Can't get activity to communicate with fragment through interface

Question

I am trying to get my activity to make a call to my fragment when my viewpager detects any swiping.

Here is my activity code.

public interface SwipeListener { void swipe(); } private SwipeListener mSwipeListener; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_catalog); ButterKnife.bind(this); setSupportActionBar(mToolbar); int startingFragmentPosition = 0; mSwipeListener = (SwipeListener) this; ClothingSectionsPagerAdapter sectionsPagerAdapter = new ClothingSectionsPagerAdapter(getSupportFragmentManager()); sectionsPagerAdapter.setContext(this); mViewPager.setAdapter(sectionsPagerAdapter); mTabLayout.setupWithViewPager(mViewPager); mViewPager.addOnPageChangeListener(new TabLayout.TabLayoutOnPageChangeListener(mTabLayout)); mTabLayout.addOnTabSelectedListener(new TabLayout.ViewPagerOnTabSelectedListener(mViewPager)); mViewPager.setCurrentItem(startingFragmentPosition); mViewPager.setOnDragListener(new View.OnDragListener() { @Override public boolean onDrag(View view, DragEvent dragEvent) { mSwipeListener.swipe(); return false; } }); }

In my fragment I implement this interface and override the method. However when I run the app. It crashes, saying that it cannot cast this to SwipeListener in the onCreate method. How do I fix this?

Can you describe what you are trying to achieve, Do you want to send swipe action to fragment? — Sasikanth
– Sasikanth, Commented Dec 29, 2017 at 8:48
I want the fragment to start loading data early. Because currently when there is a swipe event, the fragment doesn't load the data automatically. — Tom Finet
– Tom Finet, Commented Dec 29, 2017 at 8:52
You can call the load methods from activity and then directly pass it to fragment by getting registered fragment: stackoverflow.com/questions/8785221/…. The listener implementation for this is wrong AFAIK. — Sasikanth
– Sasikanth, Commented Dec 29, 2017 at 8:55
Your implementation is wrong. Here you are just type casting fragment context into SwipeListener. You need to implement interface in activity and set the interface from there. — Sunil Sunny
– Sunil Sunny, Commented Dec 29, 2017 at 8:59

Javier Ruiz Rodriguez · Accepted Answer · 2017-12-29 08:57:35Z

This is because you are trying to put your activity as SwipeListener. Your fragment should implement this interface, and then put this fragment as the SwipeListener.

I think is better to pass to your ClothingSectionsPageAdapter the list of fragments (creating them in your activity) so you can have a reference, or maybe create your SwipeListener in the same method you create your list of fragments.

SpiritCrusher · Accepted Answer · 2017-12-29 09:11:28Z

This is not how it works . You have to register callback to publisher component (In your case Activity). If you want to pass swipe action to fragment form Activity . You should implement SwipeListener in Fragment and set it to Activity . Below is an Example.

public class MainActivity extends AppCompatActivity { private List<SwipeListener> listeners = new ArrayList<>(); public void addSwipeListener(SwipeListener listener) { listeners.add(listener); } @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_new); mViewPager.setOnDragListener(new View.OnDragListener() { @Override public boolean onDrag(View view, DragEvent dragEvent) { for (SwipeListener listener : listeners) { listener.swipe(); } return false; } }); }

}

And the example fragment.

class FragmentA extends Fragment implements SwipeListener{ @Override public void onActivityCreated(@Nullable Bundle savedInstanceState) { ((MainActivity)getActivity()).addSwipeListener(this); super.onActivityCreated(savedInstanceState); } @Override public void swipe() { // here you will get callback } }

I have made a list of callback so you can manage multiple fragments, modify it as per your need . And do not forget to remove listener on fragmnent's Detach.

Couldn't thegetActivity() call potentially be null in onCreateView(). I think it should be in onActivityCreated().

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