public class MainActivity extends AppCompatActivity
 private MutableLiveData<List<Photo>> mLivePhotos;
     //some code...

 @Override
 protected void onCreate(Bundle savedInstanceState) {
    mLivePhotos = loadData();
    mLivePhotos.observe(this, photos -> {
        Log.d(TAG, "onChanged!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!");
        mProgressBar.setVisibility(View.GONE);
        mPhotos = photos;
        if (mIsInitialCall) {
            initiateAdapter();
            mIsInitialCall = false;
        } else {
            mAdapter.updateList(mPhotos.subList(mPageNumber, mPageNumber + 10));
        }
    });

mRecyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {
    @Override
    public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
        int lastPosition =
mLayoutManager.findLastCompletelyVisibleItemPosition();
        Log.d(TAG, "onScrolled - lastPosition: " + lastPosition);
        if (lastPosition == mLayoutManager.getItemCount() - 1) {
            Log.d(TAG, "onScrolled - End of list?");
            loadData();
        }
    }
});
}

private MutableLiveData<List<Photo>> loadData() {
Log.d(TAG, "loadData");
if (mArticleViewModel == null) return null;
mPageNumber += 10;
mProgressBar.setVisibility(View.VISIBLE);
return mArticleViewModel.loadPhotos();
}

public class ArticleViewModel extends ViewModel {
private MutableLiveData<List<Photo>> photos;
private ArticleRepository articleRepository;

public MutableLiveData<List<Photo>> loadPhotos() {
Log.d(TAG, "getArticleList");
//TODO; add Dagger 2
articleRepository = new ArticleRepository();
photos = articleRepository.getPhotos();
return photos;
}

public class ArticleRepository {

    public MutableLiveData<List<Photo>> getPhotos() {
    final MutableLiveData<List<Photo>> result = new MutableLiveData<>();
    Log.d(TAG, "getResults");
    ApiService.getService().getPhotos().enqueue(new Callback<List<Photo>>() {
    @Override
    public void onResponse(Call<List<Photo>> call, Response<List<Photo>> response) {
        Log.d(TAG, "onResponse");
        if (response.isSuccessful()) {
            Log.d(TAG, "isSuccessful");
            result.postValue(response.body());
        }
    }

    @Override
    public void onFailure(Call<List<Photo>> call, Throwable t) {
        Log.d(TAG, "onFailure: " + t.getMessage() + "n" +  t.getStackTrace());
    }
});
return result;
}