AndroidViewModel does not preserve state between Fragment switch

Question

I was under the impression, that a ViewModel would preserve state and data between Fragment switches, as long as they belong to the same Activity. That does not seem to be the case.

My ViewModel:

public class MainViewModel extends AndroidViewModel {

    public MainViewModel(@NonNull Application application) {
        super(application);

        Log.d("MainViewModel","constructor called");
    }
}

Access in a fragment:

MainViewModel viewModel = new ViewModelProvider(this).get(MainViewModel.class);

I noticed that the constructor is called every time I request the ViewModel. I thought that the purpose of the ViewModel was to stay alive all this time and could be used to preserve some state between fragments. Am I doing this wrong? Was my expectation wrong? How can I preserve state between fragments?

Answer 1

The issue is that you are using a ViewModelProvider that is scoped to the fragment instance rather than the parent activity instance. Replace new ViewModelProvider(this) with new ViewModelProvider(requireActivity()).

More info available in the documentation: https://developer.android.com/topic/libraries/architecture/viewmodel/viewmodel-apis#vm-api-any-owner