Navigation action/destination com.xxx:id/action_x cannot be found from the current destination

异常日志：

java.lang.IllegalArgumentException: Navigation action/destination com.xxx.xxx:id/action_AFragnebt_to_BFragment cannot be found from the current destination Destination(com.xxx.xxx:id/BFragment) label=BFragment class=com.XXX.XXX.ui.fragment.setting.BFragmentat androidx.navigation.NavController.navigate(NavController.java:938)at androidx.navigation.NavController.navigate(NavController.java:875)at androidx.navigation.NavController.navigate(NavController.java:861)at androidx.navigation.NavController.navigate(NavController.java:849)at com.XXX.XXX.ui.fragment.delivery.AFragment.onClickListener(AFragment.kt:290)at android.view.View.performClick(View.java:7259)at android.view.View.performClickInternal(View.java:7236)at android.view.View.access$3600(View.java:801)at android.view.View$PerformClick.run(View.java:27896)at android.os.Handler.handleCallback(Handler.java:883)at android.os.Handler.dispatchMessage(Handler.java:100)at android.os.Looper.loop(Looper.java:214)at android.app.ActivityThread.main(ActivityThread.java:7397)at java.lang.reflect.Method.invoke(Native Method)at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:935)

异常日志对应代码：

com.xxx.xxx:id/action_AFragnebt_to_BFragment cannot be found from the current destination Destination(com.xxx.xxx:id/BFragment)
从代码看跳转的时候是在BFragment中调用的NavHostFragment.findNavController(this).navigate(R.id.action_AFragnebt_to_BFragment)进行的页面跳转，而从异常日志却提示的是BFragment中没查找到这个action,为何是从BFragment查找而不是实际跳转位置的AFragment ？

开始分析源码：
先分析NavHostFragment.findNavController(this)是如何获取到的NavController对象

NavHostFragment.java@NonNull
public static NavController findNavController(@NonNull Fragment fragment) {Fragment findFragment = fragment;//如果当前fragment或者其parentFragment有继承了NavHostFragment的话就直接获取对应的NavControllerwhile (findFragment != null) { if (findFragment instanceof NavHostFragment) {return ((NavHostFragment) findFragment).getNavController();}Fragment primaryNavFragment = findFragment.getParentFragmentManager().getPrimaryNavigationFragment();if (primaryNavFragment instanceof NavHostFragment) {return ((NavHostFragment) primaryNavFragment).getNavController();}findFragment = findFragment.getParentFragment();}// Try looking for one associated with the view instead, if applicable//我们的AFragment和BFragment都没有继承NavHostFragment,实际走下面逻辑View view = fragment.getView();if (view != null) {return Navigation.findNavController(view);}// For DialogFragments, look at the dialog's decor viewDialog dialog = fragment instanceof DialogFragment? ((DialogFragment) fragment).getDialog(): null;if (dialog != null && dialog.getWindow() != null) {return Navigation.findNavController(dialog.getWindow().getDecorView());}throw new IllegalStateException("Fragment " + fragment+ " does not have a NavController set");}

从上面NavHostFragment源码可以分析出最终走了Navigation.findNavController(view)获取NavController。

Navigation.java@NonNull
public static NavController findNavController(@NonNull View view) {NavController navController = findViewNavController(view);if (navController == null) {throw new IllegalStateException("View " + view + " does not have a NavController set");}return navController;
}@Nullable
private static NavController findViewNavController(@NonNull View view) {//从view以及view的parentView中获取while (view != null) {NavController controller = getViewNavController(view);if (controller != null) {return controller;}ViewParent parent = view.getParent();view = parent instanceof View ? (View) parent : null;}return null;
}@Nullable
private static NavController getViewNavController(@NonNull View view) {//从view的tag中获取Object tag = view.getTag(R.id.nav_controller_view_tag);NavController controller = null;if (tag instanceof WeakReference) {controller = ((WeakReference<NavController>) tag).get();} else if (tag instanceof NavController) {controller = (NavController) tag;}return controller;
}

从上面源码可以看出，最终是通过fragment中的view，或者parentView的一个R.id.nav_controller_view_tag的tag中拿到的NavController,而在NavHostFragment的onCreate中new了一个NavHostController，并在onViewCreate中为其布局view添加了tagR.id.nav_controller_view_tag传入了这个NavHostController对象

NavHostFragment.java@CallSuper
@Override
public void onCreate(@Nullable Bundle savedInstanceState) {final Context context = requireContext();mNavController = new NavHostController(context);.....................super.onCreate(savedInstanceState);
}@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {super.onViewCreated(view, savedInstanceState);if (!(view instanceof ViewGroup)) {throw new IllegalStateException("created host view " + view + " is not a ViewGroup");}Navigation.setViewNavController(view, mNavController);// When added programmatically, we need to set the NavController on the parent - i.e.,// the View that has the ID matching this NavHostFragment.if (view.getParent() != null) {mViewParent = (View) view.getParent();if (mViewParent.getId() == getId()) {Navigation.setViewNavController(mViewParent, mNavController);}}
}//Navigation中的setViewNavController()
public static void setViewNavController(@NonNull View view,@Nullable NavController controller) {//通过R.id.nav_controller_view_tag传入了controllerview.setTag(R.id.nav_controller_view_tag, controller);
}

而AFragment和BFragment对应的同一个activity布局如下，最终获取到的是设置的同一个NavHostFragment中的NavController。

<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"><androidx.fragment.app.FragmentContainerViewandroid:id="@+id/fmContainer"android:name="androidx.navigation.fragment.NavHostFragment"android:layout_width="match_parent"android:layout_height="match_parent"app:defaultNavHost="true"app:navGraph="@navigation/navigation_XX_task"/></androidx.constraintlayout.widget.ConstraintLayout>

接下来分析的navigate跳转：
如果在AFragment连续调用两次navigate(R.id.action_AFragment_to_BFragment)，当第一次的跳转数据加入到mBackStack后，再次调用一次navigate(R.id.action_AFragment_to_BFragment)。

public void navigate(@IdRes int resId, @Nullable Bundle args, @Nullable NavOptions navOptions,@Nullable Navigator.Extras navigatorExtras) {//如果mBackStack栈不是空就获取最后的加入的NavDestination，//当第一次跳转后mBackStack.getLast().getDestination()对应的是BFragment的NavDestination NavDestination currentNode = mBackStack.isEmpty()? mGraph: mBackStack.getLast().getDestination();if (currentNode == null) {throw new IllegalStateException("no current navigation node");}//destId=R.id.action_AFragment_to_BFragment@IdRes int destId = resId;//在BFragment的NavDestination中没有R.id.action_AFragment_to_BFragment//此时navAction==nullfinal NavAction navAction = currentNode.getAction(resId);Bundle combinedArgs = null;if (navAction != null) {if (navOptions == null) {navOptions = navAction.getNavOptions();}destId = navAction.getDestinationId();Bundle navActionArgs = navAction.getDefaultArguments();if (navActionArgs != null) {combinedArgs = new Bundle();combinedArgs.putAll(navActionArgs);}}
..................................//此处返回的node==nullNavDestination node = findDestination(destId);if (node == null) {final String dest = NavDestination.getDisplayName(mContext, destId);if (navAction != null) {throw new IllegalArgumentException("Navigation destination " + dest+ " referenced from action "+ NavDestination.getDisplayName(mContext, resId)+ " cannot be found from the current destination " + currentNode);} else {//navAction为null 走这边throw new IllegalArgumentException("Navigation action/destination " + dest+ " cannot be found from the current destination " + currentNode);}}navigate(node, combinedArgs, navOptions, navigatorExtras);}@SuppressWarnings("WeakerAccess") /* synthetic access */NavDestination findDestination(@IdRes int destinationId) {//destinationId==R.id.action_AFragment_to_BFragmentif (mGraph == null) {return null;}if (mGraph.getId() == destinationId) {return mGraph;}//第二次跳转时 currentNode 对应的是BFragment的NavDestinationNavDestination currentNode = mBackStack.isEmpty()? mGraph: mBackStack.getLast().getDestination();NavGraph currentGraph = currentNode instanceof NavGraph? (NavGraph) currentNode: currentNode.getParent();//从BFragment的Graph中没有 R.id.action_AFragment_to_BFragment//返回nullreturn currentGraph.findNode(destinationId);}

结论：通过代码分析如果在AFragment执行一次到BFragment的跳转后，一定的情况下（已经跳到BFragment并将BFragment加入到BackStack队列中后）再次执行从A到B的跳转就会出现上述异常崩溃。

简单粗暴的解决方案

//将navigate这一步加上try catch，防止出现上述崩溃
fun NavController.navigateSafe(resId: Int, args: Bundle?=null,navOptions : NavOptions?=null) {try {navigate(resId, args,navOptions)} catch (e: Exception) {Pdlog.d("NavigationExt","navigate: resId = $resId args = $args")Pdlog.d("NavigationExt","navigate: "+e)}
}

其他解决思路：

//调用navigate跳转之前先判断
if (navController.currentDestination?.id == R.id.Afragment) {navController.navigate(R.id.action_Afragment_to_Bfragment)
}

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