android savedInstanceState始终为null

Question

I have the following situation:

• Activity A starts activity B
• Activity A calls onPause , onSaveInstanceState and onStop methods.
• On activity BI press the "UP button" on the action bar
• Activity A first gets destroyed (calls onDestroy ) and then recreated. (This just seems to happen this way. I have not implemented this way, just seems to be the Android way of doing things ..)
• During the onCreate method, the variable savedInstanceState is always null.

I know there have been similar topics around here, but none of them seem to have an answer for my situation. All the callback methods have log lines in them, so I'm certain that the save method is executed and destory is executed. But why is there never a savedInstanceState object?

My save method:

@Override
public void onSaveInstanceState(Bundle savedInstanceState) {
    savedInstanceState.putSerializable(OpenSessionsActivity.SESSION, session);
    System.out.println("saving ..");
    super.onSaveInstanceState(savedInstanceState);
}

Is there any other code you need me to include?

Answer 1

The top activity is recreated when navigating Up. To preserve state of the activity A you can

A) Set launch mode of the activity A to "singleTop" (add android:launchMode="singleTop" to AndroidManifed.xml)

or

B) Add FLAG_ACTIVITY_CLEAR_TOP flag to the up intent when navigating from the activity B:

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()) {
        case android.R.id.home:
            Intent up = NavUtils.getParentActivityIntent(this);
            up.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
            NavUtils.navigateUpTo(this, up);
            return true;
    }
    return super.onOptionsItemSelected(item);
}

This is a documented behavior:

Similarly, if you navigate up to an activity on the current stack, the behavior is determined by the parent activity's launch mode. If the parent activity has launch mode singleTop (or the up intent contains FLAG_ACTIVITY_CLEAR_TOP), the parent is brought to the top of the stack, and its state is preserved. The navigation intent is received by the parent activity's onNewIntent() method. If the parent activity has launch mode standard (and the up intent does not contain FLAG_ACTIVITY_CLEAR_TOP), the current activity and its parent are both popped off the stack, and a new instance of the parent activity is created to receive the navigation intent.

Answer 2

Press Up on the action bar is actually not the same thing as pressing the back button.

If you want them to behave the same, you could use this code in Activity B:

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()) {
        case android.R.id.home:
            onBackPressed();
            return true;
        default:
            return super.onOptionsItemSelected(item);
    }
}

Answer 3

You should put the

super.onSaveInstanceState(savedInstanceState);

before the code of saving your own data.

Answer 4

As mentioned in the Documentation :

protected void onSaveInstanceState (Bundle outState):

Do not confuse this method with activity lifecycle callbacks such as onPause(), which is always called when an activity is being placed in the background or on its way to destruction, or onStop() which is called before destruction. One example of when onPause() and onStop() is called and not this method is when a user navigates back from activity B to activity A: there is no need to call onSaveInstanceState(Bundle) on B because that particular instance will never be restored, so the system avoids calling it. An example when onPause() is called and not onSaveInstanceState(Bundle) is when activity B is launched in front of activity A: the system may avoid calling onSaveInstanceState(Bundle) on activity A if it isn't killed during the lifetime of B since the state of the user interface of A will stay intact.

If you do an orientation change with your device onStop() and onSavedInstanceState(...) will be called.

Answer 5

I had to add android:launchMode="singleTop" into the activity section of my manifest file to get this to work. I then only needed

NavUtils.navigateUpFromSameTask(getActivity())