android - savedInstanceState is always null

Question

This is my savedInstaceState code:

@Override
public void onSaveInstanceState(Bundle savedInstanceState) 
{
    savedInstanceState.putStringArrayList("todo_arraylist", Altodo);
    Log.v("bundle", "Saved");
    super.onSaveInstanceState(savedInstanceState);
}


public void onCreate(Bundle savedInstanceState) 
{
    super.onCreate(savedInstanceState);

    if (savedInstanceState != null) 
    {
        Altodo = savedInstanceState.getStringArrayList("todo_arraylist");
        Log.v("bundle", "Restored");
    }
    else
    {
        Log.v("bundle", "null");
    }

    setContentView(R.layout.main);
}

The logs always show the "bundle save" tag.

But in onCreate method, SavedInstanceState is always null.

Answer 1

I observed the exact same symptoms (reported as issue 133394) in a project with two Activities A and B that extend ActionBarActivity. Activity A is the main activity, and I always receive null for savedInstanceState in onCreate of its list fragment when returning from a detail view activity B. After many hours, this problem exposed itself to me as a navigation issue in disguise.

The following may be relevant to my setup and come from other answers on this page:

• Given this answer, I made sure that fragment and activity each have unique IDs set.
• There is no override of onSaveInstanceState without super call.
• Activity A is specified as acitivy B's parent in AndroidManifest.xml, using both the android:parentActivityName attribute and the corresponding meta-data tag for earlier versions of Android (see "Providing Up Navigation").

Already without any corresponding creation code such as getActionBar() .setHomeButtonEnabled(true), activity B has a functioning back button (<) in its action bar. When this button is tapped, activity A reappears but with (a) all previous instance state lost, (b) onCreate always called, and (c) savedInstanceState always null.

Interestingly, when I tap the back button provided at the bottom edge of the emulator display (an open triangle that points to the left), activity A reappears just as it was left (i.e. its instance state fully retained) without invoking onCreate. So maybe something is wrong with navigation?

After more reading, I implemented my own navigation instructions to run in response to a tap on the back-button in activity B:

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    if (item.getItemId() == android.R.id.home)
        NavUtils.navigateUpFromSameTask(this);
        return true;
    }
    return super.onOptionsItemSelected(item);
}

Nothing related to restoring instance state of activity A changed. NavUtils also provide a method getParentActivityIntent(Activity) and navigateUpTo(Activity, Intent) that allow us to modify the navigation intent to explicitly instruct that activity A is not started fresh (and thus without saved instance state provided) by setting the FLAG_ACTIVITY_CLEAR_TOP flag:

If set, and the activity being launched is already running in the current task, then instead of launching a new instance of that activity, all of the other activities on top of it will be closed and this Intent will be delivered to the (now on top) old activity as a new Intent.

In my hands, this solves problem of lost instance state and could look like:

public boolean onOptionsItemSelected(MenuItem item) {
    if (item.getItemId()== android.R.id.home) {
        Intent intent = NavUtils.getParentActivityIntent(this);
        intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
        NavUtils.navigateUpTo(this, intent);
        return true;
    }
    return super.onOptionsItemSelected(item);
}

Note that this may not be the complete solution in other cases where a user can switch directly to activity B from within a different task (see here). Also, a possibly identical solution in behavior that does not make use of NavUtils is to simply call finish():

public boolean onOptionsItemSelected(MenuItem item) {
    if (item.getItemId()== android.R.id.home) {
        finish();
        return true;
    }
    return super.onOptionsItemSelected(item);
}

Both solutions work in my hands. I am only speculating that the original issue is a slightly incorrect default implementation of the back-button, and it may be related to that implementation invoking some kind of navigateUp that misses FLAG_ACTIVITY_CLEAR_TOP.