Providing Up Navigation

All screens in your app that are not the main entrance to your app (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 . This lesson shows you how to properly implement this behavior.

Figure 1. The Up button in the action bar.

Specify the Parent Activity

To implement Up navigation, the first step is to declare which activity is the appropriate parent for each activity. Doing so allows the system to facilitate navigation patterns such as Up because the system can determine the logical parent activity from the manifest file.

Beginning in Android 4.1 (API level 16), you can declare the logical parent of each activity by specifying the android:parentActivityName attribute in the <activity> element.

If your app supports Android 4.0 and lower, include the Support Library with your app and add a <meta-data> element inside the <activity> . Then specify the parent activity as the value for android.support.PARENT_ACTIVITY , matching the android:parentActivityName attribute.

For example:

       <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>
      

With the parent activity declared this way, you can navigate Up to the appropriate parent using the NavUtils APIs, as shown in the following sections.

Add Up Action

To allow Up navigation with the app icon in the action bar, call setDisplayHomeAsUpEnabled() :

       @Override
public void onCreate(Bundle savedInstanceState) {
    ...
    getActionBar().setDisplayHomeAsUpEnabled(true);
}
      

This adds a left-facing caret alongside the app icon and enables it as an action button such that when the user presses it, your activity receives a call to onOptionsItemSelected() . The ID for the action is android.R.id.home .

Navigate Up to Parent Activity

To navigate up when the user presses the app icon, you can use the NavUtils class's static method, navigateUpFromSameTask() . When you call this method, it finishes the current activity and starts (or resumes) the appropriate parent activity. If the target parent activity is in the task's back stack, it is brought forward. The way it is brought forward depends on whether the parent activity is able to handle an onNewIntent() call:

• If the parent activity has launch mode <singleTop> , or the up intent contains FLAG_ACTIVITY_CLEAR_TOP , the parent activity is brought to the top of the stack, and receives the intent through its onNewIntent() method.
• If the parent activity has launch mode <standard> , and the up intent does not contain FLAG_ACTIVITY_CLEAR_TOP , the parent activity is popped off the stack, and a new instance of that activity is created on top of the stack to receive the intent.

For example:

       @Override
public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()) {
    // Respond to the action bar's Up/Home button
    case android.R.id.home:
        NavUtils.navigateUpFromSameTask(this);
        return true;
    }
    return super.onOptionsItemSelected(item);
}
      

However, using navigateUpFromSameTask() is suitable only when your app is the owner of the current task (that is, the user began this task from your app). If that's not true and your activity was started in a task that belongs to a different app, then navigating Up should create a new task that belongs to your app, which requires that you create a new back stack.

Navigate up with a new back stack

If your activity provides any intent filters that allow other apps to start the activity, you should implement the onOptionsItemSelected() callback such that if the user presses the Up button after entering your activity from another app's task, your app starts a new task with the appropriate back stack before navigating up.

You can do so by first calling shouldUpRecreateTask() to check whether the current activity instance exists in a different app's task. If it returns true, then build a new task with TaskStackBuilder . Otherwise, you can use the navigateUpFromSameTask() method as shown above.

For example:

       @Override
public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()) {
    // Respond to the action bar's Up/Home button
    case android.R.id.home:
        Intent upIntent = NavUtils.getParentActivityIntent(this);
        if (NavUtils.shouldUpRecreateTask(this, upIntent)) {
            // This activity is NOT part of this app's task, so create a new task
            // when navigating up, with a synthesized back stack.
            TaskStackBuilder.create(this)
                    // Add all of this activity's parents to the back stack
                    .addNextIntentWithParentStack(upIntent)
                    // Navigate up to the closest parent
                    .startActivities();
        } else {
            // This activity is part of this app's task, so simply
            // navigate up to the logical parent activity.
            NavUtils.navigateUpTo(this, upIntent);
        }
        return true;
    }
    return super.onOptionsItemSelected(item);
}
      

Note: In order for the addNextIntentWithParentStack() method to work, you must declare the logical parent of each activity in your manifest file, using the android:parentActivityName attribute (and corresponding <meta-data> element) as described above.