java - How to properly call network fetching function from view model

Question

How should I call the function QueryUtils.fetchData(REQUEST_URL) inside of ViewModel, it returns List<Earthquakes>. This code fetches data the way it should, but it doesn't display it probably because it already sets data before it is fetched.

public class MainActivity extends AppCompatActivity {

    private EarthquakeAdapter mEarthquakeAdapter;
    private EarthquakeViewModel aEarthquakeViewModel;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

// Redacted

    aEarthquakeModel = ViewModelProviders.of(this).get(EarthquakeViewModel.class);

    aEarthquakeViewModel.fetchEarthquakes().observe(this, new Observer<ArrayList<Earthquake>>() {
            @Override
            public void onChanged(ArrayList<Earthquake> earthquakes) {
                if(earthquakes != null) {
                    mEarthquakeAdapter.clear();
                    mEarthquakeAdapter.addAll(earthquakes);
                }
            }
        });
    }
}


public class EarthquakeViewModel extends AndroidViewModel {

    public MutableLiveData<ArrayList<Earthquake>> earthquakesData;
    private ArrayList<Earthquake> earthquakes;

    public EarthquakeViewModel(@NonNull Application application) {
        super(application);
        Log.i("EarthquakeViewModel", "EarthquakeViewModel constructor entered");
        earthquakesData = new MutableLiveData<>();
        doIt();
        Log.i("EarthquakeViewModel", "EarthquakeViewModel constructor finished");
    }

    public LiveData<ArrayList<Earthquake>> fetchEarthquakes() {
        earthquakesData.setValue(earthquakes);
        return earthquakesData;
    }

    public void doIt() {
        Thread thread = new Thread(new Runnable() {
            @Override
            public void run() {
                earthquakes = (ArrayList<Earthquake>) QueryUtils.fetchData(REQUEST_URL);
            }
        });
        thread.start();
    }
}

Answer 1

Rewriting your viewmodel class a bit:

 public class EarthquakeViewModel extends AndroidViewModel {

public MutableLiveData<ArrayList<Earthquake>> earthquakesData;
private ArrayList<Earthquake> earthquakes;

public EarthquakeViewModel(@NonNull Application application) {
    super(application);
    Log.i("EarthquakeViewModel", "EarthquakeViewModel constructor entered");
    earthquakesData = new MutableLiveData<>();
    doIt();
    Log.i("EarthquakeViewModel", "EarthquakeViewModel constructor finished");
}

public LiveData<ArrayList<Earthquake>> fetchEarthquakes() {
    return earthquakesData;
}

public void doIt() {
    Thread thread = new Thread(new Runnable() {
        @Override
        public void run() {
            earthquakes = (ArrayList<Earthquake>) QueryUtils.fetchData(REQUEST_URL);
          earthQuakesData.postValue(earthquakes);

        }
    });
    thread.start();
}
}

Please try this and let me know

Answer 2

You need to understand here concept of setValue() and postValue() in MutableLiveData. You have to update MutableLive data by using function setValue() when you are updating in the main thread and while in worker thread you need to use postValue(). Hope this helps. And above answer satisfies your request.