How can I change in ViewModel source of my LiveData from Room Dao

Question

How can I change in ViewModel source of my LiveData from Room Dao. In WordDao I have two queries:

@Dao
public interface WordDao {

   @Query("SELECT * FROM " + Word.TABLE_NAME + " ORDER BY text ASC")
   LiveData<List<Word>> getWordsByAsc();

   @Query("SELECT * FROM " + Word.TABLE_NAME + " ORDER BY text DESC")
   LiveData<List<Word>> getWordsByDesc();
}

I have also Repository class:

public class WordRepository {
   public LiveData<List<Word>> getWordsByAsc() {
       return wordDao.getWordsByAsc();
   }

   public LiveData<List<Word>> getWordsByDesc() {
       return wordDao.getWordsByDesc();
   }
}

and my ViewModel class:

public class WordViewModel extends AndroidViewModel {
    private boolean isSortAsc = true;
    private LiveData<Word> words;
    private WordRepository wordRepository;

    public WordViewModel(@NonNull Application application) {
        super(application);

        wordRepository = new WordRepository(application);
        words = wordRepository.getWordsByAsc();
    }

    public LiveData<List<Word>> getWords() {
       return words;
    }

    public void sortButtonClicked() {
       isSortAsc = !isSortAsc;
       //TODO how change here source of data depending on the isSortAsc
       //It works only after change configuration of screen, not insta
       words = isSortAsc ? wordRepository.getWordsByAsc() : 
            wordRepository.getWordsByDesc()
    }
}

In my Activity I have added observer:

 wordViewModel.getWords().observe(this, new Observer<List<Word>>() {
      @Override
      public void onChanged(@Nullable List<Word> words) {
          adapter.setWords(words);
      }
 });

In setWords(words) method I call also "notifyDataSetChanged()".

How change in the viewModel class source of LiveData depending on the "isSortAsc" parameter (words = isSortAsc ? wordRepository.getWordsByAsc() : wordRepository.getWordsByDesc()

It works only after change configuration of screen, not after insta change source for LiveData words

Answer 1

One approach might be to use MediatorLiveData ....for example:

val words = MediatorLiveData<Word>().apply {
    this.addSource(sortDirection) {
        if (sortDirection.value) {
             this.value = wordRepository.getWordsByAsc()
        } else {
             this.value = wordRepository.getWordsByDesc()
        }
    }
}

I do something similar in following (with setting of direction) https://github.com/joreilly/galway-bus-android/blob/master/app/src/main/java/com/surrus/galwaybus/ui/viewmodel/BusStopsViewModel.kt

Answer 2

A solution using Transformations.switchMap

First make isSortAsc LiveData

private MutableLiveData<Boolean> isSortAsc = new MutableLiveData<Boolean>(true);

Then use Transformations.switchMap on isSortAsc like so:

words = Transformations.switchMap(isSortAsc, 
    value -> value ? wordRepository.getWordsByAsc() : wordRepository.getWordsByDesc())