我在MainActivity中创建了一个BottomNavigationView（以及最新的JetPack导航组件），我需要根据用户的身份验证状态在不同的Fragment中显示/隐藏它。
就在几天前，我还只是使用requireActivity（）.findViewById（R.id.nav_view）从Fragment中检索它（我意识到这不是最佳实践，但目前还可以接受）。
在升级到Android Studio Bumblebee沿着其他一些导航库后，会出现NullPointerException。
然后我尝试使用下面的界面，但仍然得到NPE。
我真的希望仍然创建底部酒吧 * 在一个地方只 *，并能够检索它从任何地方相应。
main_activity.xml

<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingTop="?attr/actionBarSize">

    <com.google.android.material.bottomnavigation.BottomNavigationView
        android:id="@+id/nav_view"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="0dp"
        android:layout_marginEnd="0dp"
        android:background="?android:attr/windowBackground"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:menu="@menu/bottom_nav_menu" />
    <fragment
        android:id="@+id/nav_host_fragment_activity_main"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:defaultNavHost="true"
        app:layout_constraintBottom_toTopOf="@id/nav_view"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:navGraph="@navigation/mobile_navigation" />  
</androidx.constraintlayout.widget.ConstraintLayout>

MainActivity.java

public class MainActivity extends AppCompatActivity implements HomeFragment.onGetBottomNavViewListener {

private ActivityMainBinding binding;
private BottomNavigationView bottomNavigationView;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    binding = ActivityMainBinding.inflate(getLayoutInflater());
    setContentView(binding.getRoot());

    bottomNavigationView = binding.navView;

    AppBarConfiguration appBarConfiguration = new AppBarConfiguration.Builder(
            R.id.navigation_home, R.id.navigation_dashboard, R.id.navigation_notifications)
            .build();
    NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment_activity_main);
    NavigationUI.setupActionBarWithNavController(this, navController, appBarConfiguration);
    NavigationUI.setupWithNavController(bottomNavigationView, navController);
}

@Override
    public void showBottomNavView() {
       bottomNavigationView.setVisibility(View.VISIBLE);
    }

    @Override
    public void hideBottomNavView() {
      bottomNavigationView.setVisibility(View.INVISIBLE);
    }

}

HomeFragment.xml

<?xml version="1.0" encoding="utf-8"?>
<ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/home_fragment"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

<androidx.constraintlayout.widget.ConstraintLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    tools:context=".ui.home.HomeFragment"
    android:paddingStart="@dimen/activity_padding"
    android:paddingTop="@dimen/activity_padding"
    android:paddingEnd="@dimen/activity_padding"
    android:paddingBottom="100dp">

    <LinearLayout>....</LinearLayout>

    <GridLayout>....</GridLayout>
       
    <androidx.appcompat.widget.AppCompatButton
        android:id="@+id/search_btn"
        style="@style/BaseButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="45dp"
        android:layout_marginBottom="5dp"
        android:text="Go!"
        android:visibility="invisible"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@id/gridLayout" />
</androidx.constraintlayout.widget.ConstraintLayout>

</ScrollView>

HomeFragment.java

public class HomeFragment extends Fragment implements View.OnClickListener {
  private onGetBottomNavViewListener listener;

  public View onCreateView(@NonNull LayoutInflater inflater,
                             ViewGroup container, Bundle savedInstanceState) {

        binding = FragmentHomeBinding.inflate(inflater, container, false);
        user = FirebaseAuth.getInstance().getCurrentUser();
        View root = binding.getRoot();

return root;
}

 @Override
    public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);

        ((AppCompatActivity) requireActivity()).getSupportActionBar().setSubtitle("");

        // If user is logged in
        if (user != null) {
             listener.showBottomNavView();
        } else {
             listener.hideBottomNavView();
        }
   }

    // INTERFACE
    public interface onGetBottomNavViewListener {
        void showBottomNavView();
        void hideBottomNavView();
    }

    @Override
    public void onAttach(@NonNull Context context) {
        super.onAttach(context);
        // Store the listener (activity) that will have events fired once the fragment is attached
        if (context instanceof onGetBottomNavViewListener) {
            listener = (onGetBottomNavViewListener) context;
        } else {
            throw new ClassCastException(context +
                    "must implement HomeFragment.onGetBottomNavViewListener");
        }
    }
}

UPDATE：仍然无法从片段中获取bottomBar，但我通过简单地在MainActivity中移动身份验证检查沿着bar管理来解决这个特殊情况，方法是使用

navController.addOnDestinationChangedListener(new NavController.OnDestinationChangedListener() {
            @Override
            public void onDestinationChanged(@NonNull NavController controller, @NonNull NavDestination destination, @Nullable Bundle arguments) {
                if (FirebaseAuth.getInstance().getCurrentUser() == null) {
                    navView.setVisibility(View.GONE);
                } else {
                    navView.setVisibility(View.VISIBLE);

                }
            }
        });