Android onCreate() 崩溃 - 片段无法转换为 androidx.navigation.fragment.NavHostFragment

Question

我决定为我的应用程序执行 Firebase Robo 测试并注意到它总是在打开外部 activity 然后返回到应用程序后崩溃。我能够通过在开发人员选项中启用“不保留活动”来重现该问题。

崩溃：java.lang.ClassCastException: com.example.problemtesting.Fragments.Fragment_2 cannot be cast to androidx.navigation.fragment.NavHostFragment

说明问题出在这行代码：NavHostFragment navHostFragment = (NavHostFragment) getSupportFragmentManager().findFragmentById(R.id.nav_host_fragment);

如何重现问题：

启用“不保留活动”
打开应用程序，然后打开应用程序抽屉
Select“片段 2”
按分享按钮
按消息按钮（或任何其他外部应用程序）
Return 到应用（崩溃）

如果禁用“不保留活动”，应用程序可以正常运行。

MainActivity

public class MainActivity extends AppCompatActivity implements NavigationView.OnNavigationItemSelectedListener {

    DrawerLayout drawer;
    NavigationView navigationView;
    FragmentManager fragmentManager;
    AppBarConfiguration mAppBarConfiguration;
    NavController navController;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        fragmentManager = getSupportFragmentManager();
        drawer = findViewById(R.id.drawer_layout);
        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_fragment_1, R.id.nav_fragment_2)
                .setOpenableLayout(drawer)
                .build();

        NavHostFragment navHostFragment = (NavHostFragment) getSupportFragmentManager().findFragmentById(R.id.nav_host_fragment);
        if (navHostFragment != null)
            navController = navHostFragment.getNavController();
        else navController = Navigation.findNavController(this, R.id.nav_host_fragment);

        NavigationUI.setupActionBarWithNavController(MainActivity.this, navController, mAppBarConfiguration);
        NavigationUI.setupWithNavController(navigationView, navController);

        setNavigationViewListener();
    }

    @Override
    public boolean onSupportNavigateUp() {
        return NavigationUI.navigateUp(navController, mAppBarConfiguration)
                || super.onSupportNavigateUp();
    }

    private void setNavigationViewListener() {
        navigationView.setNavigationItemSelectedListener(MainActivity.this);
    }

    @Override
    public boolean onNavigationItemSelected(@NonNull MenuItem menuItem) {
        switch (menuItem.getItemId()) {
            case R.id.nav_fragment_1: {
                fragmentManager.beginTransaction()
                        .replace(R.id.nav_host_fragment, new Fragment_1())
                        .commitNow();
                MainActivity.this.setTitle("Fragment 1");
                break;
            }
            case R.id.nav_fragment_2: {
                fragmentManager.beginTransaction()
                        .replace(R.id.nav_host_fragment, new Fragment_2())
                        .commitNow();
                MainActivity.this.setTitle("Fragment 2");
                break;
            }
        }
        drawer.closeDrawers();
        return true;
    }
}

片段 1

public class Fragment_1 extends Fragment{

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

        container.removeAllViews();
        View root = inflater.inflate(R.layout.fragment_1, container, false);

        return root;
    }
}

片段 2

public class Fragment_2 extends Fragment {

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

        container.removeAllViews();
        View root = inflater.inflate(R.layout.fragment_2, container, false);

        Button share = root.findViewById(R.id.share);

        share.setOnClickListener(view -> {
            Intent intent = new Intent(Intent.ACTION_SEND);
            intent.setType("text/plain");
            intent.putExtra(Intent.EXTRA_SUBJECT, "Subject");
            intent.putExtra(Intent.EXTRA_TEXT, "Message");
            startActivity(Intent.createChooser(intent, "Title"));
        });

        return root;
    }
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.drawerlayout.widget.DrawerLayout 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/drawer_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    tools:openDrawer="start"
    >

    <androidx.fragment.app.FragmentContainerView
        android:id="@+id/nav_host_fragment"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:tag="my_fragment"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintBottom_toBottomOf="parent"
        app:defaultNavHost="true"
        app:navGraph="@navigation/mobile_navigation"
        />

    <com.google.android.material.navigation.NavigationView
        android:id="@+id/nav_view"
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:layout_gravity="start"
        android:fitsSystemWindows="true"
        app:headerLayout="@layout/nav_header_main"
        app:menu="@menu/main_drawer"
        />
</androidx.drawerlayout.widget.DrawerLayout>

mobile_navigation.xml

<?xml version="1.0" encoding="utf-8"?>
<navigation 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"
    app:startDestination="@+id/nav_fragment_1">

    <fragment
        android:id="@+id/nav_fragment_1"
        android:name="com.example.problemtesting.Fragments.Fragment_1"
        tools:layout="@layout/fragment_1"
        />
    <fragment
        android:id="@+id/nav_fragment_2"
        android:name="com.example.problemtesting.Fragments.Fragment_2"
        tools:layout="@layout/fragment_2"
        />
</navigation>

main_drawer.xml

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    tools:showIn="navigation_view">

    <group android:checkableBehavior="single">
        <item
            android:id="@+id/nav_fragment_1"
            android:title="Fragment 1" />
        <item
            android:id="@+id/nav_fragment_2"
            android:title="Fragment 2" />
    </group>
</menu>

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.problemtesting">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name="com.example.problemtesting.MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>
</manifest>

这是一个令人沮丧的问题，我想不通。我已尝试删除 NavigationView.OnNavigationItemSelectedListener 侦听器并且它工作正常，但我需要该侦听器用于其他控件，例如片段切换动画和抽屉布局转换。

Answer 1

问题是您的 onNavigationItemSelected 正在执行 FragmentTransaction，用片段替换整个 NavHostFragment。在使用 NavHostFragment 时，这始终是更改片段的错误方法 - 您应该始终是 navigating to a destination.

但是，在您的情况下，您所做的工作比实际需要的多得多。相反，您应该：

完全删除您的 OnNavigationItemSelectedListener 代码。当您调用 NavigationUI.setupWithNavController(navigationView, navController); 时，导航已经为您设置好了，其中包括关闭抽屉并根据 Material 设计规范使用正确的淡入淡出动画。
根据 top app bar guide，您应该 而不是 手动设置活动的标题，而是添加一个 android:label到图表中的每个目的地，您完成的 setupActionBarWithNavController 调用将为您完成。

Android onCreate() 崩溃 - 片段无法转换为 androidx.navigation.fragment.NavHostFragment

Android crash onCreate() - Fragment cannot be cast to androidx.navigation.fragment.NavHostFragment

java

android

android-fragments

navigation-drawer

fragmentmanager