重新创建一个Activity

Recreating an Activity

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This lesson teaches you to

Save Your Activity
State
Restore Your Activity
State

You should also read

Supporting Different Screens
Handling Runtime Changes
Activities

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 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 Backor 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 

android:id

 attribute.
To save additional data about the activity state, you must override the 

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()

:

@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
    }
    ...
}

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(STATE_SCORE);
    mCurrentLevel = savedInstanceState.getInt(STATE_LEVEL);
}

Caution: Always call the superclass implementation of 

onRestoreInstanceState()

 so
the default implementation can restore the state of the view hierarchy.
To learn more about recreating your activity due to a restart event at runtime (such as when the screen rotates), read Handling
Runtime Changes.