Android – how i can break things with Fragments with setRetainInstance(true) and adding them to backstack

androidandroid-fragmentsandroid-orientation

the docs on setRetainInstance say :

This can only be used with fragments not in the back stack.

so I started playing with it.

I have one Activity with adds first frag A

FragmentManager fm = getSupportFragmentManager();
FragmentTransaction ft = fm.beginTransaction();
ft.replace(R.id.content, new PackageFragment());
ft.commit

then from this frag I run a method from parent Activity which adds frag B to backstack

FragmentManager fm = getSupportFragmentManager();
FragmentTransaction ft = fm.beginTransaction();
ft.replace(R.id.content, new OrderFragment());
ft.addToBackStack(null);
ft.commit();

then I create log msg from onCreate,onDestroy,onSaveInstanceState,onActivityCreated…etc

I try two versions of this process. Rotating the device on each fragment.

  1. default

Everything is as expected. onCreate, onDestroy on fragments fire

  1. setRetainInstance(true)

Everything is as expected?. onCreate, onDestroy on fragments dont fire

and all seems to work while fragments are in the backstack.. so why the docs say I shouldnt use it?
What are the scenarios where I might get in trouble?

thanks

Best Answer

Updated answer:

What are the scenarios where I might get in trouble?

When adding a Fragment to the back stack and passing a Bundle in the Fragment from onSaveInstanceState() to onCreateView() on configuration change. Calling setRetainInstance(true) will set the Bundle to null on configuration change.

(I'm not sure a developer would actually attempt this since using setRetainInstance(true) makes onSaveInstanceState() kind of redundant, but I didn't see the behaviour documented in the API docs so I wrote up this answer).

If both addToBackStack() and setRetainInstance(true) are called, setRetainInstance() partly alters the Fragment lifecycle method calls and parameter values on configuration changes, compared to calling only addToBackStack().

Specifically, in the test below, looking a differences between calling only addToBackStack() and calling setRetainInstance(true) as well, and seeing what happens on configuration change:

Calling addToBackStack() but not setRetainInstance(true);

  • onCreate() and onDestroy() are called.
  • a bundle passed from onSaveInstanceState() is received as a parameter in onCreateView().

Calling both addToBackStack() and setRetainInstance(true):

  • onCreate() and onDestroy() are not called. This is metioned in the API docs.
  • a bundle passed from onSaveInstanceState() is not received in onCreateView(). The passed-in Bundle is null.

A test with logged method calls and parameters tested for null:

In the Activity:

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    MyFragment fragment;
    if (savedInstanceState != null) {
        fragment = (MyFragment) getFragmentManager().findFragmentByTag("my_fragment_tag");
    } else {
        fragment = new MyFragment();
        FragmentTransaction t = getFragmentManager().beginTransaction();
        t.addToBackStack(null);//toggle this
        t.add(android.R.id.content, fragment, "my_fragment_tag").commit(); 
    }
}

In the Fragment:

@Override
public void onActivityCreated(Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);
    setRetainInstance(true);//toggle this
}

and

@Override
public void onSaveInstanceState(Bundle outState) {
    outState.putString("test", "value");
    super.onSaveInstanceState(outState);
}

Test 1: Fragment lifecycle when addToBackStack() is called , and setRetainInstance(true) is not called

  • onAttach()
  • onCreate()
  • onCreateView()
  • onActivityCreated()
  • onStart()
  • onResume()

[Device rotated from portrait to landscape]

  • onPause()
  • onSaveInstanceState()
  • onStop()
  • onDestroyView()
  • onDestroy()
  • onDetach()
  • onAttach()
  • onCreate()
  • onCreateView() with bundle param != null
  • onStart()
  • onResume()

Test 2 & 3: Fragment lifecycle calls with setRetainInstance(true) called, addToBackStack() called / not called (same result):

  • onAttach()
  • onCreateView()
  • onActivityCreated()
  • onStart()
  • onResume()

[Device rotated from portrait to landscape]

  • onPause()
  • onSaveInstanceState()
  • onStop()
  • onDestroyView()
  • onDetach()
  • onAttach()
  • onCreateView() with bundle param == null
  • onStart()
  • onResume()
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