layout and tell the system to use one on "small" screens and the other on "large" screens. For more information, see the class about Supporting Different Devices.
Figure 1. Illustration of how ViewGroup objects form branches in the layout and contain other View objects.

Create a Linear Layout

  1. In Android Studio, from the res/layout directory, open the activity_my.xml file.
    The BlankActivity template you chose when you created this project includes the activity_my.xml file with aRelativeLayout root view and a TextView child view.
  2. In the Preview pane, click the Hide icon  to close the Preview pane.
    In Android Studio, when you open a layout file, you’re first shown the Preview pane. Clicking elements in this pane opens the WYSIWYG tools in the Design pane. For this lesson, you’re going to work directly with the XML.
  3. Delete the <TextView> element.
  4. Change the <RelativeLayout> element to <LinearLayout>.
  5. Add the android:orientation attribute and set it to "horizontal".
  6. Remove the android:padding attributes and the tools:context attribute.
The result looks like this:
res/layout/activity_my.xml
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="horizontal" >
</LinearLayout>
LinearLayout is a view group (a subclass of ViewGroup) that lays out child views in either a vertical or horizontal orientation, as specified by the android:orientation attribute. Each child of a LinearLayout appears on the screen in the order in which it appears in the XML.
Two other attributes, android:layout_width and android:layout_height, are required for all views in order to specify their size.
Because the LinearLayout is the root view in the layout, it should fill the entire screen area that's available to the app by setting the width and height to "match_parent". This value declares that the view should expand its width or height to match the width or height of the parent view.
For more information about layout properties, see the Layout guide.

Add a Text Field

As with every View object, you must define certain XML attributes to specify the EditText object's properties.
  1. In the activity_my.xml file, within the <LinearLayout> element, define an <EditText> element with the idattribute set to @+id/edit_message.
  2. Define the layout_width and layout_height attributes as wrap_content.
  3. Define a hint attribute as a string object named edit_message.
The <EditText> element should read as follows:
res/layout/activity_my.xml
<EditText android:id="@+id/edit_message"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:hint="@string/edit_message" />
Here are the <EditText> attributes you added:
android:id
This provides a unique identifier for the view, which you can use to reference the object from your app code, such as to read and manipulate the object (you'll see this in the next lesson).
The at sign (@) is required when you're referring to any resource object from XML. It is followed by the resource type (id in this case), a slash, then the resource name (edit_message).

Resource Objects

A resource object is a unique integer name that's associated with an app resource, such as a bitmap, layout file, or string.
Every resource has a corresponding resource object defined in your project's gen/R.java file. You can use the object names in the R class to refer to your resources, such as when you need to specify a string value for theandroid:hint attribute. You can also create arbitrary resource IDs that you associate with a view using theandroid:id attribute, which allows you to reference that view from other code.
The SDK tools generate the R.java file each time you compile your app. You should never modify this file by hand.
For more information, read the guide toProviding Resources.
The plus sign (+) before the resource type is needed only when you're defining a resource ID for the first time. When you compile the app, the SDK tools use the ID name to create a new resource ID in your project's gen/R.java file that refers to the EditText element. With the resource ID declared once this way, other references to the ID do not need the plus sign. Using the plus sign is necessary only when specifying a new resource ID and not needed for concrete resources such as strings or layouts. See the sidebox for more information about resource objects.
android:layout_width and android:layout_height
Instead of using specific sizes for the width and height, the"wrap_content" value specifies that the view should be only as big as needed to fit the contents of the view. If you were to instead use "match_parent", then the EditText element would fill the screen, because it would match the size of the parentLinearLayout. For more information, see the Layouts guide.
android:hint
This is a default string to display when the text field is empty. Instead of using a hard-coded string as the value, the"@string/edit_message" value refers to a string resource defined in a separate file. Because this refers to a concrete resource (not just an identifier), it does not need the plus sign. However, because you haven't defined the string resource yet, you’ll see a compiler error at first. You'll fix this in the next section by defining the string.
Note: This string resource has the same name as the element ID: edit_message. However, references to resources are always scoped by the resource type (such as id or string), so using the same name does not cause collisions.

Add String Resources

By default, your Android project includes a string resource file at res/values/strings.xml. Here, you'll add a new string named "edit_message" and set the value to "Enter a message."
  1. In Android Studio, from the res/values directory, open strings.xml.
  2. Add a line for a string named "edit_message" with the value, "Enter a message".
  3. Add a line for a string named "button_send" with the value, "Send".
    You'll create the button that uses this string in the next section.
  4. Remove the line for the "hello world" string.
The result for strings.xml looks like this:
res/values/strings.xml
<?xml version="1.0" encoding="utf-8"?>
<resources>
    <string name="app_name">My First App</string>
    <string name="edit_message">Enter a message</string>
    <string name="button_send">Send</string>
    <string name="action_settings">Settings</string>
    <string name="title_activity_main">MainActivity</string>
</resources>
For text in the user interface, always specify each string as a resource. String resources allow you to manage all UI text in a single location, which makes the text easier to find and update. Externalizing the strings also allows you to localize your app to different languages by providing alternative definitions for each string resource.
For more information about using string resources to localize your app for other languages, see the Supporting Different Devices class.

Add a Button

  1. In Android Studio, from the res/layout directory, edit the activity_my.xml file.
  2. Within the <LinearLayout> element, define a <Button> element immediately following the <EditText>element.
  3. Set the button's width and height attributes to "wrap_content" so the button is only as big as necessary to fit the button's text label.
  4. Define the button's text label with the android:text attribute; set its value to the button_send string resource you defined in the previous section.
Your <LinearLayout> should look like this:
res/layout/activity_my.xml
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="horizontal" >
      <EditText android:id="@+id/edit_message"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:hint="@string/edit_message" />
      <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button_send" />
</LinearLayout>
Note: This button doesn't need the android:id attribute, because it won't be referenced from the activity code.
The layout is currently designed so that both the EditText and Button widgets are only as big as necessary to fit their content, as shown in figure 2.
Figure 2. The EditText and Button widgets have their widths set to "wrap_content".
This works fine for the button, but not as well for the text field, because the user might type something longer. It would be nice to fill the unused screen width with the text field. You can do this inside a LinearLayout with theweight property, which you can specify using the android:layout_weight attribute.
The weight value is a number that specifies the amount of remaining space each view should consume, relative to the amount consumed by sibling views. This works kind of like the amount of ingredients in a drink recipe: "2 parts soda, 1 part syrup" means two-thirds of the drink is soda. For example, if you give one view a weight of 2 and another one a weight of 1, the sum is 3, so the first view fills 2/3 of the remaining space and the second view fills the rest. If you add a third view and give it a weight of 1, then the first view (with weight of 2) now gets 1/2 the remaining space, while the remaining two each get 1/4.
The default weight for all views is 0, so if you specify any weight value greater than 0 to only one view, then that view fills whatever space remains after all views are given the space they require.

Make the Input Box Fill in the Screen Width

To fill the remaining space in your layout with the EditText element, do the following:
  1. In the activity_my.xml file, assign the <EditText> element's layout_weight attribute a value of 1.
  2. Also, assign <EditText> element's layout_width attribute a value of 0dp.
    res/layout/activity_my.xml
    <EditText
    android:layout_weight="1"
    android:layout_width="0dp"
    ... />
    To improve the layout efficiency when you specify the weight, you should change the width of the EditText to be zero (0dp). Setting the width to zero improves layout performance because using "wrap_content" as the width requires the system to calculate a width that is ultimately irrelevant because the weight value requires another width calculation to fill the remaining space.
    Figure 3 shows the result when you assign all weight to the EditText element.
    Figure 3. The EditText widget is given all the layout weight, so it fills the remaining space in the LinearLayout.
Here’s how your complete activity_my.xmllayout file should now look:
res/layout/activity_my.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="horizontal">
    <EditText android:id="@+id/edit_message"
        android:layout_weight="1"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:hint="@string/edit_message" />
    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button_send" />
</LinearLayout>

Run Your App

This layout is applied by the default Activity class that the SDK tools generated when you created the project. Run the app to see the results:
  • In Android Studio, from the toolbar, click Run .
  • Or from a command line, change directories to the root of your Android project and execute:
    ant debug
adb install bin/MyFirstApp-debug.apk
Continue to the next lesson to learn how to respond to button presses, read content from the text field, start another activity, and more.

Starting Another Activity

PREVIOUSNEXT
After completing the previous lesson, you have an app that shows an activity (a single screen) with a text field and a button. In this lesson, you’ll add some code to MyActivity that starts a new activity when the user clicks the Send button.

Respond to the Send Button

  1. In Android Studio, from the res/layout directory, edit theactivity_my.xml file.
  2. To the <Button> element, add the android:onClick attribute.
    res/layout/activity_my.xml
    <Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/button_send"
    android:onClick="sendMessage" />
    The android:onClick attribute’s value, "sendMessage", is the name of a method in your activity that the system calls when the user clicks the button.
  3. In the java/com.mycompany.myfirstapp directory, open the MyActivity.java file.
  4. Within the MyActivity class, add the sendMessage() method stub shown below.
    java/com.mycompany.myfirstapp/MyActivity.java
    /** Called when the user clicks the Send button */
public void sendMessage(View view) {
    // Do something in response to button
}
    In order for the system to match this method to the method name given to android:onClick, the signature must be exactly as shown. Specifically, the method must:
    • Be public
    • Have a void return value
    • Have a View as the only parameter (this will be the View that was clicked)
Next, you’ll fill in this method to read the contents of the text field and deliver that text to another activity.

Build an Intent

  1. In MyActivity.java, inside the sendMessage() method, create an Intent to start an activity calledDisplayMessageActivity with the following code:
    java/com.mycompany.myfirstapp/MyActivity.java
    public void sendMessage(View view) {
  Intent intent = new Intent(this, DisplayMessageActivity.class);
}

    Intents

    An Intent is an object that provides runtime binding between separate components (such as two activities). The Intent represents an app’s "intent to do something." You can use intents for a wide variety of tasks, but most often they’re used to start another activity. For more information, seeIntents and Intent Filters.
    Note: The reference to DisplayMessageActivity will raise an error if you’re using an IDE such as Android Studio because the class doesn’t exist yet. Ignore the error for now; you’ll create the class soon.
    The constructor used here takes two parameters:
    • A Context as its first parameter (this is used because theActivity class is a subclass of Context)
    • The Class of the app component to which the system should deliver the Intent (in this case, the activity that should be started)
    Android Studio indicates that you must import the Intent class.
  2. At the top of the file, import the Intent class:
    java/com.mycompany.myfirstapp/MyActivity.java
    import android.content.Intent;
    Tip: In Android Studio, press Alt + Enter (option + return on Mac) to import missing classes.
  3. Inside the sendMessage() method, use findViewById() to get the EditText element.
    java/com.mycompany.myfirstapp/MyActivity.java
    public void sendMessage(View view) {
  Intent intent = new Intent(this, DisplayMessageActivity.class);
  EditText editText = (EditText) findViewById(R.id.edit_message);
}
  4. At the top of the file, import the EditText class.
    In Android Studio, press Alt + Enter (option + return on Mac) to import missing classes.
  5. Assign the text to a local message variable, and use the putExtra() method to add its text value to the intent.
    java/com.mycompany.myfirstapp/MyActivity.java
    public void sendMessage(View view) {
  Intent intent = new Intent(this, DisplayMessageActivity.class);
  EditText editText = (EditText) findViewById(R.id.edit_message);
  String message = editText.getText().toString();
  intent.putExtra(EXTRA_MESSAGE, message);
}
    An Intent can carry data types as key-value pairs called extras. The putExtra() method takes the key name in the first parameter and the value in the second parameter.
  6. At the top of the MyActivity class, add the EXTRA_MESSAGE definition as follows:
    java/com.mycompany.myfirstapp/MyActivity.java
    public class MyActivity extends ActionBarActivity {
    public final static String EXTRA_MESSAGE = "com.mycompany.myfirstapp.MESSAGE";
    ...
}
    For the next activity to query the extra data, you should define the key for your intent's extra using a public constant. It's generally a good practice to define keys for intent extras using your app's package name as a prefix. This ensures the keys are unique, in case your app interacts with other apps.
  7. In the sendMessage() method, to finish the intent, call the startActivity() method, passing it the Intentobject created in step 1.
With this new code, the complete sendMessage() method that's invoked by the Send button now looks like this:
java/com.mycompany.myfirstapp/MyActivity.java
/** Called when the user clicks the Send button */
public void sendMessage(View view) {
    Intent intent = new Intent(this, DisplayMessageActivity.class);
    EditText editText = (EditText) findViewById(R.id.edit_message);
    String message = editText.getText().toString();
    intent.putExtra(EXTRA_MESSAGE, message);
    startActivity(intent);
}
The system receives this call and starts an instance of the Activity specified by the Intent. Now you need to create the DisplayMessageActivity class in order for this to work.

Create the Second Activity

All subclasses of Activity must implement the onCreate() method. This method is where the activity receives the intent with the message, then renders the message. Also, the onCreate() method must define the activity layout with the setContentView() method. This is where the activity performs the initial setup of the activity components.

Create a new activity using Android Studio


Figure 1. The new activity wizard in Android Studio.
Android Studio includes a stub for theonCreate() method when you create a new activity.
  1. In Android Studio, in the java directory, select the package,com.mycompany.myfirstapp, right-click, and select New > Activity > Blank Activity.
  2. In the Choose options window, fill in the activity details:
    • Activity Name: DisplayMessageActivity
    • Layout Name: activity_display_message
    • Title: My Message
    • Hierarchical Parent: com.mycompany.myfirstapp.MyActivity
    • Package name: com.mycompany.myfirstapp
    Click Finish.
  3. Open the DisplayMessageActivity.java file.
    The class already includes an implementation of the required onCreate() method. You will update the implementation of this method later. It also includes an implementation of onOptionsItemSelected(), which handles the action bar's Up behavior. Keep these two methods as they are for now.
  4. Remove the onCreateOptionsMenu() method.
    You won't need it for this app.
If you're developing with Android Studio, you can run the app now, but not much happens. Clicking the Send button starts the second activity, but it uses a default "Hello world" layout provided by the template. You'll soon update the activity to instead display a custom text view.

Create the activity without Android Studio

If you're using a different IDE or the command line tools, do the following:
  1. Create a new file named DisplayMessageActivity.java in the project's src/ directory, next to the originalMyActivity.java file.
  2. Add the following code to the file:
    public class DisplayMessageActivity extends ActionBarActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_display_message);

        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment()).commit();
        }
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {

        public PlaceholderFragment() { }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                  Bundle savedInstanceState) {
              View rootView = inflater.inflate(R.layout.fragment_display_message,
                      container, false);
              return rootView;
        }
    }
}
    Note: If you are using an IDE other than Android Studio, your project does not contain theactivity_display_message layout that's requested by setContentView(). That's OK because you will update this method later and won't be using that layout.
  3. To your strings.xml file, add the new activity's title as follows:
    <resources>
    ...
    <string name="title_activity_display_message">My Message</string>
</resources>
  4. In your manifest file, AndroidManifest.xml, within the Application element, add the <activity> element for your DisplayMessageActivity class, as follows:
    <application ... >
    ...
    <activity
        android:name="com.mycompany.myfirstapp.DisplayMessageActivity"
        android:label="@string/title_activity_display_message"
        android:parentActivityName="com.mycompany.myfirstapp.MyActivity" >
        <meta-data
            android:name="android.support.PARENT_ACTIVITY"
            android:value="com.mycompany.myfirstapp.MyActivity" />
    </activity>
</application>
The android:parentActivityName attribute declares the name of this activity's parent activity within the app's logical hierarchy. The system uses this value to implement default navigation behaviors, such as Up navigationon Android 4.1 (API level 16) and higher. You can provide the same navigation behaviors for older versions of Android by using the Support Library and adding the <meta-data> element as shown here.
Note: Your Android SDK should already include the latest Android Support Library, which you installed during the Adding SDK Packages step. When using the templates in Android Studio, the Support Library is automatically added to your app project (you can see the library's JAR file listed under Android Dependencies). If you're not using Android Studio, you need to manually add the library to your project—follow the guide forsetting up the Support Library then return here.
If you're using a different IDE than Android Studio, don't worry that the app won't yet compile. You'll soon update the activity to display a custom text view.

Receive the Intent

Every Activity is invoked by an Intent, regardless of how the user navigated there. You can get the Intent that started your activity by calling getIntent() and retrieve the data contained within the intent.
  1. In the java/com.mycompany.myfirstapp directory, edit the DisplayMessageActivity.java file.
  2. In the onCreate() method, remove the following line:
      setContentView(R.layout.activity_display_message);
  3. Get the intent and assign it to a local variable.
    Intent intent = getIntent();
  4. At the top of the file, import the Intent class.
    In Android Studio, press Alt + Enter (option + return on Mac) to import missing classes.
  5. Extract the message delivered by MyActivity with the getStringExtra() method.
    String message = intent.getStringExtra(MyActivity.EXTRA_MESSAGE);

Display the Message

  1. In the onCreate() method, create a TextView object.
    TextView textView = new TextView(this);
  2. Set the text size and message with setText().
    textView.setTextSize(40);
textView.setText(message);
  3. Then add the TextView as the root view of the activity’s layout by passing it to setContentView().
    setContentView(textView);
  4. At the top of the file, import the TextView class.
    In Android Studio, press Alt + Enter (option + return on Mac) to import missing classes.
The complete onCreate() method for DisplayMessageActivity now looks like this:
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    // Get the message from the intent
    Intent intent = getIntent();
    String message = intent.getStringExtra(MyActivity.EXTRA_MESSAGE);

    // Create the text view
    TextView textView = new TextView(this);
    textView.setTextSize(40);
    textView.setText(message);

    // Set the text view as the activity layout
    setContentView(textView);
}
You can now run the app. When it opens, type a message in the text field, click Send, and the message appears on the second activity.

Adding the Action Bar

GET STARTED

DEPENDENCIES AND PREREQUISITES

  • Android 2.1 or higher

YOU SHOULD ALSO READ

DESIGN GUIDE

Action Bar
The action bar is one of the most important design elements you can implement for your app's activities. It provides several user interface features that make your app immediately familiar to users by offering consistency between other Android apps. Key functions include:
  • A dedicated space for giving your app an identity and indicating the user's location in the app.
  • Access to important actions in a predictable way (such as Search).
  • Support for navigation and view switching (with tabs or drop-down lists).
This training class offers a quick guide to the action bar's basics. For more information about action bar's various features, see the Action Bar guide.

Lessons

Setting Up the Action Bar
Learn how to add a basic action bar to your activity, whether your app supports only Android 3.0 and higher or also supports versions as low as Android 2.1 (by using the Android Support Library).
Adding Action Buttons
Learn how to add and respond to user actions in the action bar.
Styling the Action Bar
Learn how to customize the appearance of your action bar.
Overlaying the Action Bar
Learn how to overlay the action bar in front of your layout, allowing for seamless transitions when hiding the action bar.

Setting Up the Action Bar

PREVIOUSNEXT
In its most basic form, the action bar displays the title for the activity and the app icon on the left. Even in this simple form, the action bar is useful for all activities to inform users about where they are and to maintain a consistent identity for your app.

Figure 1. An action bar with the app icon and activity title.
Setting up a basic action bar requires that your app use an activity theme that enables the action bar. How to request such a theme depends on which version of Android is the lowest supported by your app. So this lesson is divided into two sections depending on which Android version is your lowest supported.

Support Android 3.0 and Above Only

Beginning with Android 3.0 (API level 11), the action bar is included in all activities that use the Theme.Holotheme (or one of its descendants), which is the default theme when either the targetSdkVersion orminSdkVersion attribute is set to "11" or greater.
So to add the action bar to your activities, simply set either attribute to 11 or higher. For example:
<manifest ... >
    <uses-sdk android:minSdkVersion="11" ... />
    ...</manifest>
Note: If you've created a custom theme, be sure it uses one of the Theme.Holo themes as its parent. For details, see Styling the Action Bar.
Now the Theme.Holo theme is applied to your app and all activities show the action bar. That's it.

Support Android 2.1 and Above

Adding the action bar when running on versions older than Android 3.0 (down to Android 2.1) requires that you include the Android Support Library in your application.
To get started, read the Support Library Setup document and set up the v7 appcompat library (once you've downloaded the library package, follow the instructions for Adding libraries with resources).
Once you have the Support Library integrated with your app project:
  1. Update your activity so that it extends ActionBarActivity. For example:
    public class MainActivity extends ActionBarActivity { ... }
  2. In your manifest file, update either the <application> element or individual <activity> elements to use one of the Theme.AppCompat themes. For example:
    <activity android:theme="@style/Theme.AppCompat.Light" ... >
    Note: If you've created a custom theme, be sure it uses one of the Theme.AppCompat themes as its parent. For details, see Styling the Action Bar.
Now your activity includes the action bar when running on Android 2.1 (API level 7) or higher.
Remember to properly set your app's API level support in the manifest:
<manifest ... >
    <uses-sdk android:minSdkVersion="7"  android:targetSdkVersion="18" />
    ...</manifest>







Adding Action Buttons





PREVIOUSNEXT










The action bar allows you to add buttons for the most important action items relating to the app's current context. Those that appear directly in the action bar with an icon and/or text are known as action buttons. Actions that can't fit in the action bar or aren't important enough are hidden in the action overflow.





Figure 1. An action bar with an action button for Search and the action overflow, which reveals additional actions.



Specify the Actions in XML





All action buttons and other items available in the action overflow are defined in an XML menu resource. To add actions to the action bar, create a new XML file in your project's res/menu/ directory.



Add an <item> element for each item you want to include in the action bar. For example:



res/menu/main_activity_actions.xml


<menu xmlns:android="http://schemas.android.com/apk/res/android" >
    <!-- Search, should appear as action button -->
    <item android:id="@+id/action_search"
          android:icon="@drawable/ic_action_search"
          android:title="@string/action_search"
          android:showAsAction="ifRoom" />
    <!-- Settings, should always be in the overflow -->
    <item android:id="@+id/action_settings"
          android:title="@string/action_settings"
          android:showAsAction="never" />
</menu>





Download action bar icons



To best match the Androidiconography guidelines, you should use icons provided in the Action Bar Icon Pack.





This declares that the Search action should appear as an action button when room is available in the action bar, but the Settings action should always appear in the overflow. (By default, all actions appear in the overflow, but it's good practice to explicitly declare your design intentions for each action.)



The icon attribute requires a resource ID for an image. The name that follows @drawable/ must be the name of a bitmap image you've saved in your project's res/drawable/ directory. For example, "@drawable/ic_action_search" refers to ic_action_search.png. Likewise, the title attribute uses a string resource that's defined by an XML file in your project's res/values/ directory, as discussed inBuilding a Simple User Interface.



Note: When creating icons and other bitmap images for your app, it's important that you provide multiple versions that are each optimized for a different screen density. This is discussed more in the lesson aboutSupporting Different Screens.



If your app is using the Support Library for compatibility on versions as low as Android 2.1, the showAsActionattribute is not available from the android: namespace. Instead this attribute is provided by the Support Library and you must define your own XML namespace and use that namespace as the attribute prefix. (A custom XML namespace should be based on your app name, but it can be any name you want and is only accessible within the scope of the file in which you declare it.) For example:



res/menu/main_activity_actions.xml


<menu xmlns:android="http://schemas.android.com/apk/res/android"
      xmlns:yourapp="http://schemas.android.com/apk/res-auto" >
    <!-- Search, should appear as action button -->
    <item android:id="@+id/action_search"
          android:icon="@drawable/ic_action_search"
          android:title="@string/action_search"
          yourapp:showAsAction="ifRoom"  />
    ...</menu>



Add the Actions to the Action Bar





To place the menu items into the action bar, implement the onCreateOptionsMenu() callback method in your activity to inflate the menu resource into the given Menu object. For example:


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu items for use in the action bar
    MenuInflater inflater = getMenuInflater();
    inflater.inflate(R.menu.main_activity_actions, menu);
    return super.onCreateOptionsMenu(menu);
}



Respond to Action Buttons





When the user presses one of the action buttons or another item in the action overflow, the system calls your activity's onOptionsItemSelected() callback method. In your implementation of this method, call getItemId()on the given MenuItem to determine which item was pressed—the returned ID matches the value you declared in the corresponding <item> element's android:id attribute.


@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle presses on the action bar items
    switch (item.getItemId()) {
        case R.id.action_search:
            openSearch();
            return true;
        case R.id.action_settings:
            openSettings();
            return true;
        default:
            return super.onOptionsItemSelected(item);
    }
}



Add Up Button for Low-level Activities









Figure 4. The Up button in Gmail.





All screens in your app that are not the main entrance to your app (activities that are not the "home" screen) should offer the user a way to navigate to the logical parent screen in the app's hierarchy by pressing the Up button in the action bar.



When running on Android 4.1 (API level 16) or higher, or when usingActionBarActivity from the Support Library, performing Up navigation simply requires that you declare the parent activity in the manifest file and enable the Up button for the action bar.



For example, here's how you can declare an activity's parent in the manifest:


<application ... >
    ...
    <!-- The main/home activity (it has no parent activity) -->
    <activity
        android:name="com.example.myfirstapp.MainActivity" ...>
        ...
    </activity>
    <!-- A child of the main activity -->
    <activity
        android:name="com.example.myfirstapp.DisplayMessageActivity"
        android:label="@string/title_activity_display_message"
        android:parentActivityName="com.example.myfirstapp.MainActivity" >
        <!-- Parent activity meta-data to support 4.0 and lower -->
        <meta-data
            android:name="android.support.PARENT_ACTIVITY"
            android:value="com.example.myfirstapp.MainActivity" />
    </activity>
</application>



Then enable the app icon as the Up button by calling setDisplayHomeAsUpEnabled():


@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_displaymessage);

    getSupportActionBar().setDisplayHomeAsUpEnabled(true);
    // If your minSdkVersion is 11 or higher, instead use:
    // getActionBar().setDisplayHomeAsUpEnabled(true);
}



Because the system now knows MainActivity is the parent activity for DisplayMessageActivity, when the user presses the Up button, the system navigates to the parent activity as appropriate—you do not need to handle theUp button's event.



For more information about up navigation, see Providing Up Navigation.

Styling the Action Bar

PREVIOUSNEXT
The action bar provides your users a familiar and predictable way to perform actions and navigate your app, but that doesn't mean it needs to look exactly the same as it does in other apps. If you want to style the action bar to better fit your product brand, you can easily do so using Android's style and themeresources.
Android includes a few built-in activity themes that include "dark" or "light" action bar styles. You can also extend these themes to further customize the look for your action bar.
Note: If you are using the Support Library APIs for the action bar, then you must use (or override) the Theme.AppCompatfamily of styles (rather than the Theme.Holo family, available in API level 11 and higher). In doing so, each style property that you declare must be declared twice: once using the platform's style properties (the android: properties) and once using the style properties included in the Support Library (theappcompat.R.attr properties—the context for these properties is actually your app). See the examples below for details.

Use an Android Theme

Android includes two baseline activity themes that dictate the color for the action bar:
You can apply these themes to your entire app or to individual activities by declaring them in your manifest file with the android:theme attribute for the<application> element or individual <activity>elements.
For example:
<application android:theme="@android:style/Theme.Holo.Light" ... />
You can also use a dark action bar while the rest of the activity uses the light color scheme by declaring theTheme.Holo.Light.DarkActionBar theme.
When using the Support Library, you must instead use the Theme.AppCompat themes:
Be sure that you use action bar icons that properly contrast with the color of your action bar. To help you, theAction Bar Icon Pack includes standard action icons for use with both the Holo light and Holo dark action bar.

Customize the Background

To change the action bar background, create a custom theme for your activity that overrides theactionBarStyle property. This property points to another style in which you can override the backgroundproperty to specify a drawable resource for the action bar background.
If your app uses navigation tabs or the split action bar, then you can also specify the background for these bars using the backgroundStacked and backgroundSplit properties, respectively.
Caution: It's important that you declare an appropriate parent theme from which your custom theme and style inherit their styles. Without a parent style, your action bar will be without many style properties unless you explicitly declare them yourself.

For Android 3.0 and higher only

When supporting Android 3.0 and higher only, you can define the action bar's background like this:
res/values/themes.xml
<?xml version="1.0" encoding="utf-8"?>
<resources>
    <!-- the theme applied to the application or activity -->
    <style name="CustomActionBarTheme"
           parent="@android:style/Theme.Holo.Light.DarkActionBar">
        <item name="android:actionBarStyle">@style/MyActionBar</item>
    </style>

    <!-- ActionBar styles -->
    <style name="MyActionBar"
           parent="@android:style/Widget.Holo.Light.ActionBar.Solid.Inverse">
        <item name="android:background">@drawable/actionbar_background</item>
    </style>
</resources>
Then apply your theme to your entire app or individual activities:
<application android:theme="@style/CustomActionBarTheme" ... />

For Android 2.1 and higher

When using the Support Library, the same theme as above must instead look like this:
res/values/themes.xml
<?xml version="1.0" encoding="utf-8"?>
<resources>
    <!-- the theme applied to the application or activity -->
    <style name="CustomActionBarTheme"
           parent="@style/Theme.AppCompat.Light.DarkActionBar">
        <item name="android:actionBarStyle">@style/MyActionBar</item>

        <!-- Support library compatibility -->
        <item name="actionBarStyle">@style/MyActionBar</item>
    </style>

    <!-- ActionBar styles -->
    <style name="MyActionBar"
           parent="@style/Widget.AppCompat.Light.ActionBar.Solid.Inverse">
        <item name="android:background">@drawable/actionbar_background</item>

        <!-- Support library compatibility -->
        <item name="background">@drawable/actionbar_background</item>
    </style>
</resources>
Then apply your theme to your entire app or individual activities:
<application android:theme="@style/CustomActionBarTheme" ... />

Customize the Text Color

To modify the color of text in the action bar, you need to override separate properties for each text element:

For Android 3.0 and higher only

When supporting Android 3.0 and higher only, your style XML file might look like this:
res/values/themes.xml
<?xml version="1.0" encoding="utf-8"?>
<resources>
    <!-- the theme applied to the application or activity -->
    <style name="CustomActionBarTheme"
           parent="@style/Theme.Holo">
        <item name="android:actionBarStyle">@style/MyActionBar</item>
        <item name="android:actionBarTabTextStyle">@style/MyActionBarTabText</item>
        <item name="android:actionMenuTextColor">@color/actionbar_text</item>
    </style>

    <!-- ActionBar styles -->
    <style name="MyActionBar"
           parent="@style/Widget.Holo.ActionBar">
        <item name="android:titleTextStyle">@style/MyActionBarTitleText</item>
    </style>

    <!-- ActionBar title text -->
    <style name="MyActionBarTitleText"
           parent="@style/TextAppearance.Holo.Widget.ActionBar.Title">
        <item name="android:textColor">@color/actionbar_text</item>
    </style>

    <!-- ActionBar tabs text styles -->
    <style name="MyActionBarTabText"
           parent="@style/Widget.Holo.ActionBar.TabText">
        <item name="android:textColor">@color/actionbar_text</item>
    </style>
</resources>

For Android 2.1 and higher

When using the Support Library, your style XML file might look like this:
res/values/themes.xml
<?xml version="1.0" encoding="utf-8"?>
<resources>
    <!-- the theme applied to the application or activity -->
    <style name="CustomActionBarTheme"
           parent="@style/Theme.AppCompat">
        <item name="android:actionBarStyle">@style/MyActionBar</item>
        <item name="android:actionBarTabTextStyle">@style/MyActionBarTabText</item>
        <item name="android:actionMenuTextColor">@color/actionbar_text</item>

        <!-- Support library compatibility -->
        <item name="actionBarStyle">@style/MyActionBar</item>
        <item name="actionBarTabTextStyle">@style/MyActionBarTabText</item>
        <item name="actionMenuTextColor">@color/actionbar_text</item>
    </style>

    <!-- ActionBar styles -->
    <style name="MyActionBar"
           parent="@style/Widget.AppCompat.ActionBar">
        <item name="android:titleTextStyle">@style/MyActionBarTitleText</item>

        <!-- Support library compatibility -->
        <item name="titleTextStyle">@style/MyActionBarTitleText</item>
    </style>

    <!-- ActionBar title text -->
    <style name="MyActionBarTitleText"
           parent="@style/TextAppearance.AppCompat.Widget.ActionBar.Title">
        <item name="android:textColor">@color/actionbar_text</item>
        <!-- The textColor property is backward compatible with the Support Library -->
    </style>

    <!-- ActionBar tabs text -->
    <style name="MyActionBarTabText"
           parent="@style/Widget.AppCompat.ActionBar.TabText">
        <item name="android:textColor">@color/actionbar_text</item>
        <!-- The textColor property is backward compatible with the Support Library -->
    </style>
</resources>

Customize the Tab Indicator

To change the indicator used for the navigation tabs, create an activity theme that overrides theactionBarTabStyle property. This property points to another style resource in which you override thebackground property that should specify a state-list drawable.
Note: A state-list drawable is important so that the tab currently selected indicates its state with a background different than the other tabs. For more information about how to create a drawable resource that handles multiple button states, read the State List documentation.
For example, here's a state-list drawable that declares a specific background image for several different states of an action bar tab:
res/drawable/actionbar_tab_indicator.xml
<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
<!-- STATES WHEN BUTTON IS NOT PRESSED -->

    <!-- Non focused states -->
    <item android:state_focused="false" android:state_selected="false"
          android:state_pressed="false"
          android:drawable="@drawable/tab_unselected" />
    <item android:state_focused="false" android:state_selected="true"
          android:state_pressed="false"
          android:drawable="@drawable/tab_selected" />

    <!-- Focused states (such as when focused with a d-pad or mouse hover) -->
    <item android:state_focused="true" android:state_selected="false"
          android:state_pressed="false"
          android:drawable="@drawable/tab_unselected_focused" />
    <item android:state_focused="true" android:state_selected="true"
          android:state_pressed="false"
          android:drawable="@drawable/tab_selected_focused" />

<!-- STATES WHEN BUTTON IS PRESSED -->

    <!-- Non focused states -->
    <item android:state_focused="false" android:state_selected="false"
          android:state_pressed="true"
          android:drawable="@drawable/tab_unselected_pressed" />
    <item android:state_focused="false" android:state_selected="true"
        android:state_pressed="true"
        android:drawable="@drawable/tab_selected_pressed" />

    <!-- Focused states (such as when focused with a d-pad or mouse hover) -->
    <item android:state_focused="true" android:state_selected="false"
          android:state_pressed="true"
          android:drawable="@drawable/tab_unselected_pressed" />
    <item android:state_focused="true" android:state_selected="true"
          android:state_pressed="true"
          android:drawable="@drawable/tab_selected_pressed" />
</selector>

For Android 3.0 and higher only

When supporting Android 3.0 and higher only, your style XML file might look like this:
res/values/themes.xml
<?xml version="1.0" encoding="utf-8"?>
<resources>
    <!-- the theme applied to the application or activity -->
    <style name="CustomActionBarTheme"
           parent="@style/Theme.Holo">
        <item name="android:actionBarTabStyle">@style/MyActionBarTabs</item>
    </style>

    <!-- ActionBar tabs styles -->
    <style name="MyActionBarTabs"
           parent="@style/Widget.Holo.ActionBar.TabView">
        <!-- tab indicator -->
        <item name="android:background">@drawable/actionbar_tab_indicator</item>
    </style>
</resources>

For Android 2.1 and higher

When using the Support Library, your style XML file might look like this:
res/values/themes.xml
<?xml version="1.0" encoding="utf-8"?>
<resources>
    <!-- the theme applied to the application or activity -->
    <style name="CustomActionBarTheme"
           parent="@style/Theme.AppCompat">
        <item name="android:actionBarTabStyle">@style/MyActionBarTabs</item>

        <!-- Support library compatibility -->
        <item name="actionBarTabStyle">@style/MyActionBarTabs</item>
    </style>

    <!-- ActionBar tabs styles -->
    <style name="MyActionBarTabs"
           parent="@style/Widget.AppCompat.ActionBar.TabView">
        <!-- tab indicator -->
        <item name="android:background">@drawable/actionbar_tab_indicator</item>

        <!-- Support library compatibility -->
        <item name="background">@drawable/actionbar_tab_indicator</item>
    </style>
</resources>





Overlaying the Action Bar





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By default, the action bar appears at the top of your activity window, slightly reducing the amount of space available for the rest of your activity's layout. If, during the course of user interaction, you want to hide and show the action bar, you can do so by calling hide() and show() on the ActionBar. However, this causes your activity to recompute and redraw the layout based on its new size.







Figure 1. Gallery's action bar in overlay mode.





To avoid resizing your layout when the action bar hides and shows, you can enable overlay mode for the action bar. When in overlay mode, your activity layout uses all the space available as if the action bar is not there and the system draws the action bar in front of your layout. This obscures some of the layout at the top, but now when the action bar hides or appears, the system does not need to resize your layout and the transition is seamless.



Tip: If you want your layout to be partially visible behind the action bar, create a custom style for the action bar with a partially transparent background, such as the one shown in figure 1. For information about how to define the action bar background, read Styling the Action Bar.




Enable Overlay Mode





To enable overlay mode for the action bar, you need to create a custom theme that extends an existing action bar theme and set the android:windowActionBarOverlay property to true.




For Android 3.0 and higher only



If your minSdkVersion is set to 11 or higher, your custom theme should use Theme.Holo theme (or one of its descendants) as your parent theme. For example:


<resources>
    <!-- the theme applied to the application or activity -->
    <style name="CustomActionBarTheme"
           parent="@android:style/Theme.Holo">
        <item name="android:windowActionBarOverlay">true</item>
    </style>
</resources>




For Android 2.1 and higher



If your app is using the Support Library for compatibility on devices running versions lower than Android 3.0, your custom theme should use Theme.AppCompat theme (or one of its descendants) as your parent theme. For example:


<resources>
    <!-- the theme applied to the application or activity -->
    <style name="CustomActionBarTheme"
           parent="@android:style/Theme.AppCompat">
        <item name="android:windowActionBarOverlay">true</item>

        <!-- Support library compatibility -->
        <item name="windowActionBarOverlay">true</item>
    </style>
</resources>



Also notice that this theme includes two definitions for the windowActionBarOverlay style: one with the android:prefix and one without. The one with the android: prefix is for versions of Android that include the style in the platform and the one without the prefix is for older versions that read the style from the Support Library.




Specify Layout Top-margin





When the action bar is in overlay mode, it might obscure some of your layout that should remain visible. To ensure that such items remain below the action bar at all times, add either margin or padding to the top of the view(s) using the height specified by actionBarSize. For example:


<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingTop="?android:attr/actionBarSize">
    ...</RelativeLayout>



If you're using the Support Library for the action bar, you need to remove the android: prefix. For example:


<!-- Support library compatibility -->
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingTop="?attr/actionBarSize">
    ...</RelativeLayout>



In this case, the ?attr/actionBarSize value without the prefix works on all versions, including Android 3.0 and higher.

Supporting Different Devices

GET STARTED

DEPENDENCIES AND PREREQUISITES

  • Android 1.6 or higher

YOU SHOULD ALSO READ

Android devices come in many shapes and sizes all around the world. With a wide range of device types, you have an opportunity to reach a huge audience with your app. In order to be as successful as possible on Android, your app needs to adapt to various device configurations. Some of the important variations that you should consider include different languages, screen sizes, and versions of the Android platform.
This class teaches you how to use basic platform features that leverage alternative resources and other features so your app can provide an optimized user experience on a variety of Android-compatible devices, using a single application package (APK).

Lessons

Supporting Different Languages
Learn how to support multiple languages with alternative string resources.
Supporting Different Screens
Learn how to optimize the user experience for different screen sizes and densities.
Supporting Different Platform Versions
Learn how to use APIs available in new versions of Android while continuing to support older versions of Android.

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