如何在Fragment中设置BottomNavigationView

Question

我只有一项活动和几个片段。NavController我使用这样的导航图设置我的活动：

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    binding = ActivityMainBinding.inflate(layoutInflater)
    setContentView(binding.root)

    val navHostFragment = supportFragmentManager
                        .findFragmentById(R.id.nav_host_activity_main) as NavHostFragment
    navController = navHostFragment.findNavController()

}

override fun onSupportNavigateUp(): Boolean {
    return navController.navigateUp() || super.onSupportNavigateUp()
}

在此导航图中，我负责启动屏幕、登录、注册片段以及转到具有BottomNavigationView两个子片段的主片段。

我为 BottomNavView 片段制作了一个菜单，并为它们制作了一个图表，菜单 id 与底部导航图 id 相匹配。

我的布局如下所示：

<androidx.fragment.app.FragmentContainerView
    android:id="@+id/nav_host_fragment_main"
    android:name="androidx.navigation.fragment.NavHostFragment"
    android:layout_width="0dp"
    android:layout_height="0dp"
    app:defaultNavHost="true"
    app:layout_constraintBottom_toTopOf="@+id/bottom_Navigation_view"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent"
    app:navGraph="@navigation/bottom_view_nav_graph" />

<com.google.android.material.bottomnavigation.BottomNavigationView
    android:id="@+id/bottom_Navigation_view"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:background="@color/white"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:menu="@menu/bottom_nav_menu" />

</androidx.constraintlayout.widget.ConstraintLayout>

在主要片段中，我尝试使用导航图设置底部导航视图，如下所示：

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)

    val appCompat = requireActivity() as AppCompatActivity

    val navHostFragment = appCompat.supportFragmentManager.findFragmentById(R.id.nav_host_fragment_main) as NavHostFragment
    val navController = navHostFragment.findNavController()

    binding.bottomNavigationView.setupWithNavController(navController)

}

但是我不断收到此异常：

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.ak.chatter, PID: 1937
java.lang.NullPointerException: null cannot be cast to non-null type androidx.navigation.fragment.NavHostFragment
    at com.ak.chatter.ui.main.MainFragment.onViewCreated(MainFragment.kt:34)
    at androidx.fragment.app.FragmentStateManager.createView(FragmentStateManager.java:332)
    at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1187)
    at androidx.fragment.app.FragmentManager.addAddedFragments(FragmentManager.java:2224)
    at androidx.fragment.app.FragmentManager.executeOpsTogether(FragmentManager.java:1997)
    at androidx.fragment.app.FragmentManager.removeRedundantOperationsAndExecute(FragmentManager.java:1953)
    at androidx.fragment.app.FragmentManager.execPendingActions(FragmentManager.java:1849)
    at androidx.fragment.app.FragmentManager$4.run(FragmentManager.java:413)
    at android.os.Handler.handleCallback(Handler.java:900)
    at android.os.Handler.dispatchMessage(Handler.java:103)
    at android.os.Looper.loop(Looper.java:219)
    at android.app.ActivityThread.main(ActivityThread.java:8347)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:513)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1055)

Answer 1

java.lang.NullPointerException: null cannot be cast to non-null type 
androidx.navigation.fragment.NavHostFragment

您尝试将 null 转换为非 null。

这null是从appCompat.supportFragmentManager.findFragmentById(R.id.nav_host_fragment_main)声明中获得的。

因为被用作由fragment 托管的supportFragmentManagerfragments 的fragment 管理器。尽管应该管理由活动托管的片段。supportFragmentManager

而childFragmentManager应该使用 来管理由另一个片段托管的片段。

因此，要解决这个问题，supportFragmentManager请childFragmentManager在主片段类中替换为：

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)

    val appCompat = requireActivity() as AppCompatActivity

    val navHostFragment = appCompat.childFragmentManager.findFragmentById(R.id.nav_host_fragment_main) as NavHostFragment
    val navController = navHostFragment.findNavController()

    binding.bottomNavigationView.setupWithNavController(navController)

}