Android View - What is automatically saved and restored in an Activity

Question

I am a beginner with Android.

In Android, some generic elements can be automatically saved/restored in onSaveInstanceState/onRestoreInstanceState.

For example, an EditText saves/restores the Text property, a RatingBar saves/restores the Rating property...

I see from some tests but I can't find anything about this in the documentation.

How can I know what is saved/restored implicitly, without my intervention?

For example, where I can find that the EditText.Text is automatically saved/restored?

I specifically don't want test all properties.

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Edit from JRG answer :

https://developer.android.com/guide/components/activities/activity-lifecycle.html

Save your activity state As your activity begins to stop, the system calls the onSaveInstanceState() method<...> The default implementation of this method saves transient 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 widget.

How I can know what is the default implementation of save/restore?

Second edit after reread JRG answer :

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 widget).

The default implementation saves/restores all state of element view.

Answer 1

Android Documentation that explains about saving states and a very good article on saving states in activity and fragment.

• Android Documentation: https://developer.android.com/guide/components/activities/activity-lifecycle.html
• Additional Article: https://inthecheesefactory.com/blog/fragment-state-saving-best-practices/en

Saving and restoring activity state 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 the finish() method. The system may also destroy the process containing your activity to recover memory if the activity is in the Stopped state and hasn't been used in a long time, or if the foreground activity requires more resources.

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.

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 widget). 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.

Save your activity state As your activity begins to stop, the system calls the onSaveInstanceState() method so your activity can save state information with a collection of key-value pairs. The default implementation of this method saves transient 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 widget.

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

To save additional state information for your activity, you must override onSaveInstanceState() and add key-value pairs to the Bundle object that is saved in the event that your activity is destroyed unexpectedly. 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);
}

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 persistent data, such as user preferences or data for a database, you should take appropriate opportunities when your activity is in the foreground. If no such opportunity arises, you should save such data during the onStop() method.

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 to 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, the following code snippet shows 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.