Post or Set MutableLiveData - Observer onChanged not called

Question

I discover the new android architecture component and I want to test the couple ViewModel / LiveData through a small test application. The latter has two fragments (in a ViewPager), the first creates/updates a list of cards (via an EditText) and the second displays all the cards.

My ViewModel:

public class CardsScanListViewModel extends AndroidViewModel {

    private MutableLiveData> cardsLiveData = new MutableLiveData();
    private HashMap cardsMap = new HashMap();

    public CardsScanListViewModel(@NonNull Application application) {
        super(application);
    }

    public MutableLiveData> getCardsLiveData() {
        return this.cardsLiveData;
    }

    public void saveOrUpdateCard(String id) {
        if(!cardsMap.containsKey(id)) {
            cardsMap.put(id, new Card(id, new AtomicInteger(0)));
        }
        cardsMap.get(id).getCount().incrementAndGet();
        this.cardsLiveData.postValue(cardsMap);
    }
}

My second fragment:

public class CardsListFragment extends Fragment {

    CardsAdapter cardsAdapter;

    RecyclerView recyclerCardsList;

    public CardsListFragment() {}

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        final CardsScanListViewModel viewModel =
                ViewModelProviders.of(this).get(CardsScanListViewModel.class);

        observeViewModel(viewModel);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View v = inflater.inflate(R.layout.fragment_cards_list, container, false);
        recyclerCardsList = v.findViewById(R.id.recyclerCardsList);
        recyclerCardsList.setLayoutManager(new LinearLayoutManager(getActivity()));
        cardsAdapter = new CardsAdapter(getActivity());
        recyclerCardsList.setAdapter(cardsAdapter);

        return v;
    }

    private void observeViewModel(CardsScanListViewModel viewModel) {
        viewModel.getCardsLiveData().observe(this, new Observer  > () {
            @Override
            public void onChanged(@Nullable HashMap  cards) {
                if (cards != null) {
                    cardsAdapter.setCardsList(cards.values());
                }
            }
        });
    }
}

TheHashMap, like my MutableLiveData, update well but my second fragment doesn't receive the information via observer.

Answer 1

You are observing the new instance of ViewModel instead of observing the same ViewModel used by your First Fragment.

final CardsScanListViewModel viewModel =
            ViewModelProviders.of(this).get(CardsScanListViewModel.class);

Above code initialize new instance of CardsScanListViewModel for your second fragment CardsListFragment, because you passed this as context. If you update any data from this fragment it will update in this instance of ViewModel.

It works in your first Fragment because it updates data and observes data from same instance of ViewModel

To keep data common among ViewModels initiate view model by passing activity context instead of fragment context in both the fragments.

final CardsScanListViewModel viewModel =
            ViewModelProviders.of(getActivity()).get(CardsScanListViewModel.class);

This will create single instance of CardsScanListViewModel and data will be shared between fragments as they are observing LiveData from single instance of ViewModel.

For confirmation, you need to add notifyDataSetChanged() after updating the list if you haven't done that in adapter itself

private void observeViewModel(CardsScanListViewModel viewModel) {
    viewModel.getCardsLiveData().observe(this, new Observer  > () {
        @Override
        public void onChanged(@Nullable HashMap  cards) {
            if (cards != null) {
                cardsAdapter.setCardsList(cards.values());
                cardsAdapter.notifyDataSetChanged();
            }
        }
    });
}