android - Share ViewModel between fragments that are in different Activity

Question

I have a ViewModel named SharedViewModel:

public class SharedViewModel<T> extends ViewModel {

    private final MutableLiveData<T> selected = new MutableLiveData<>();


    public void select(T item) {
        selected.setValue(item);
    }

    public LiveData<T> getSelected() {
        return selected;
    }
}

I've implemented it based on SharedViewModel example on the Google's Arch ViewModel reference page:

https://developer.android.com/topic/libraries/architecture/viewmodel.html#sharing_data_between_fragments

It is very common that two or more fragments in an activity need to communicate with each other. This is never trivial as both fragments need to define some interface description and the owner activity must bind the two together. Moreover, both fragments must handle the case where the other fragment is not yet created or not visible.

I have two fragments, called ListFragment and DetailFragment.

Until now I used these two fragments inside an activity called MasterActivity, and everything worked well.

I got the ViewModel in ListFragment, selected the value to use it on DetailFragment.

mStepSelectorViewModel = ViewModelProviders.of(getActivity()).get(SharedViewModel.class);

However, now, in certain cases, I need that ListFragment (a layout to a different device configuration) will be added to a different activity, called DetailActivity. Is there a way to do that similarly to the above example?

Answer 1

A little late but you can accomplish this using a shared ViewModelStore. Fragments and activities implement the ViewModelStoreOwner interface. In those cases fragments have a store per instance and activities save it in a static member (I guess so it can survive configuration changes).

Getting back to the shared ViewModelStore, let say for example that you want it to be your Application instance. You need your application to implement ViewModelStoreOwner.

class MyApp: Application(), ViewModelStoreOwner {
    private val appViewModelStore: ViewModelStore by lazy {
        ViewModelStore()
    }

    override fun getViewModelStore(): ViewModelStore {
        return appViewModelStore
    }
}

Then in the cases when you know that you need to share ViewModels between activity boundaries you do something like this.

val viewModel = ViewModelProvider(myApp, viewModelFactory).get(CustomViewModel::class.java)

So now it will use the Store defined in your app. That way you can share ViewModels.

Very important. Because in this example the ViewModels live in your application instance they won't be destroyed when the fragment/activity that uses them gets destroyed. So you will have to link them to the lifecycle of the last fragment/activity that will use them, or manually destroy them.