导航组件没有NavController集，在androidx.Navigation.Navigation.findNav

Question

在等待5秒钟后，我基本上正在尝试导航到其他片段。当我在onViewCreated中进行导航过程时，它是有效的，但当我在on Finish中进行导航时，它会给我以下错误。

我试图将View对象保留为全局变量，但它也不起作用。

E/AndroidRuntime：致命异常：main流程：com.example.ui，PID:17838java.lang.IollegalStateException：查看androidx.constraintlayout.widget.constraintlayout｛8dcd038 V.E…….D 0,0-10801908｝没有NavController集位于androidx.navigation.navigation.findNavController(navigation.java:84(网址：com.example.ui.sign_in.LoadingFragment$1.onFinish(LoadingFragment.java:39(在android.os.CountDownTimer$1.handleMessage(CountDownTimer.java:127(在android.os.Handler.dispatchMessage(Handler.java:106(在android.os.Looper.loop(Looper.java：201(在android.app.ActivityThread.main(ActivityThreads.java:6810(位于java.lang.reflect.Method.ioke(本机方法(网址：com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547(网址：com.android.internal.os.ZygoteInit.main(ZygoteNit.java:873(I/Process：发送信号。PID：17838 SIG:9

**私有视图；

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
AppCompatDelegate.setDefaultNightMode(AppCompatDelegate.MODE_NIGHT_NO);
// Inflate the layout for this fragment
return inflater.inflate(R.layout.fragment_loading, container, false);
}

@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
this.view = view;
new CountDownTimer(5000, 1000) { //Wait 5 seconds
public void onTick(long millisUntilFinished) {
}
public void onFinish() { // Then navigate to other page.
navigate();
}
}.start();
super.onViewCreated(view, savedInstanceState);
}

private void navigate() { 
Navigation.findNavController(this.view).navigate(LoadingFragmentDirections.actionLoadingFragmentToSignInPagerFragment());
}**

Answer 1

解决方案1(

在onFinish()被调用之前，您需要检查碎片是否可见

添加一个布尔值，在onResume()中初始化它，并在onPause()中使其为false，就像这个一样

private boolean isFragmentVisible=false;
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
AppCompatDelegate.setDefaultNightMode(AppCompatDelegate.MODE_NIGHT_NO);
// Inflate the layout for this fragment
return inflater.inflate(R.layout.fragment_loading, container, false);
}

@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
this.view = view;
new CountDownTimer(5000, 1000) { //Wait 5 seconds
public void onTick(long millisUntilFinished) {
}
public void onFinish() { // Then navigate to other page.
//check if fragment is visible or not
if(isFragmentVisible){
navigate();
}

}
}.start();
super.onViewCreated(view, savedInstanceState);
}
@Override
public void onResume() {
super.onResume();
isFragmentVisible=true;
}
@Override
public void onPause() {
super.onPause();
isFragmentVisible=false;
}
private void navigate() { 
Navigation.findNavController(this.view).navigate(LoadingFragmentDirections.actionLoadingFragmentToSignInPagerFragment());
}**

解决方案2(您需要声明CountDownTimer的一个实例并初始化它一次，当片段不再可见时使其为空

Answer 2

从片段中获取导航控制器的两种方法：

1. NavHostFragment.findNavController(this);

2. Navigation.findNavController(requireActivity(), R.id.MAIN_NAV_HOST_FRAGMENT);

Answer 3

我不确定这是否适用于您，但我也遇到过类似的问题，我注意到视图对象正在被破坏。所以它以我自己的方式保存：(

此变量在MyExampleFragment类的成员变量下声明

private View mFragView;

在onCreateView中，我只是添加了一个null检查，所以不会再次创建它。

@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
if (mFragView != null) {
return mFragView;
}
mFragView = inflater.inflate(R.layout.fragment_dashboard, container, false);
ButterKnife.bind(this, mFragView);

return mFragView;
}

Answer 4

我更改代码：

val action =
StartFragmentDirections.actionStartFragmentToLoginFragment()
Navigation.findNavController(view).navigate(action);

到onViewCreated