onResume() not called on ViewPager fragment when using custom Loader

问题

Short version:

I have a fragment that maintains a ViewPager for displaying two other fragments, let's call them FragmentOne and FragmentTwo. When starting the app FragmentOne is visible and FragmentTwo is off-screen, becoming visible only as one swipes the view to the left.

Normally onStart() and onResume() get invoked immediately for both fragments as soon as the app gets started.

The problem I have is when FragmentOne starts a custom Loader then onResume() does not get called on FragmentTwo until it becomes fully visible.

Questions:

Is this a problem with my code or a bug in the Android Support Library? (The problem did not occur with revision 12 of the library, it started with revision 13.)

If it's a bug in revisons 13 and 18, is there a workaround?

Is there something wrong with my custom Loader?

Long version:

I have built a sample application that demonstrates the problem. I have tried to reduce the code to the bare minimum but it's still a lot so please bear with me.

I have a MainActivity that loads a MainFragment which creates a ViewPager. It is important for my app that the ViewPager is maintained by a Fragment instead of an Activity.

MainFragment creates a FragmentPagerAdapter that in turn creates the fragments FragmentOne and FragmentTwo.

Let's start with the interesting bit, the two fragments:

FragmentOne is a ListFragment that uses a custom Loader to load the content:

public class FragmentOne extends ListFragment implements LoaderCallbacks<List<String>> {
    private ArrayAdapter<String> adapter;

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);

        adapter = new ArrayAdapter<String>(getActivity(), android.R.layout.simple_list_item_1);
        setListAdapter(adapter);

        setEmptyText("Empty");
    }

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

        // initializing the loader seems to cause the problem!
        getLoaderManager().initLoader(0, null, this);
    }

    @Override
    public Loader<List<String>> onCreateLoader(int id, Bundle args) {
        return new MyLoader(getActivity());
    }

    @Override
    public void onLoadFinished(Loader<List<String>> loader, List<String> data) {
        adapter.clear();
        adapter.addAll(data);
    }

    @Override
    public void onLoaderReset(Loader<List<String>> loader) {
        adapter.clear();
    }

    public static class MyLoader extends AsyncTaskLoader<List<String>> {
        public MyLoader(Context context) {
            super(context);
        }

        @Override
        protected void onStartLoading() {
            forceLoad();
        }

        @Override
        public List<String> loadInBackground() {
            return Arrays.asList("- - - - - - - - - - - - - - - - - - - foo",
                    "- - - - - - - - - - - - - - - - - - - bar",
                    "- - - - - - - - - - - - - - - - - - - baz");
        }
    }
}

It is that Loader that seems to cause the problem. Commenting out the initLoader line makes the fragment life-cycle work as expected again.

FragmentTwo changes its content based on whether onResume() has been invoked or not:

public class FragmentTwo extends Fragment {
    private TextView text;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        text = new TextView(container.getContext());
        text.setText("onCreateView() called");
        return text;
    }

    @Override
    public void onResume() {
        super.onResume();
        Log.i("Fragment2", "onResume() called");
        text.setText("onResume() called");
    }
}

And here is the boring rest of the code.

MainActivity:

public class MainActivity extends FragmentActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        Fragment fragment = new MainFragment();
        getSupportFragmentManager().beginTransaction().add(R.id.container, fragment).commit();
    }
}

Layout activity_main:

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

MainFragment:

public class MainFragment extends Fragment {
    private ViewPager viewPager;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View layout = inflater.inflate(R.layout.frag_master, container, false);
        viewPager = (ViewPager) layout.findViewById(R.id.view_pager);
        return layout;
    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);

        viewPager.setAdapter(new MyPagerAdapter(getChildFragmentManager()));
    }

    private static final class MyPagerAdapter extends FragmentPagerAdapter {
        public MyPagerAdapter(FragmentManager fragmentManager) {
            super(fragmentManager);
        }

        @Override
        public int getCount() {
            return 2;
        }

        @Override
        public Fragment getItem(int position) {
            if (position == 0)
                return new FragmentOne();
            else
                return new FragmentTwo();
        }
    }
}

Layout frag_master:

<android.support.v4.view.ViewPager xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/view_pager"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

回答1:

It appears to be a bug in support library. The change below solves the issue.

// FragmentOne.java

@Override
public void onResume() {
    super.onResume();
    Handler handler = getActivity().getWindow().getDecorView().getHandler();
    handler.post(new Runnable() {
        @Override public void run() {
            // initialize the loader here!
            getLoaderManager().initLoader(0, null, FragmentOne.this);
        }
    });
}

回答2:

Another workaround that worked for me was to use the MainFragment's loader manager:

getParentFragment().getLoaderManager().initLoader(0, null, this);

来源：https://stackoverflow.com/questions/18301979/onresume-not-called-on-viewpager-fragment-when-using-custom-loader