ViewPager里面的片段，如何保留状态？

Question

In my application the fragment activity holds two fragments, Fragment A and Fragment B. Fragment B is a view pager that contains 3 fragments.

In my activity, to prevent that the fragment is recreated on config changes:

if(getSupportFragmentManager().findFragmentByTag(MAIN_TAB_FRAGMENT) == null) {
    getSupportFragmentManager().beginTransaction().replace(R.id.container, new MainTabFragment(), MAIN_TAB_FRAGMENT).commit();
}

Code for Fragment B:

public class MainTabFragment extends Fragment {

    private PagerSlidingTabStrip mSlidingTabLayout;
    private LfPagerAdapter adapter;
    private ViewPager mViewPager;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setRetainInstance(true);
    }

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

    @Override
    public void onViewCreated(View view, Bundle savedInstanceState) {

        this.adapter = new LfPagerAdapter(getChildFragmentManager());

        this.mViewPager = (ViewPager) view.findViewById(R.id.viewpager);
        this.mViewPager.setAdapter(adapter);

        this.mSlidingTabLayout = (PagerSlidingTabStrip) view.findViewById(R.id.sliding_tabs);
        this.mSlidingTabLayout.setViewPager(this.mViewPager);
    }
}

Code for the adapter:

public class LfPagerAdapter extends FragmentPagerAdapter {

    private static final int NUM_ITEMS = 3;

    private FragmentManager fragmentManager;

    public LfPagerAdapter(FragmentManager fm) {
        super(fm);
        this.fragmentManager = fm;
    }

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

    @Override
    public Fragment getItem(int position) {
        Log.d("TEST","TEST");
        switch (position) {
            case 1:
                return FragmentC.newInstance();
            case 2:
                return FragmentD.newInstance();
            default:
                return FragmentE.newInstance();
        }
    }
}

My problem is that I am not able to retain the state of the view pager an its child fragments on orientation changes.

Obviously this is called on every rotation:

this.adapter = new LfPagerAdapter(getChildFragmentManager());

which will cause the whole pager to be recreated, right? As a result

getItem(int position)

will be called on every rotation and the fragment will be created from scratch and losing his state:

return FragmentC.newInstance();

I tried solving this with:

if(this.adapter == null)
    this.adapter = new LfPagerAdapter(getChildFragmentManager());

in onViewCreated but the result was that on rotation the fragments inside the pager where removed.

Any ideas how to correctly retain the state inside the pager?

Answer 1

You will need to do 2 things to resolve the issue:

1) You should use onCreate method instead of onViewCreated to instantiate LfPagerAdapter ;

ie:

    public class MainTabFragment extends Fragment {

    private PagerSlidingTabStrip mSlidingTabLayout;
    private LfPagerAdapter adapter;
    private ViewPager mViewPager;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setRetainInstance(true);
        this.adapter = new LfPagerAdapter(getChildFragmentManager());
    }

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

    @Override
    public void onViewCreated(View view, Bundle savedInstanceState) {


        this.mViewPager = (ViewPager) view.findViewById(R.id.viewpager);
        this.mViewPager.setAdapter(adapter);

        this.mSlidingTabLayout = (PagerSlidingTabStrip) view.findViewById(R.id.sliding_tabs);
        this.mSlidingTabLayout.setViewPager(this.mViewPager);
    }
}

2) You will need to extend FragmentStatePagerAdapter instead of FragmentPagerAdapter

Answer 2

Android will automatically recreate your activity on configuration without this line android:configChanges="keyboardHidden|orientation|screenSize" in you manifest so that you can handle onConfiguration change yourself.

The only way then is to use onSaveInstanceState() in both your activityFragement to save viewPager state(current position for example) and in fragments where you need to save stuff

Example on how you can save current position of viewpager and restore it onConfiguration change

    @Override
    public void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
            int position = mViewPager.getCurrentItem();
            outState.Int("Key", position );
    }

    @Override //then restore in on onCreate();
    public void onCreated(Bundle savedInstanceState) {
        super.onCreated(savedInstanceState);
        // do stuff
        if (savedInstanceState != null) {
              int  position= savedInstanceState.getInt("Key");
            mViewPager.setCurrentItem(position)
        }
    }

Of course, this is a very basic example.

Ps: to restore in fragment use onActivityCreated() instead of onCreate() method.

Here is another example on how to retain state : Click me!