onSaveInstanceState () and onRestoreInstanceState ()

Question

I\'m trying to save and restore the state of an Activity using the methods onSaveInstanceState() and onRestoreInstanceState().

Answer 1

As a workaround, you could store a bundle with the data you want to maintain in the Intent you use to start activity A.

Intent intent = new Intent(this, ActivityA.class);
intent.putExtra("bundle", theBundledData);
startActivity(intent);

Activity A would have to pass this back to Activity B. You would retrieve the intent in Activity B's onCreate method.

Intent intent = getIntent();
Bundle intentBundle;
if (intent != null)
    intentBundle = intent.getBundleExtra("bundle");
// Do something with the data.

Another idea is to create a repository class to store activity state and have each of your activities reference that class (possible using a singleton structure.) Though, doing so is probably more trouble than it's worth.

Answer 2

If you are handling activity's orientation changes with android:configChanges="orientation|screenSize" and onConfigurationChanged(Configuration newConfig), onRestoreInstanceState() will not be called.

Answer 3

From the documentation Restore activity UI state using saved instance state it is stated as:

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:

IMO, this is more clear way than checking this at onCreate, and better fits with single responsiblity principle.

Answer 4

Usually you restore your state in onCreate(). It is possible to restore it in onRestoreInstanceState() as well, but not very common. (onRestoreInstanceState() is called after onStart(), whereas onCreate() is called before onStart().

Use the put methods to store values in onSaveInstanceState():

protected void onSaveInstanceState(Bundle icicle) {
  super.onSaveInstanceState(icicle);
  icicle.putLong("param", value);
}

And restore the values in onCreate():

public void onCreate(Bundle icicle) {
  if (icicle != null){
    value = icicle.getLong("param");
  }
}

Answer 5

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 (i.e. think of it as exiting desktop application) and next time you start your app it is started "fresh", i.e. 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, i.e. 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 6

In my case, onRestoreInstanceState was called when the activity was reconstructed after changing the device orientation. onCreate(Bundle) was called first, but the bundle didn't have the key/values I set with onSaveInstanceState(Bundle).

Right after, onRestoreInstanceState(Bundle) was called with a bundle that had the correct key/values.