Why it is not possible to use ViewPager within a F

问题:

There are information that it is impossible to use ViewPager within a Fragment in many sources like "The Busy Coders Guide for Android Developers" by Mark Murphy, or posts like this on SO. I'm confused because I don't have such a problem and I successfully use ViewPager within my Fragment. The only distinction is that I instantiate a ViewPager not in onCreateView() method but in onActivityCreated(). And everything works perfectly fine.

So the question is - may be I just don't know something and this is not recommended for some reason to make UI instantiations in onActivityCreated()? But again - everything works just fine.

Here is the listing of the class and xml:

Class:

public class ViewPagerFragment extends Fragment {

    static final int NUM_ITEMS = 2;

    private ViewPagerAdapter mAdapter;
    private ViewPager mPager;

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

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

        mAdapter = new ViewPagerAdapter(getFragmentManager());

        mPager = (ViewPager) getView().findViewById(R.id.pager);
        mPager.setAdapter(mAdapter);
    }

    public static class ViewPagerAdapter extends FragmentPagerAdapter {
        public ViewPagerAdapter(FragmentManager fm) {
            super(fm);
        }

        @Override
        public Fragment getItem(int num) {
            if (num == 0) {
                return new ItemsListFragment();
            } else {
                return new FavsListFragment();
            }
        }

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

Layout:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:layout_width="fill_parent"
              android:layout_height="fill_parent"
              android:orientation="vertical">

    <android.support.v4.view.ViewPager
        android:id="@+id/pager"
        android:layout_width="fill_parent"
        android:layout_height="0dp"
        android:layout_weight="1"/>

</LinearLayout>

回答1:

UPDATE: Since this answer was originally written, you now can have nested fragments, which means it is possible to have a ViewPager use fragments for pages and be in a fragment itself. This sample project demonstrates the technique.

I now return you to your regularly-scheduled answer, presented in its entirety...

---

Quoting myself from the book:

The simplest way to use a ViewPager is to have it page fragments in and out of the screen based on user swipes. This only works if the ViewPager itself is not contained within a fragment, as you cannot have fragments nested inside of other fragments.

Quoting Dianne Hackborn:

Nested fragments are not currently supported. Trying to put a fragment within the UI of another fragment will result in undefined and likely broken behavior.

It is perfectly possible to put a ViewPager inside a Fragment, so long as the contents of the ViewPager do not themselves contain fragments. Since the concrete implementations of PagerAdapter supplied by the Android Support package use fragments, you have to roll your own fragment-less PagerAdapter to put the ViewPager in a fragment.

I will endeavor to make this point clearer in the next edition of the book (unless you're British, in which case I'll endeavour to make this point clearer :-).

回答2:

To implement the View pager within a fragment use getChildFragmentManager() instead of getFragmentManager(). You can call setAdapter() for the ViewPager from onCreateView() or onActivityCreated(), that is not a matter.

I hope this will help you guys.

回答3:

While Initializing the SetionPageAdapter for ViewPager use "getChildFragmentManager()".

mSectionsPagerAdapter = new SectionsPagerAdapter(getChildFragmentManager());

回答4:

it is possible try to do this code and save view_pager_fragment.xml

<android.support.design.widget.AppBarLayout
    android:id="@+id/appBarLayout"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar">

    <android.support.design.widget.TabLayout
        android:id="@+id/tab"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />

</android.support.design.widget.AppBarLayout>

<android.support.v4.view.ViewPager
    android:id="@+id/viewpager"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    app:layout_behavior="@string/appbar_scrolling_view_behavior" />

ViewPagerFragment

ViewPager viewPager;
TabLayout tabLayout;

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

    viewPager = (ViewPager) view.findViewById(R.id.viewpager);
    tabLayout = (TabLayout) view.findViewById(R.id.tab);

    return view;
}

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

    setupViewPager(viewPager);
    tabLayout.setupWithViewPager(viewPager);

    tabLayout.setOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
        @Override
        public void onTabSelected(TabLayout.Tab tab) {

        }

        @Override
        public void onTabUnselected(TabLayout.Tab tab) {

        }

        @Override
        public void onTabReselected(TabLayout.Tab tab) {

        }
    });
}

private void setupViewPager(ViewPager viewPager) {
    ViewPagerAdapter viewPagerAdapter = new ViewPagerAdapter(getChildFragmentManager());``

    viewPagerAdapter.addFragment(new FirstFragment(), "First");
    viewPagerAdapter.addFragment(new SecoundFragment(), "Secound");
    viewPagerAdapter.addFragment(new LoginFragment(), "Login");

    viewPager.setAdapter(viewPagerAdapter);
}

private class ViewPagerAdapter extends FragmentPagerAdapter {

    List<Fragment> fragmentList = new ArrayList<>();
    List<String> fragmentTitles = new ArrayList<>();

    public ViewPagerAdapter(FragmentManager fragmentManager) {
        super(fragmentManager);
    }

    @Override
    public Fragment getItem(int position) {
        return fragmentList.get(position);
    }

    @Override
    public int getCount() {
        return fragmentList.size();
    }

    @Override
    public CharSequence getPageTitle(int position) {
        return fragmentTitles.get(position);
    }

    public void addFragment(Fragment fragment, String name) {
        fragmentList.add(fragment);
        fragmentTitles.add(name);
    }
}