Android ViewModel with LiveData transformations


Example

public class BaseViewModel extends ViewModel {
    private static final int TAG_SEGMENT_INDEX = 2;
    private static final int VIDEOS_LIMIT = 100;

    // We save input params here
    private final MutableLiveData<Pair<String, String>> urlWithReferrerLiveData = new MutableLiveData<>();

    // transform specific uri param to "tag"
    private final LiveData<String> currentTagLiveData = Transformations.map(urlWithReferrerLiveData, pair -> {
        Uri uri = Uri.parse(pair.first);
        List<String> segments = uri.getPathSegments();
        if (segments.size() > TAG_SEGMENT_INDEX)
            return segments.get(TAG_SEGMENT_INDEX);
        return null;
    });

    // transform "tag" to videos list
    private final LiveData<List<VideoItem>> videoByTagData = Transformations.switchMap(currentTagLiveData, tag -> contentRepository.getVideoByTag(tag, VIDEOS_LIMIT));

    ContentRepository contentRepository;

    public BaseViewModel() {
        // some inits
    }

    public void setUrlWithReferrer(String url, String referrer) {
        // set value activates observers and transformations
        urlWithReferrerLiveData.setValue(new Pair<>(url, referrer));
    }

    public LiveData<List<VideoItem>> getVideoByTagData() {
        return videoByTagData;
    }
}

Somewhere in UI:

public class VideoActivity extends BaseCompatLifecycleActivity {
    private VideoViewModel viewModel;

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

        // Get ViewModel
        viewModel = ViewModelProviders.of(this).get(BaseViewModel.class);
        // Add observer
        viewModel.getVideoByTagData().observe(this, data -> {
            // some checks
            adapter.updateData(data);
        });

        ...
        if (savedInstanceState == null) {
            // init loading only at first creation
            // you just set params and 
            viewModel.setUrlWithReferrer(url, referrer);
        }
}