Question

in my app I’m using one activity and two fragments. The app uses a layout with a container so the fragments are added via transactions. The first fragment contains a listview and the other fragment a detail view for the listview items.
Both fragments use setRetainInstance(true). The fragments are added via a replace transaction and addToBackStack(null) is set. The listfragment contains an instance variable which holds some infos for the list. Now I’m changing to detail and press back and the instance variable is null. I read about setRetainInstance and addToBackStack and removed addToBackStack, but even then the instance variable is null.

Does anyone know what I might be doing wrong?

regards,
Thomas

Answer 1

setRetainInstance(true) will tell the FragmentManager to keep the fragment around when the containing Activity is killed and rebuilt for some reason. It doesn’t guarantee that the Fragment instance will stick around after a transaction to add or replace. It sounds like your adapter is being garbage collected and you’re not creating a new one.

A more generally easy solution would be to make a viewless Fragment to retain your ListAdapter. The way you do this is to create the Fragment, set the retain instance to true, and return null in the method onCreateView(). To add it, just called addFragment(Fragment, String) via the FragmentTransaction. You never remove or replace it, so it will always stay in memory for the length of the app. Screen rotations won’t kill it.

Whenever your ListFragment is created, in onCreateView() get the FragmentManager and use either the method findFragmentById() or FindFragmentByTag() to retrieve your retained fragment from memory. Then get the adapter from that fragment and set it as your adapter for the list.

public class ViewlessFragment extends Fragment {

   public final static string TAG = "ViewlessFragment";

   private ListAdapter mAdapter;

   @Override
   public ViewlessFragment() {
      mAdapter = createAdater();
      setRetainInstance(true);
   }

   @Override
   public void onCreateView (LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
      return null;
   }

   public ListAdapter getAdapter() {
      return mAdapter;
   }
}

public class MyListFragment extends ListFragment {

   final public static String TAG = "MyListFragment";

   @Override
   public void onCreateView (LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
      final View returnView = getMyView();
      final ViewlessFragment adapterFragment = (ViewlessFragment) getFragmentManager().findFragmentByTag(ViewlessFragment.TAG);
      setListAdapter(ViewlessFragment.getAdapter());
      return returnView;
   }
}

public class MainActivity extends FragmentActivity {

   @Override
   public void onCreate(Bundle icicle) {
      // ... setup code...
      final FragmentManager fm = getSupportFragmentManager();
      final FragmentTransaction ft = fm.beginTransaction();
      ViewlessFragment adapterFragment = fm.findFragmentByTag(ViewlessFragment.TAG);
      if(adapterFragment == null) {
         ft.add(new ViewlessFragment(), ViewlessFragment.TAG);
      }
      ft.add(R.id.fragmentContainer, new MyListFragment(), MyListFragment.TAG);
      ft.commit();
   }
}