LiveData 观察者在片段中多次触发

Question

I was trying to implement SignIn feature in a fragment . Following is the code for that:

@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
    MainViewModel mainViewModel = new ViewModelProvider(requireActivity()).get(MainViewModel.class);
    String email = ....;
    String password = ....;
    SignInLiveData signInStatusObserver = mainViewModel.getSignInStatusLiveData(email, password);
    signInStatusObserver.observe(getViewLifeCycleOwer(), new Observer<Boolean>{
            @Override
            public void onChanged(Boolean isSignedIn) {
                if(isSignedIn)
                    Toast.makeText(requireContext(), "SignIn Successful", Toast.LENGTH_LONG).show();
                else
                    Toast.makeText(requireContext(), "SignIn Unsuccessful", Toast.LENGTH_LONG).show();
            }
    }
}

It is working fine but when I lock the phone and then unlock it (with the app on the screen), the Toast message is show again. Again when I lock and then unlock the device (for any number of time), the toast is shown again and again.

Why is it happening like that?
How to show the toast only once?

Answer 1

I believe it goes like this in your case:

1. You lock your device -> at some point fragment's view is destroyed (view model is not)
2. You unlock your device and back to the app -> your fragment gets new view, tries to observe live data (remember view model is the same as in p1) -> the event gets emitted.

How to deal with it: to keep it short, pls check here https://medium.com/androiddevelopers/livedata-with-snackbar-navigation-and-other-events-the-singleliveevent-case-ac2622673150 (I am not the author)