This lesson teaches you to
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finish(). The system may also destroy your activity if it's
currently stopped and hasn't been used in a long time or the foreground activity requires more
resources so the system must shut down background processes to recover memory.When your activity is destroyed because the user presses Back or the activity finishes itself, the system's concept of that
Activity instance is gone forever because
the behavior indicates the activity is no longer needed. 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 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 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
To save additional data about the activity state, you must override
the android:id attribute.onSaveInstanceState() callback 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 unexpectedly. 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. For example:static final String STATE_SCORE = "playerScore"; static final String STATE_LEVEL = "playerLevel"; ... @Override public void onSaveInstanceState(Bundle savedInstanceState) { // Save the user's current game state savedInstanceState.putInt(STATE_SCORE, mCurrentScore); savedInstanceState.putInt(STATE_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():Instead of restoring the state during@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(STATE_SCORE); mCurrentLevel = savedInstanceState.getInt(STATE_LEVEL); } else { // Probably initialize members with default values for a new instance } ... }
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(STATE_SCORE); mCurrentLevel = savedInstanceState.getInt(STATE_LEVEL); }
Caution: Always call the superclass implementation of
To learn more about recreating your activity due to a
restart event at runtime (such as when the screen rotates), read Handling Runtime Changes.onRestoreInstanceState() so the default implementation
can restore the state of the view hierarchy.
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