帶片段的導航抽屜（帶 ViewPager）重新打開時消失

Question

我試圖為我的應用程序創建一個導航，使用導航抽屜，我的片段之一（HomeFragment）有一個包含 3 個片段（邦迪時鍾、公告、日歷）的 ViewPager

當我在開始時運行我的應用程序時它工作正常，我可以在我的 HomeFragment 的 3 個片段（Bundy 時鍾、公告、日歷）之間切換。 但是當我在導航抽屜中選擇項目（主頁）時，我的 HomeFragment 中的 3 個 Fragment 消失了，我的 logcat 中沒有錯誤，並且應用程序不會停止它，只是我的 ViewPager 中的 3 個 Fragment 消失了。

主活動.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

 viewPager.setAdapter(new ViewPagerAdapter(getActivity().getSupportFragmentManager()));

用

viewPager.setAdapter(new ViewPagerAdapter(getChildFragmentManager()));

我遇到了這個問題，我使用上面的解決方案解決了它

Answer 2

而不是使用 FragmentPagerAdapater 我切換到 FragmentStatePagerAdapter

代替

public class ViewPagerAdapter extends FragmentPagerAdapter { }

我用

public class ViewPagerAdapter extends FragmentStatePagerAdapter { }