带有片段的导航抽屉（带有ViewPager），重新打开后就消失了

Question

我尝试为我的应用创建导航，使用导航抽屉，我的一个片段（HomeFragment）具有一个ViewPager，该ViewPager可以容纳3个片段（邦迪时钟，公告，日历）

当我在启动时运行我的应用程序时，它工作正常，我可以在HomeFragment的3个片段之间切换（邦迪时钟，公告，日历）。但是，当我在导航抽屉中选择项目（Home）时，HomeFragment中的3 Fragments不见了，我的logcat也没有错误，并且应用程序也没有停止它，只是ViewPager中的3 Fragments不见了。

MainActivity.java

public class MainActivity extends AppCompatActivity {

private AppBarConfiguration mAppBarConfiguration;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);
    FloatingActionButton fab = findViewById(R.id.fab);
    fab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                    .setAction("Action", null).show();
        }
    });
    DrawerLayout drawer = findViewById(R.id.drawer_layout);
    NavigationView navigationView = findViewById(R.id.nav_view);
    // Passing each menu ID as a set of Ids because each
    // menu should be considered as top level destinations.
    mAppBarConfiguration = new AppBarConfiguration.Builder(
            R.id.nav_home, R.id.nav_gallery, R.id.nav_slideshow,
            R.id.nav_tools, R.id.nav_share, R.id.nav_send)
            .setDrawerLayout(drawer)
            .build();
    NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment);
    NavigationUI.setupActionBarWithNavController(this, navController, mAppBarConfiguration);
    NavigationUI.setupWithNavController(navigationView, navController);
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

@Override
public boolean onSupportNavigateUp() {
    NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment);
    return NavigationUI.navigateUp(navController, mAppBarConfiguration)
            || super.onSupportNavigateUp();
}
}

HomeFragment.java

public class HomeFragment extends Fragment {

private HomeViewModel homeViewModel;

public View onCreateView(@NonNull LayoutInflater inflater,
                         ViewGroup container, Bundle savedInstanceState) {
    homeViewModel =
            ViewModelProviders.of(this).get(HomeViewModel.class);
    View root = inflater.inflate(R.layout.fragment_home, container, false);

    ViewPager viewPager = root.findViewById(R.id.view_pager);
    viewPager.setAdapter(new ViewPagerAdapter(getActivity().getSupportFragmentManager()));
    return root;
}
}

ViewPagerAdapter.java

public class ViewPagerAdapter extends FragmentPagerAdapter {

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

@Override
public Fragment getItem(int position) {
    switch (position)
    {
        case 0:
            return new BundyFragment(); //ChildFragment1 at position 0
        case 1:
            return new AnnouncementFragment(); //ChildFragment2 at position 1
        case 2:
            return new CalendarFragment(); //ChildFragment3 at position 2
        default:
    }
    return null; //does not happen
}

@Override
public int getCount() {
    return 3; //three fragments
}
}

fragment_home.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent">
<androidx.viewpager.widget.ViewPager
    android:id="@+id/view_pager"
    android:layout_width="match_parent"
    android:layout_height="match_parent"/>
</LinearLayout>

Answer 1

而不是使用FragmentPagerAdapater我切换到FragmentStatePagerAdapter

而不是

public class ViewPagerAdapter extends FragmentPagerAdapter { }

我使用

public class ViewPagerAdapter extends FragmentStatePagerAdapter { }