android - Retained Fragments with UI and memory leaks

Question

I've read that setting .setOnRetainInstance(true) on fragments presenting UI may lead to memory leaks.

Could somebody please explain why and how this would happen? I didn't find a detailed explanation anywhere.

Answer 1

In a Fragment with UI you often save some Views as instance state to speed up access. For example a link to your EditText so you don't have to findViewById it all the time.

The problem is that a View keeps a reference to the Activity context. Now if you retain a View you also retain a reference to that context.

That is no problem if the context is still valid but the typical retain case is restarting the Activity. Very often for a screen rotation for example. Activity recreation will create a new context and old contexts are intended to be garbage collected. But it can't be garbage collected now since your Fragment still has a reference to the old one.

Following example shows how not to do it

public class LeakyFragment extends Fragment {

    private View mLeak; // retained

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

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        mLeak = inflater.inflate(R.layout.whatever, container, false);
        return mLeak;
    }

    @Override
    public void onDestroyView() {
        super.onDestroyView();
        // not cleaning up.
    }
}

To get rid of that problem, you need to clear all references to your UI in onDestroyView. Once the Fragment instance is re-used you will be asked to create a new UI on onCreateView. There is also no point in keeping the UI after onDestroyView. The Ui is not going to be used.

The fix in this example is just changing onDestroyView to

@Override
public void onDestroyView() {
    super.onDestroyView();
    mLeak = null; // now cleaning up!
}

And besides keeping references to Views you should obviously not keep references to the Activity (e.g. from onAttach - clean on onDetach) or any Context (unless it's the Application context).