【错误记录】Navigation 导航组件报错 ( Activity xxActivity@3f does not have a NavController set on 2131xx )

一、报错信息

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在 Android 中 , 使用 Navigation 导航组件 , 运行应用时报错 , 报错信息如下 :

FATAL EXCEPTION: main
Process: kim.hsl.nav, PID: 30009
java.lang.RuntimeException: Unable to start activity ComponentInfo{kim.hsl.nav/kim.hsl.nav.MainActivity}: java.lang.IllegalStateException: Activity kim.hsl.nav.MainActivity@3f810b3 does not have a NavController
	at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2951)
	at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3086)
	at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
	at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
	at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
	at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1816)
	at android.os.Handler.dispatchMessage(Handler.java:106)
	at android.os.Looper.loop(Looper.java:193)
	at android.app.ActivityThread.main(ActivityThread.java:6718)
	at java.lang.reflect.Method.invoke(Native Method)
	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
Caused by: java.lang.IllegalStateException: Activity kim.hsl.nav.MainActivity@3f810b3 does not have a NavController set on 2131231211
	at androidx.navigation.Navigation.findNavController(Navigation.kt:50)
	at kim.hsl.nav.MainActivity.onCreate(MainActivity.kt:15)
	at android.app.Activity.performCreate(Activity.java:7144)
	at android.app.Activity.performCreate(Activity.java:7135)
	at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
	at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2931)
	at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3086) 
	at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) 
	at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108) 
	at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68) 
	at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1816) 
	at android.os.Handler.dispatchMessage(Handler.java:106) 
	at android.os.Looper.loop(Looper.java:193) 
	at android.app.ActivityThread.main(ActivityThread.java:6718) 

                                                   	at java.lang.reflect.Method.invoke(Native Method) 
	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) 
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858) 

Activity 布局文件代码如下 : 这是 Android Studio 中通过可视化操作自动生成的代码 ;

<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:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

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

androidx.constraintlayout.widget.ConstraintLayout>

二、问题分析

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该错误 通常是由于 Activity 中没有 获取到正确的 导航控制器（NavController）导致的 , 为了解决这个问题，需要在 Activity 中获取正确的 NavController ;

想要在 Activity 中获取正确的 NavController 就需要设置正确的 NavHostFragment ;

首先 , 确保 Activity 继承自 , 不要继承 android.app.Activity , 否则将无法获取 NavController , 报如下错误 :

Type mismatch.
Required:
AppCompatActivity
Found:
MainActivity

然后 , 确保 Activity 的布局文件中 , 设置了正确的 NavHostFragment 组件 ;

这里有一个大坑 , 就是 Android Studio 自动生成的代码 , 在 布局文件的 Design 模式下 , 将 Container 下的 NavHostFragment 组件拖入到布局文件中 , 生成的代码是错误的 ;

生成了如下代码 : 如果使用这个代码 , 是无法获取到 NavController 的 ;

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

必须使用 fragment 标签 , 设置的 NavHostFragment , 才能获取 正确的 NavController ;

    <fragment
        android:id="@+id/fragment"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:defaultNavHost="true"
        app:navGraph="@navigation/navigation_graph" />

三、解决方案

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在 Activity 布局文件中 , 使用 fragment 标签 , 并设置 android:name=“androidx.navigation.fragment.NavHostFragment” 属性 ;

<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:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <fragment
        android:id="@+id/fragment"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:defaultNavHost="true"
        app:navGraph="@navigation/navigation_graph" />

androidx.constraintlayout.widget.ConstraintLayout>

之后编译运行通过 ;