With reference to the
ViewModel is scoped to the lifecycle of the UI component it relates to, so in a
Fragment-based app, that will be the fragment lifecycle. This is a good thing.
In some cases one wants to share a
ViewModel instance between multiple fragments. Specifically I am interested in the case where many screens relate to the same underlying data.
(The docs suggest similar approach when multiple related fragments are displayed on the same screen but this can be worked around by using a single host fragment as per answer below.)
This is discussed in the official ViewModel documentation:
ViewModels can also be used as a communication layer between different Fragments of an Activity. Each Fragment can acquire the ViewModel using the same key via their Activity. This allows communication between Fragments in a de-coupled fashion such that they never need to talk to the other Fragment directly.
In other words, to share information between fragments that represent different screens, the
ViewModel should be scoped to the
Activity lifecycle (and according to Android docs this can also be used in other shared instances).
Now in the new Jetpack Navigation pattern, it is recommended to use a "One Activity / Many Fragments" architecture. This means that the activity lives for the whole time the app is being used.
i.e. any shared
ViewModel instances that are scoped to
Activity lifecycle will never be cleared - the memory remains in constant use.
With a view to preserving memory and using as little as required at any point in time, it would be nice to be able to clear shared
ViewModel instances when no longer required.
How can one manually clear a
ViewModel from it's
ViewModelStore or holder fragment?
If you don't want the
ViewModel to be scoped to the
Activity lifecycle, you can scope it to the parent fragment's lifecycle. So if you want to share an instance of the
ViewModel with multiple fragments in a screen, you can layout the fragments such that they all share a common parent fragment. That way when you instantiate the
ViewModel you can just do this:
CommonViewModel viewModel = ViewModelProviders.of(getParentFragment()).class(CommonViewModel.class);
Hopefully this helps!
There's nothing wrong with using multiple viewmodels as well. First one could be scoped to the Activity while another one could be scoped to the fragment.
Try to use the Activity scoped Viewmodel only for things that need to be shared. And put as many things as possible in the Fragment Scoped Viewmodel. The Fragment scoped viewmodel will be cleared when the fragment is destroyed. Reducing the overall memory footprint.