Android Nested Fragment and Interface

Question

I have a Fragment A which has an interface that will return the value of a TextView. The Fragment A is initialized and Attached to FragmentB. Here is the code for Fragment A and B.

Fragment A

public class FragmentA extends Fragment {
    ...

    public interface Listener {
       void onValue(int value);
    }

    @Override
    public void onAttach(Context context) {
        super.onAttach(context);
        if (context instanceof Listener) {
            listener = (Listener) context;
        } else {
            throw new RuntimeException(context.toString() + " must implement Listener");
        }
    }
}

Fragment B

public class FragmentB extends Fragment implements FragmentA.Listener {    
    ...

    private void initFragmentA() {
        FragmentManager fragmentManager = getChildFragmentManager();
        fragmentA = FragmentA.newInstance();
        fragmentManager.beginTransaction().add(container, fragmentA, TAG).commit();
    }

    @Override
    public void onValue(int value) {

    }
}

When I start the app, a error occurred:

Java.lang.RuntimeException: ####.page.MainActivity@1f7f316c must implement Listener 
    at ####.widget.FragmentA.onAttach(FragmentA.java:66)

The MainActivity contains the Fragment B, but the Fragment B has already implemented the interface of Fragment A. Why the error occurred? The interface implemented in parent Fragment doesn't work?

Answer 1

From Android Developer website, it says:

Often you will want one Fragment to communicate with another, for example to change the content based on a user event. All Fragment-to-Fragment communication is done through the associated Activity. Two Fragments should never communicate directly.

To allow a Fragment to communicate up to its Activity, you can define an interface in the Fragment class and implement it within the Activity. The Fragment captures the interface implementation during its onAttach() lifecycle method and can then call the Interface methods in order to communicate with the Activity.

Therefore, when you implement that interface in Fragment B, it throws a RuntimeException error.

Summary: you have to implement that interface in your fragment hosting activity.

You can learn more on how to pass data between fragments thru a hosting activity (usually we make use of Bundle) at http://www.vogella.com/tutorials/AndroidFragments/article.html