Shared ViewModel's onCleared() is never called

Question

I have a single Activity application. In one part of the app there is a master-detail combination of Fragments that use a shared ViewModel to share some data between them. The problem I face is that the ViewModel's onCleared() method is not called even when both of the Fragments have been destroyed. onCleared() is only called when the activity is destroyed.

Is this how it is supposed to work? Because that is quite useless in a single Activity model, because the Activity is always alive. Or am I missing something?

Answer 1

It would be a little hard to give a solid answer without seeing a little bit of code. My first guess would be that you might have scoped the ViewModel to your Activity and not the Fragment's themselves.

//inside of fragment onCreate()

//scoped to fragment
viewModel = ViewModelProviders.of(this).get(SharedViewModel::class.java)

//scoped to activity
viewModel = ViewModelProviders.of(requireActivity()).get(SharedViewModel::class.java)

If this is the case, if you look at the diagram for the scope of a ViewModel. Then the reason why onCleared() never gets called is because your Activity has never technically gotten destroyed since it is what keeps your app on the foreground.

If this isn't the right solution to your problem, then I think the docs on ViewModel might be a good place to start looking for the right answer. Happy coding!