IssueTracker

Component used: androidx.lifecycle:lifecycle-livedata

Right now, code that uses a MediatorLiveData not only needs the MediatorLiveData but needs to keep track of all sources added to it, if you ever want to remove those sources.

https://issuetracker.google.com/issues/62910664 calls for exposing the sources via a getSources() method. Since that request has been outstanding for two years, my guess is that it ain't happening.

A less dramatic change would be to add a removeAllSources() or clear() method to MediatorLiveData that removes all outstanding sources, reverting the MediatorLiveData to its original state.

Scenario:

You have a ViewModel that is supporting an MVI-style app, so the ViewModel has a LiveData<ViewState>, where the ViewState represents an immutable set of data to populate in the UI (e.g., via data binding). The source for that LiveData, though, varies based on user input (search strings, filter criteria, etc.), so the repository exposes a function that returns a LiveData<Model> based on that input criteria. We want the fragment to have a consistent source of truth for the UI data, so we want the LiveData<ViewState> to be the same LiveData instance throughout the fragment's life. So, we make it be a MediatorLiveData<ViewState>, where we use addSource() to attach the LiveData<Model> and use the observer lambda to map the Model to the ViewState. When the user changes the input data, we can call addSource() for the new LiveData<Model> that we get from the repository... but we also need to remove the earlier source. Right now, that means we have to hold onto the earlier LiveData<Model> to be able to call removeSource(). Ideally, we could just call removeAllSources() instead.

If you elected to offer getSources() as per https://issuetracker.google.com/issues/62910664, we could roll our own removeAllSources() implementation.

Thanks for considering this!