ViewPager内部片段，如何保留状态？

Question

在我的应用程序中，fragment活动包含两个片段，片段A和片段B，fragment B是一个包含3个片段的视图分页器。

在我的活动中，为了防止在配置更改时重新创建片段：

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

片段B的代码：

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);
    }
}

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();
        }
    }
}

显然，在每次轮换时都要这样做：

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

这会导致整个寻呼机被重新创建，对吗？结果是

getItem(int position)

将在每次循环时被调用,碎片将从头创建并失去状态：

return FragmentC.newInstance();

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

在onviewcreated中，但其结果是，在循环时，页面中的片段被移除。

有什么想法如何正确地保留在寻呼机内的状态吗？

Answer 1

您需要做两件事来解决这个问题：

1）应该使用oncreate方法而不是onviewcreate实例化LFPagerAdapter；

即：

    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）您将需要扩展FragmentStatePagerAdapter而不是FragmentPagerAdapter