来自viewpager getActivity的Android childfragments返回null

Question

I have an activity and with a button I am switching between the two fragments (MAIN & SETTINGS). In the MAIN fragment I have a ViewPager with 4 child fragments.

At first run everything works fine, but if I rotate the screen, the getActivity() for fragments within the ViewPager is returning null.

ActivityMain :

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    // Add or show the fragments
    showHideScreenFragment(FRAGMENT_MAIN);
}

private void showHideScreenFragment(String tag) {   

    FragmentManager fm = getSupportFragmentManager();
    FragmentTransaction ft = fm.beginTransaction();

    ft.setCustomAnimations(android.R.anim.fade_in, android.R.anim.fade_out);

    // Get the fragment from the backstack if it is existing
    BaseFragment oldFragment = getFragmentFromBackstack(tag);

    // Get the current fragment from the layout
    BaseFragment currentFragment = getCurrentFragment();

    if (oldFragment == null) {
        if (currentFragment != null) {
             ft.hide(currentFragment);
        }

        ft.add(getMainContainerId(), getFragmentInstance(tag), tag);
    }
    else {

        if (currentFragment != null) {
            if (isSameFragment(oldFragment, currentFragment))
                return;

            ft.hide(currentFragment);
        }

        if (oldFragment.isHidden())
            ft.show(oldFragment);
    }

    ft.commit();
    fm.executePendingTransactions();
}

private BaseFragment getFragmentInstance(String tag) {
    if (tag.equals(FRAGMENT_MAIN)) return getFragmentMain();
    if (tag.equals(FRAGMENT_SETTINGS)) return getFragmentSettings();

    throw new RuntimeException("Fragment not found !");
}

private FragmentMain getFragmentMain() {
    return new FragmentMain();
}

private FragmentSettings getFragmentSettings() {
    return new FragmentSettings();
}

private BaseFragment getFragmentFromBackstack(String tag) {
    if (tag.equals(FRAGMENT_MAIN)) return getFragmentMainFromBackstack();
    if (tag.equals(FRAGMENT_SETTINGS)) return getFragmentSettingsFromBackstack();

    throw new RuntimeException("Fragment not found !");
}

private FragmentMain getFragmentMainFromBackstack() {
    return (FragmentMain) getSupportFragmentManager().findFragmentByTag(FRAGMENT_MAIN);
}

private FragmentSettings getFragmentSettingsFromBackstack() {
    return (FragmentSettings) getSupportFragmentManager().findFragmentByTag(FRAGMENT_SETTINGS);
}

private boolean isSameFragment(Fragment f1, Fragment f2) {
    return f1.getTag().equals(f2.getTag());
}

FragmentMain :

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {         
        View view = inflater.inflate(R.layout.fragment_main, container, false);

        viewPager = (ViewPager) view.findViewById(R.id.viewPager);

        // Add the 4 child fragments to the viewpager
        populateViewPager();

        // Debugging
        new Handler().postDelayed(new Runnable() {
            @Override
            public void run() {
                _printFragmentStates();
            }
        }, 2500);

        return view;
    }

    private void populateViewPager() {      
        ArrayList<BaseMainFragment> fragments = new ArrayList<BaseMainFragment>();

        fragments.add(new FragmentSearch());
        fragments.add(new FragmentFavorites());
        fragments.add(new FragmentHouse());
        fragments.add(new FragmentRoom());

        adapterMain = new AdapterMain(getChildFragmentManager(), fragments);

        viewPager.setOffscreenPageLimit(4);
        viewPager.setAdapter(adapterMain);
    }

    // DEBUGGING
    private void _printFragmentStates() {
        Activity actSearch = null;
        Activity actFav = null;
        Activity actHouse = null;
        Activity actRoom = null;

        actSearch = getFragmentSearch().getActivity();
        actFav = getFragmentFavorites().getActivity();
        actHouse = getFragmentHouse().getActivity();
        actRoom = getFragmentRoom().getActivity();

        Functions.logd("~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~");
        Functions.logd("~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~");
        Functions.logd("Main fragment act, is null: " + (getActivity() == null));
        Functions.logd("Search act, is null: " + (actSearch == null));
        Functions.logd("Favorite act, is null: " + (actFav == null));
        Functions.logd("House act, is null: " + (actHouse == null));
        Functions.logd("Room act, is null: " + (actRoom == null));
        Functions.logd("~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~");
        Functions.logd("~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~");
}

private FragmentSearch getFragmentSearch() {
    return (FragmentSearch) adapterMain.getItem(0);
}

private FragmentFavorite getFragmentFavorite() {
    return (FragmentFavorite) adapterMain.getItem(1);
}

private FragmentHouse getFragmentHouse() {
    return (FragmentHouse) adapterMain.getItem(2);
}

private FragmentRoom getFragmentHouse() {
    return (FragmentRoom) adapterMain.getItem(3);
}

As I said, at first run everything works fine, but after I rotate the screen, I am getting null for getActivity(); in the 4 child fragments: Search, Favorite, House and Room.

Logcat debug

1 run:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Main fragment act, is null: false
Search act, is null: false
Favorite act, is null: false
House act, is null: false
Room act, is null: false
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

After screen orientation changed:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Main fragment act, is null: false
Search act, is null: true
Favorite act, is null: true
House act, is null: true
Room act, is null: true
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

What am I doing wrong?

Answer 1

After hours of debugging, I figured out that if you're having only 1 fragment (without child or nested fragments) attached to your activity, then you don't need to re-add your fragment.

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    // Add or show the fragments if the savedInstance is null, otherwise let the system reattach your fragment.
    if (savedIstance == null)
       showHideScreenFragment(FRAGMENT_MAIN);
}

You don't need to reattach the fragment, the android system will do this for you.

And the solution for getting NPE at getActivity(); in child fragments is:

Use FragmentStatePagerAdapter for your ViewPager 's adapter.

and override the saved state method:

@Override
public Parcelable saveState() {
    return null;
}

I don't know why, but setRetainInstance(false); does not helped me, and I think this will remain a mystery for me.

Answer 2

It's a good practice to follow a pattern like this when working with fragments :

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

@Override
public void onViewCreated(View view, Bundle savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);
    view = view.findViewById(R.id.view);
}

@Override
public void onActivityCreated(@Nullable Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);
// do your thing here
populateViewPager();
}

1) Use OnCreateView to set your fragment's view, 2) OnViewCreated to initialize view and 3) OnActivityCreated to setup things.

Using getActivity() inside OnActivityCreated ensures that getActivity() does not return null. This method gets called only after the activity is fully initialized. OnAttach , OnCreate , OnCreateView may get even before the activity is created.