How to save activity state?

Question

In my android app there are 2 activities. I am navigating from activity1 to activity2 . I am returining to activity1 by pressing a button in activity2 . I want the activity1 to retain the previous state. I am using following code to achieve this.

This code to save the state.

@Override
public void onSaveInstanceState(Bundle savedInstanceState1) {
    super.onSaveInstanceState(savedInstanceState1);
    if (mode == 3) {
        savedInstanceState1.putInt("totscore", totscore);
        for (int i = 0; i < 6; i++) {
            for (int j = 0; j < 6; j++) {
                savedInstanceState1.putInt("ballmatrix" + i + "" + j, ballmatrix[i][j][2]);
            }
        }
    }
}

This code to retrieve the state.

@Override
public void onRestoreInstanceState(Bundle savedInstanceState1) {
    super.onRestoreInstanceState(savedInstanceState1);
    // Restore UI state from the savedInstanceState.
    // This bundle has also been passed to onCreate.
    if (mode == 3 && restore == 1) {
        Log.d("ok", "2");
        if (savedInstanceState1 != null) {
            Log.d("ok", "3");
            totscore = savedInstanceState1.getInt("totscore");

            for (int i = 0; i < 6; i++) {
                for (int j = 0; j < 6; j++) {
                    ballmatrix[i][j][2] = savedInstanceState1.getInt("ballmatrix" + i + "" + j);
                }
            }
        }
    }
    drawmatrix();
}

But when retrieving is done, savedInstanceState1 != null turns out to be false everytime. What is wrong here ?

Answer 1

The answer below was taken from here: onSaveInstanceState () and onRestoreInstanceState ()

onRestoreInstanceState() is called only when recreating activity after it was killed by the OS. Such situation happen when:

• orientation of the device changes (your activity is destroyed and recreated)
• there is another activity in front of yours and at some point the OS kills your activity in order to free memory (for example). Next time when you start your activity onRestoreInstanceState() will be called.

In contrast: if you are in your activity and you hit Back button on the device, your activity is finish()ed (ie think of it as exiting desktop application) and next time you start your app it is started "fresh", ie without saved state because you intentionally exited it when you hit Back.

Other source of confusion is that when an app loses focus to another app onSaveInstanceState() is called but when you navigate back to your app onRestoreInstanceState() may not be called. This is the case described in the original question, ie if your activity was NOT killed during the period when other activity was in front onRestoreInstanceState() will NOT be called because your activity is pretty much "alive".

All in all, as stated in the documentation for onRestoreInstanceState():

Most implementations will simply use onCreate(Bundle) to restore their state, but it is sometimes convenient to do it here after all of the initialization has been done or to allow subclasses to decide whether to use your default implementation. The default implementation of this method performs a restore of any view state that had previously been frozen by onSaveInstanceState(Bundle).

As I read it: There is no reason to override onRestoreInstanceState() unless you are subclassing Activity and it is expected that someone will subclass your subclass.

Answer 2

Simply navigating to another Activity isn't going to destroy the current one. This means when you're launching activity2 , activity1 is still in the background (in stopped state, assuming activity2 is full-screen). It has not been destroyed, so all of its state is still around. onSaveInstanceState() is called, in case your Activity does get destroyed. However, unless it has been destroyed, onRestoreInstanceState() will not be called, as your Activity instance is still around -- therefore there is no need to restore state.

If this is just for testing, you can force the Activity to be destroyed when the new one arrives by going to your Developer Options and enabling "Don't Keep Activities". However, what you're seeing is expected behavior. To test that save and restore are working properly, you can either change your device orientation (which will destroy and recreate the activity), or enable the "Don't Keep Activities" option in Developer Options, and launch activity2 .