OS杀死活动后，片段savedInstanceState不为null

Question

I have a fragment which saves the state via setRetainInstance(true) . This is my fragment's very simplified code:

public class MyFragment extends Fragment {
  private SomeData mData;

  public void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setRetainInstance(true);
  }

  @Override
  public void onActivityCreated(@Nullable Bundle savedInstanceState) {
      if (savedInstanceState == null) {
        mData = new SomeData(getView());
      } else {
        mData.refresh(getView());
      }
      // More awesome code
    }
  }
}

Sometimes the app crashes with NullPointerException - my mData suddenly becomes null . This happens when I fold the application and return after some time.

I have some theory. After a while OS kills the Activity and the Fragment (despite setRetainInstance (true) ). Thus creates a new object of my fragment where mData initialized by null . But savedInstanseState not equals to null. Thus the new fragment skips initialization and attempts to call refresh(View) on the null reference.

My question: What will contain the savedInstanseState variable in the onActivityCreated(savedInstanceState) method in my snippet when OS kills the activity (when app not on the screen) and recreate after return?

Answer 1

Not sure about the answer fully but here it is What I can guess

setRetainInstance (boolean retain) Control whether a fragment instance is retained across Activity re-creation (such as from a configuration change). This can only be used with fragments not in the back stack.

So I think that When you fold the activity the views get destroyed and instance data (setReetainInstance value)is saved in the bundle ( making savedInstanceState != null while loading). However, when you load the activity (since previously the retainInstance was set to true) onCreate() does not get called and since savedInstanceState != null and mData == null you get the error.