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Recreating an Activity

There are a few scenarios in which your activity is destroyed due to normal app behavior, such as when the user presses the Back button or your activity signals its own destruction by calling finish(). The system may also destroy your activity if it's currently stopped and hasn't been used in a long time or the system needs to free up resources and must shut down cached processes to recover memory.

When your activity is destroyed because the user presses Back or the activity finishes itself, all traces of the Activity instance is gone forever. However, if the system destroys the activity due to system constraints (rather than normal app behavior), then although the actual Activity instance is gone, the system remembers that it existed such that if the user navigates back to it, the system creates a new instance of the activity using a set of saved data that describes the state of the activity when it was destroyed. The saved data that the system uses to restore the previous state is called the "instance state" and is a collection of key-value pairs stored in a Bundle object.

Caution: Your activity will be destroyed and recreated each time the device configuration changes, like when the user rotates the screen. When the screen changes orientation, the system destroys and recreates the foreground activity because the screen configuration has changed and your activity might need to load alternative resources (such as the layout).

By default, the system uses the Bundle instance state to save information about each View object in your activity layout (such as the text value entered into an EditText object). So, if your activity instance is destroyed and recreated, the state of the layout is restored to its previous state with no code required by you. However, your activity might have more transient state information that you'd like to restore, such as member variables that track the user's progress in the activity.

Note: In order for the Android system to restore the state of the views in your activity, each view must have a unique ID, supplied by the android:id attribute.

To save additional data about the activity state, you must override the onSaveInstanceState() method. The system calls this method when the user is leaving your activity and passes it the Bundle object that will be saved in the event that your activity is destroyed. If the system must recreate the activity instance later, it passes the same Bundle object to both the onRestoreInstanceState() and onCreate() methods.

Figure 2. As the system begins to stop your activity, it calls onSaveInstanceState() (1) so you can specify additional state data you'd like to save in case the Activity instance must be recreated. If the activity is destroyed and the same instance must be recreated, the system passes the state data defined at (1) to both the onCreate() method (2) and the onRestoreInstanceState() method (3).

Save Your Activity State

As your activity begins to stop, the system calls onSaveInstanceState() so your activity can save state information with a collection of key-value pairs. The default implementation of this method saves information about the state of the activity's view hierarchy, such as the text in an EditText widget or the scroll position of a ListView.

To save additional state information for your activity, you must implement onSaveInstanceState() and add key-value pairs to the Bundle object. It is important to note that you must only save transient state of your activity at this time, state that the user would expect to be present when they navigate back to your activity. Typically this is the UI state which is mostly saved automatically by the super class implementation or some other non persistent state. For example:

static final String PLAYER_SCORE = "playerScore";
static final String PLAYER_LEVEL = "playerLevel";
...

@Override
public void onSaveInstanceState(Bundle savedInstanceState) {
    // Save the user's current game state
    savedInstanceState.putInt(PLAYER_SCORE, mCurrentScore);
    savedInstanceState.putInt(PLAYER_LEVEL, mCurrentLevel);

    // Always call the superclass so it can save the view hierarchy state
    super.onSaveInstanceState(savedInstanceState);
}

Caution: Always call the superclass implementation of onSaveInstanceState() so the default implementation can save the state of the view hierarchy.

Restore Your Activity State

When your activity is recreated after it was previously destroyed, you can recover your saved state from the Bundle that the system passes your activity. Both the onCreate() and onRestoreInstanceState() callback methods receive the same Bundle that contains the instance state information.

Because the onCreate() method is called whether the system is creating a new instance of your activity or recreating a previous one, you must check whether the state Bundle is null before you attempt to read it. If it is null, then the system is creating a new instance of the activity, instead of restoring a previous one that was destroyed.

For example, here's how you can restore some state data in onCreate():

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState); // Always call the superclass first

    // Check whether we're recreating a previously destroyed instance
    if (savedInstanceState != null) {
        // Restore value of members from saved state
        mCurrentScore = savedInstanceState.getInt(PLAYER_SCORE);
        mCurrentLevel = savedInstanceState.getInt(PLAYER_LEVEL);
    } else {
        // Probably initialize members with default values for a new instance
    }
    ...
}

Instead of restoring the state during onCreate() you may choose to implement onRestoreInstanceState(), which the system calls after the onStart() method. The system calls onRestoreInstanceState() only if there is a saved state to restore, so you do not need to check whether the Bundle is null:

public void onRestoreInstanceState(Bundle savedInstanceState) {
    // Always call the superclass so it can restore the view hierarchy
    super.onRestoreInstanceState(savedInstanceState);

    // Restore state members from saved instance
    mCurrentScore = savedInstanceState.getInt(PLAYER_SCORE);
    mCurrentLevel = savedInstanceState.getInt(PLAYER_LEVEL);
}

Caution: Always call the superclass implementation of onRestoreInstanceState() so the default implementation can restore the state of the view hierarchy.

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