Android getParentFragment（）在Fragment中的ViewPager中返回null

Question

I have an Activity that contains a Fragment with a ViewPager . Then I call a method of a Fragment within the ViewPager. But if that Fragment then calls getParentFragment() , it returns null.

Why is getParentFragment() null?

---

The main Fragment that contains a ViewPager:

public class MyFragment extends Fragment {
    private TabLayout mTabLayout;
    private ViewPager mViewPager;
    private ViewPagerAdapter mAdapter;

    @Override
    public void onActivityCreated(@Nullable Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        View view = getView();

        // Setup the ViewPager
        mViewPager = (ViewPager) view.findViewById(R.id.container);

        ViewPagerAdapter adapter = new ViewPagerAdapter(getChildFragmentManager(), getActivity());
        mViewPager.setAdapter(adapter);
        //setupViewPager(mViewPager);

        // Setup the TabLayout
        mTabLayout = (TabLayout) view.findViewById(R.id.tabs);
        mTabLayout.setupWithViewPager(mViewPager);
    }

    // This method is called from Activity.
    public void callNestedFragment() {
        if (isDetached()) {
            return;
        }

        // This fragment cannot be null, because it doesn't crash here. (this is just a sample).
        ((Fragment0) mViewPager.getItem(0)).testMethod();
    }
}

The nested Fragment (inside mViewPager ):

public class Fragment0 extends Fragment {

    ...

    public void testMethod() {
        if (isDetached()) {
            return;
        }

        // Why is getParentFragment() null? This is the log: "Parent fragment: null"
        Log.i(TAG, "Parent fragment: " + getParentFragment());
        return;
    }
}

The ViewPagerAdapter :

public class ViewPagerAdapter extends FragmentPagerAdapter {
    private Context mContext;

    public ViewPagerAdapter(FragmentManager manager, Context context) {
        super(manager);
        mContext = context;
    }

    @Override
    public Fragment getItem(int position) {
        switch (position) {
            case 0:
                return new Fragment0();
            case 1:
                return new Fragment1();
            case 2:
                return new Fragment2();
            case 3:
                return new Fragment3();
            case 4:
                return new Fragment4();
        }

        return null;
    }

    @Override
    public int getCount() {
        return 5;
    }
}

If more code is needed, please let me know. Thanks for your help.

Answer 1

I found the answer. If isDetached() is true, it doesn't mean getParentFragment() is not null.

So instead of the previous check in testMethod() , this is the new code:

public void testMethod() {
    if (isDetached() && getParentFragment() != null) {
        return;
    }

    ...
}

I also now call the same method again in the onStart() method within Fragment0 , where getParentFragment() should not be null.

Answer 2

在我的情况下，我使用FragmentManager而不是ChildFragmentManager导致此问题。