NullPointerException when listening to fragments

Question

I have activity with ViewPager, in the ViewPager adapter I provide a fragment for each position.

An example fragment is DebugFragment. I wrote the source code below.

public class DebugFragment extends android.support.v4.app.Fragment {

private OnFragmentInteractionListener mListener;

public interface OnFragmentInteractionListener {
    void onFragmentInteraction(int someValue);
}

public static DebugFragment newInstance() {
    DebugFragment fragment = new DebugFragment();
    Bundle args = new Bundle();
    fragment.setArguments(args);
    return fragment;
}

private BroadcastReceiver mMessageReceiver = new BroadcastReceiver() {
    @Override
    public void onReceive(Context context, Intent intent) {
        mListener.onFragmentInteraction(0);
    }
};

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    LocalBroadcastManager.getInstance(getContext()).registerReceiver(mMessageReceiver,
            new IntentFilter("com.android.example.INITIAL_REQUEST"));
}

@Override
public void onDestroy() {
    LocalBroadcastManager.getInstance(getContext()).unregisterReceiver(mMessageReceiver);
    super.onDestroy();
}

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

@Override
public void onDetach() {
    super.onDetach();
    mListener = null;
}

@Override
public void onResume() {
    super.onResume();
    getUserData();
}

public void getUserData() {
 // Inside Background Thread
    if (getActivity() == null) {
        return;
    }
    getActivity().runOnUiThread(new Runnable() {
        @Override
        public void run() {
            mListener.onFragmentInteraction(0); // This line throws NPE
        }
    });
}

Below is my work.

public class DebugActivity extends AppCompatActivity implements
    DebugFragment.OnFragmentInteractionListener {

    // Other Activity Callback

    @Override
    public void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        if (requestCode == REQUEST_CODE) {
            if (resultCode == Activity.RESULT_OK) {
                DebugFragment debugFragment = ((DebugFragment) mViewPagerAdapter.getRegisteredFragment(2));
                if (debugFragment != null) {
                    debugFragment.getUserData();
                }
            }
        }
    }
}

I call the getUserData of my DebugFragment from onResume of Fragment, BroadcastReceiver, OnActivityResult of Activity.

Sometime I get a NullPointerException in getUserData trying to access the FragmentListener ie mListener. I want to know why?

As I already check for Activity null. Isn't that enough. Should I also check for a zero value of mListener? It would be great if someone explains to me the case when the activity is not zero, but my mListener is zero. I kept my activity only in portrait mode.

Edit

public abstract class TabPagerAdapter extends FragmentPagerAdapter {

    public TabPagerAdapter(FragmentManager fm) {
        super(fm);
    }

    public abstract View getTabView(int position);
}

public class SecondaryPagerAdapter extends TabPagerAdapter {

    private static final int NUM_PAGES = 5;

    private String tabTitles[] = new String[] { "Today", "New", "Calendar", "In-progress", "Invoices" };
    private int[] imageResId = { R.drawable.ic_tab_hired_pro, R.drawable.ic_tab_history,
            R.drawable.ic_tab_today, R.drawable.ic_tab_inprogress, R.drawable.ic_tab_invoices };
    SparseArray<Fragment> registeredFragments = new SparseArray<Fragment>();

    public SecondaryPagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int position) {
        switch (position) {
            case 0:
            case 1:
            case 3:
                return ServiceRequestFragment.newInstance(tabTitles[position]);
            case 2:
                return DebugFragment.newInstance();
            case 4:
                return InvoicesFragment.newInstance();
            default:
                throw new RuntimeException("No fragment for this position");
        }
    }

    @Override
    public int getCount() {
        return NUM_PAGES;
    }

    @Override
    public View getTabView(int position) {
        CustomTab customTab = new CustomTab(DashBoardActivity.this);
        customTab.bindWith(imageResId[position], tabTitles[position]);
        return customTab;
    }

    @Override
    public Object instantiateItem(ViewGroup container, int position) {
        Fragment fragment = (Fragment) super.instantiateItem(container, position);
        registeredFragments.put(position, fragment);
        return fragment;
    }

    @Override
    public void destroyItem(ViewGroup container, int position, Object object) {
        registeredFragments.remove(position);
        super.destroyItem(container, position, object);
    }

    public Fragment getRegisteredFragment(int position) {
        return registeredFragments.get(position);
    }
}

mSecondaryPagerAdapter = new SecondaryPagerAdapter(getSupportFragmentManager());
mSecondaryPager = (ViewPager) findViewById(R.id.dashboard_pager);
mSecondaryPager.setOffscreenPageLimit(4);
mSecondaryPager.setAdapter(mSecondaryPagerAdapter);

Answer 1

getActivity().runOnUiThread(new Runnable() {}), , Runnable , , onDestroy(), mListener null. , . , mListener, , Runnable , , .

NPE, mListener == null Runnable. , , - . Handler Runnable runOnUiThread(). onDestroy() mHandler.removeCallbacksAndMessages(null), , Runnable .

Answer 2

mMessageReceiver = new BroadcastReceiver() {
    @Override
    public void onReceive(Context context, Intent intent) {
      if(mListener != null){
        mListener.onFragmentInteraction(0);
      }
    }
};

, ,

Answer 3

@Jschools . Eventbus . Eventbus , , , NPE .

Answer 4

onAttach , . , , , , onAttach ( ) DebugFragment , mListener?

, onAttach ( ) . . https://code.google.com/p/android/issues/detail?id=183358

mListener newInstance().

public static DebugFragment newInstance(Context context) {
    DebugFragment fragment = new DebugFragment();
    fragment.setListener(context)
    Bundle args = new Bundle();
    fragment.setArguments(args);
    return fragment;
}

private void setListener(Context context){
if (context.instanceof(OnFragmentInteractionListener) {
        mListener = (OnFragmentInteractionListener) context;
     } 
     else {
        throw new RuntimeException(context.toString()
                + " must implement OnFragmentInteractionListener");
     }
}