导航组件，未设置 NavController，未在 androidx.navigation.Navigation.findNavController 设置 NavController

Question

我基本上是在等待 5 后尝试导航到其他片段 seconds.When 我在我的 onViewCreated 中执行导航过程它有效但是当我在 onFinish 中执行时它给我以下错误。

我试图将 View 对象保留为全局变量，但它也没有用。

E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.ui, PID: 17838 java.lang.IllegalStateException: View androidx.constraintlayout.widget.ConstraintLayout{8dcd038 V.E...... .......D 0,0-1080,1908} does not have a NavController set at androidx.navigation.Navigation.findNavController(Navigation.java:84) at com.example.ui.sign_in.LoadingFragment.onFinish(LoadingFragment.java:39) at android.os.CountDownTimer.handleMessage(CountDownTimer.java:127) at android.os.Handler.dispatchMessage(Handler.java:106) at android.os.Looper.loop(Looper.java:201) at android.app.ActivityThread.main(ActivityThread.java:6810) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:873) I/Process: Sending signal. PID: 17838 SIG: 9

** 私密查看；

@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

我不确定这是否适合您，但我遇到过类似的问题实例，我注意到视图对象正在被销毁。所以它以我自己的方式保存它:)

这个变量声明在MyExampleFragment的成员变量下class

 private View mFragView;

在 onCreateView 中，我只是添加了一个 null 检查，所以它不会被再次创建。

 @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 2

从片段中获取导航控制器的两种方法：

NavHostFragment.findNavController(this);
Navigation.findNavController(requireActivity(), R.id.MAIN_NAV_HOST_FRAGMENT);

Answer 3

解1)

在 onFinish() 被调用之前，您需要检查片段是否可见或不再可见

添加一个布尔值，在 onResume() 中对其进行初始化，并在 onPause() 中将其设置为假

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) 您需要声明一个 CountDownTimer 实例并将其初始化一次，并在片段不再可见时将其设为 null

Answer 4

我更改我的代码：

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

到onViewCreated

导航组件，未设置 NavController，未在 androidx.navigation.Navigation.findNavController 设置 NavController

Navigation Component, does not have a NavController set , does not have a NavController set at androidx.navigation.Navigation.findNavController

android

android-studio

android-navigation

android-architecture-navigation