尝试获取 NavController 时出现 NullPointerException

Question

我无法理解错误的原因java.lang.NullPointerException: null cannot be cast to non-null type androidx.navigation.fragment.NavHostFragment

这是我的主机片段：

class HostFragment : Fragment() {
    
    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        val root = inflater.inflate(R.layout.fragment_host, container, false)

        // error here
        val navHostFragment = requireActivity().supportFragmentManager.findFragmentById(R.id.hostFragment) as NavHostFragment
        //

        val navController = navHostFragment.navController

        root.host_text_view.setOnClickListener {
            Log.d("Tag", "clicked")

            navController.navigate(R.id.action_hostFragment_to_secondFragment)
        }

        return root
    }
}

我想通过单击 textView 导航到 SecondFragment。然后回来。

和导航图。xml 文件：

<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"
    android:id="@+id/nav_graph"
    app:startDestination="@id/hostFragment">

    <fragment
        android:id="@+id/hostFragment"
        android:name="com.example.test.HostFragment"
        android:label="fragment_host"
        tools:layout="@layout/fragment_host" >
        <action
            android:id="@+id/action_hostFragment_to_secondFragment"
            app:destination="@id/secondFragment" />
    </fragment>

    <fragment
        android:id="@+id/secondFragment"
        android:name="com.example.test.SecondFragment"
        android:label="fragment_second"
        tools:layout="@layout/fragment_second" >
        <action
            android:id="@+id/action_secondFragment_to_hostFragment"
            app:destination="@id/hostFragment" />
    </fragment>
</navigation>

导航图的最终形式如下：

Answer 1

您无需找到 navHostFragment 即可导航

val root = inflater.inflate(R.layout.fragment_host, container, false)

        
        root.host_text_view.setOnClickListener {
            Log.d("Tag", "clicked")


            if (findNavController().currentDestination?.id == hostFragment)
            findNavController().navigate(R.id.action_hostFragment_to_secondFragment)

        }

您可以检查 Kotlin 方法来导航 https://developer.android.com/guide/navigation/navigation-navigate

Answer 2

使用导航图的最佳方法是在创建视图之后，因为某些视图需要时间来创建，因此您可能会遇到此异常，因为您的视图仍在处理或创建视图中。

为了避免这种情况，您可以使用 OnViewCreated() as

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
       super.onViewCreated(view, savedInstanceState)
       navController = Navigation.findNavController(view)
    
       view.imgNumbrLookUp.setOnClickListener { 
            navController.navigate(R.id.action_homeFragment_to_webActivity) 
       }       
}

尝试获取 NavController 时出现 NullPointerException

NullPointerException when trying to get NavController

android

kotlin

android-navigation-graph