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

Question

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

Mih*_*a M 5 java android android-fragments navigation-drawer fragmentmanager

我决定为我的应用程序执行 Firebase Robo 测试，并注意到它在打开外部活动然后返回应用程序后总是崩溃。我能够通过在开发人员选项中启用“不要保留活动”来复制问题。

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

它表明问题是由这行代码引起的： NavHostFragment navHostFragment = (NavHostFragment) getSupportFragmentManager().findFragmentById(R.id.nav_host_fragment);

如何复制问题：

启用“不保持活动”
打开应用程序，然后打开应用程序抽屉
选择“片段 2”
按共享按钮
按消息按钮（或任何其他外部应用程序）
返回应用程序（崩溃）

如果禁用“不保留活动”，则该应用程序运行良好。

主要活动

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;
    }
}

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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;
    }
}

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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;
    }
}

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活动_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>

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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>

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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>

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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>

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这是一个令人沮丧的问题，我无法弄清楚。我试图删除 NavigationView.OnNavigationItemSelectedListener 侦听器，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

ian*_*ake 3

问题是你onNavigationItemSelected正在做一个，用片段FragmentTransaction替换整个。NavHostFragment使用 a 时，这始终是更改片段的错误方法NavHostFragment- 您应该始终导航到目的地。

然而，就您而言，您所做的工作比实际需要的要多得多。相反，您应该：

OnNavigationItemSelectedListener完全删除您的代码。当您致电时，导航已经为您设置了一项，NavigationUI.setupWithNavController(navigationView, navController);其中包括关闭抽屉并根据材料设计规范使用正确的交叉淡入淡出动画。
根据顶部应用程序栏指南，您不应手动设置活动的标题，而应向android:label图表中的每个目的地添加，setupActionBarWithNavController您所做的调用将为您完成此操作。

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