ViewModel observer method returns null

Question

I'm getting this following error

Attempt to invoke virtual method 'void android.arch.lifecycle.LiveData.observe on a null object reference

From this following part of code in my Main Fragment

    mReleasesViewModel = ViewModelProviders.of(this).get(ReleasesViewModel.class);
    mReleasesViewModel.getUpcomingReleases(filter).observe(this, new Observer<List<_Release>>() {
        @Override
        public void onChanged(@Nullable List<_Release> releases) {
            // whenever the list is changed
            if (releases != null) {
                mUpcomingGamesAdapter.setData(releases);
                mUpcomingGamesAdapter.notifyDataSetChanged();
            }
            mDatabaseLoading.setVisibility(View.GONE);
        }
    }); 

The error is specifically thrown on this line (when I attach the observer)

mReleasesViewModel.getUpcomingReleases(filter) ...

My ViewModel class:

public class ReleasesViewModel extends ViewModel {
    // fragment name and list
    private HashMap<String, MutableLiveData<List<_Release>>> upcomingReleasesListMap = new HashMap<>();

    private ReleasesRepository releasesRepository;
    private ArrayList<Integer> platforms;
    private String region;

    public ReleasesViewModel() {
        // Shared to all fragments : User settings region & platforms
        region = SharedPrefManager.read(SharedPrefManager.KEY_PREF_REGION, "North America");
        Set<String> defaultPlatformsSet = new HashSet<>();
        platforms = SharedPrefManager.read(SharedPrefManager.PLATFORM_IDS, defaultPlatformsSet);
    }

    public MutableLiveData<List<_Release>> getUpcomingReleases(String filter) {
        // ReleasesRepository takes a different monthly filter
        releasesRepository = new ReleasesRepository(region, filter, platforms);

        if (upcomingReleasesListMap.containsKey(filter)) {
            // Double check if it isn't null, just in case
            if (upcomingReleasesListMap.get(filter) == null) {
                // if null; try again to send a new request
                loadReleases(filter);
            } // else just don't do anything, the list is already in the Map
        } else {
            // Load it in if this filter was never added to the map [New filter and new list]
            loadReleases(filter);
        }
        return upcomingReleasesListMap.get(filter);
    }


    private void loadReleases(final String filter) {
        releasesRepository.addListener(new FirebaseDatabaseRepository.FirebaseDatabaseRepositoryCallback<_Release>() {
            @Override
            public void onSuccess(List<_Release> result) {
                // sort by release date
                if (platforms.size() > 1) {
                    // Will only sort for multiple platforms filter
                    Collections.sort(result);
                }
                MutableLiveData<List<_Release>> releases = new MutableLiveData<>();
                releases.setValue(result);
                upcomingReleasesListMap.put(filter, releases);
            }

            @Override
            public void onError(Exception e) {
                // Log.e(TAG, e.getMessage());
                MutableLiveData<List<_Release>> releases = new MutableLiveData<>();
                releases.setValue(null);
                upcomingReleasesListMap.put(filter, releases);
            }
        });
    }
}

The Hashmap is an integral part of how I'll serve data to my fragments. Because of how I'm now maintaining 6 fragments in my app, I need to use one ViewModel for all six fragments. Each of the fragments has a recyclerview with the same kind of data (objects and views), but they apply different filters to that data. Please take a look at the String filter parameter in the ViewModel, this param applies the filter to the data. And of course I'm maintaining all 6 fragments in a ViewModel. because they all need updating at the same time (when the user completes an action like changing the platforms list)

Answer 1

You have asynchronous code which you didn't take in consideration when you implemented those methods and you end up with a null LiveData on which you call observe() throwing the exception. The flow you currently use:

• call getUpcomingReleases() with an unknown filter
• this filter is not in the map so you call loadReleases()
• loadReleases() just set a listener for a background operation(firebase) and returns immediately
• you get to the line return upcomingReleasesListMap.get(filter); which will return null as the firebase listener most likely didn't completed and you din't put anything in the map for that filter value
• use the null LiveData and fail

You'll need something like the code below to make it work:

public MutableLiveData<List<_Release>> getUpcomingReleases(String filter) {
   ...
   if (upcomingReleasesListMap.get(filter) == null) {
       // we don't have a mapping for this filter so create one in the map
       MutableLiveData<List<_Release>> releases = new MutableLiveData<>();
       upcomingReleasesListMap.put(filter, releases);  
       // also call this method to update the LiveData
       loadReleases(filter);
    }
    // for now just return the empty LiveData so our ui can use it
    // when the firebase listener returns we will update it
    return upcomingReleasesListMap.get(filter);
}

private void loadReleases(final String filter) {
    releasesRepository.addListener(new FirebaseDatabaseRepository.FirebaseDatabaseRepositoryCallback<_Release>() {
        @Override
        public void onSuccess(List<_Release> result) {
            // sort by release date
            if (platforms.size() > 1) {
                // Will only sort for multiple platforms filter
                Collections.sort(result);
            }
            // just use the previous created LiveData, this time with the data we got 
            MutableLiveData<List<_Release>> releases = upcomingReleasesListMap.get(filter);
            releases.setValue(result);
        }

        @Override
        public void onError(Exception e) {
            // Log.e(TAG, e.getMessage());
            MutableLiveData<List<_Release>> releases = upcomingReleasesListMap.get(filter);
            releases.setValue(null);                
        }
    });
}