android - 导航组件，没有设置 NavController，在 androidx.navigation.Navigation.findNavController 没有设置 NavController

Question

我基本上是在等待 5 秒后尝试导航到其他 fragment 。当我在 onViewCreated 中执行导航过程时，它可以工作，但是当我在 onFinish 中执行时，它给了我下面的错误。
我试图将 View 对象保留为全局变量，但它也不起作用。

E/AndroidRuntime: FATAL EXCEPTION: mainProcess: com.example.ui, PID: 17838java.lang.IllegalStateException: View androidx.constraintlayout.widget.ConstraintLayout{8dcd038 V.E...... .......D 0,0-1080,1908} does not have a NavController setat androidx.navigation.Navigation.findNavController(Navigation.java:84)at com.example.ui.sign_in.LoadingFragment$1.onFinish(LoadingFragment.java:39)at android.os.CountDownTimer$1.handleMessage(CountDownTimer.java:127)at android.os.Handler.dispatchMessage(Handler.java:106)at android.os.Looper.loop(Looper.java:201)at android.app.ActivityThread.main(ActivityThread.java:6810)at java.lang.reflect.Method.invoke(Native Method)at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547)at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:873)I/Process: Sending signal. PID: 17838 SIG: 9

** 私有(private) View View ；

@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)
您需要在 onFinish() 之前检查 fragment 是否可见。被叫
添加一个 bool 值，在 onResume() 中初始化它并在 onPause() 中设为假像这样

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