Trigger update for a LiveData member when another LiveData is updated in the view model

Question

In a word game app I share a model between an activity and a fragment:

public class MainViewModel extends AndroidViewModel {
    private LiveData<List<Game>> mGames;
    private final MutableLiveData<Game> mDisplayedGame = new MutableLiveData<>();

(please excuse the non-english text in the screenshot)

The activity observes mGames being currently played by the user and updates the navigational drawer menu (see the left side of the above screenshot).

The fragment observes mDisplayedGame and displays it in a custom view (see the right side of the above screenshot).

My problem is that when the list of the games is updated at the server (and the activity receives new list of games via Websocket and stores it in the Room), I need to post an update to the fragment: "Hey, the game you are displaying was updated, redraw it!"

Is it possible to do that from within the shared view model?

I know that I could observe mGames in the fragment too and add a code there iterating through them and then finding out if the displayed game was updated at the server.

But I would prefer to do it in the MainViewModel because I have a feeling that the fragment should only observe the one game it is displaying and that's it.

TL;DR

Whenever mGames is updated in the view model via Room, I need to notify the mDisplayedGame observers too!

Answer 1

You should use a MediatorLiveData for this.

The way it works is that

public class MainViewModel extends AndroidViewModel {
    private final LiveData<List<Game>> mGames;
    private final MutableLiveData<Game> mSelectedGame = new MutableLiveData<>();

    private final MediatorLiveData<Game> mDisplayedGame = new MediatorLiveData<>();

    {
        mDisplayedGame.addSource(mGames, (data) -> {
            // find the new value of the selected game in the list
            mSelectedGame.setValue(newSelectedGame);
        });

        mDisplayedGame.addSource(mSelectedGame, (data) -> {
            mDisplayedGame.setValue(data);
        });
    }

And then you expose mDisplayedGame as a LiveData<Game> and it should just work.