Android LiveData和Room：getValue返回NULL

Question

使用ROOM从DB读取数据并返回LiveData时，getValue（）方法返回null。我一直无法理解一会儿出了什么问题。你能帮忙解决这个问题吗？数据库中有数据，似乎更像是我如何使用LiveData对象。

活动：

public class ExercisesViewActivity extends AppCompatActivity implements View.OnClickListener {

  private ExerciseViewModel exerciseViewModel;
  private ExercisesAdapter recyclerViewerAdapter;

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

    Toolbar toolbar = findViewById(R.id.toolbar_exercise_view_activity);
    toolbar.setTitle("Exercises");
    setSupportActionBar(toolbar);

    ActionBar actionBar = getSupportActionBar();
    if (actionBar != null) {
      actionBar.setDisplayHomeAsUpEnabled(true);
      actionBar.setDisplayShowHomeEnabled(true);
    }

    RecyclerView recyclerView = findViewById(R.id.exercise_view_recycle_viewer);
    RecyclerView.LayoutManager layoutManager = new LinearLayoutManager(getApplicationContext());
    recyclerView.setLayoutManager(layoutManager);
    this.recyclerViewerAdapter = new ExercisesAdapter();
    recyclerView.setAdapter(recyclerViewerAdapter);

    this.exerciseViewModel = ViewModelProviders.of(this).get(ExerciseViewModel.class);
    this.exerciseViewModel.setFilters("", "");
//    this.exerciseViewModel.selectAll();
    this.exerciseViewModel.select().observe(this, exercises -> {
      if (exercises != null) {
        this.recyclerViewerAdapter.updateDataset(exercises);
      }
    });

    Button button = findViewById(R.id.test_button);
    button.setOnClickListener(this);
  }

  @Override
  public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()) {
      case android.R.id.home:
        onBackPressed();
        break;
    }

    return true;
  }

  @Override
  public void onClick(View v) {
    this.exerciseViewModel.setFilters("", "");
//    this.exerciseViewModel.select().observe(this, exercises -> {
//      if (exercises != null) {
//        this.recyclerViewerAdapter.updateDataset(exercises);
//      }
//    });
  }
}

视图模型：

public class ExerciseViewModel extends AndroidViewModel {
  ExercisesRepository repository;
  MutableLiveData<List<Exercise>> data;

  public ExerciseViewModel(Application application) {
    super(application);
    this.data = new MutableLiveData<>();
    this.repository = new ExercisesRepository(application);
  }

  public void setFilters(String muscleGroups, String type) {
    LiveData<List<Exercise>> listLiveData = this.repository.filterSelect(muscleGroups, type);
    this.data.setValue(listLiveData.getValue());
  }

  public void selectAll() {
//    this.data.setValue(this.repository.selectAll().getValue());
  }

  public LiveData<List<Exercise>> select() {
    return data;
  }

  public void insert(Exercise exercise) {
    this.repository.insert(exercise);
  }
}

库：

public class ExercisesRepository {
  private ExerciseDao dao;

  public ExercisesRepository(Application context) {
    WorkoutRoomDatabase database = WorkoutRoomDatabase.getDb(context);
    this.dao = database.exerciseDao();
  }

  public LiveData<List<Exercise>> filterSelect(String muscleGroups, String type) {
    return this.dao.filterSelect("%" + muscleGroups + "%", "%" + type + "%");
  }

  public LiveData<List<Exercise>> selectAll() {
    return this.dao.selectAll();
  }

  public void insert(Exercise exercise) {
    new insertAsyncTask(this.dao).execute(exercise);
  }

  private static class insertAsyncTask extends AsyncTask<Exercise, Void, Void> {

    private ExerciseDao exerciseDao;

    insertAsyncTask(ExerciseDao  dao) {
      exerciseDao = dao;
    }

    @Override
    protected Void doInBackground(final Exercise... params) {
      exerciseDao.insert(params[0]);
      return null;
    }
  }
}

DAO:

@Dao
public interface ExerciseDao {
  @Query("SELECT * FROM exercises WHERE muscleGroups LIKE :muscleGroup AND type LIKE :type")
  LiveData<List<Exercise>> filterSelect(String muscleGroup, String type);
  @Query("SELECT * FROM exercises")
  LiveData<List<Exercise>> selectAll();
  @Insert
  void insert(Exercise exercise);
  @Update
  void update(Exercise exercise);
  @Delete
  void delete(Exercise exercise);
  @Query("DELETE FROM exercises")
  void deleteAll();
}

更新：

如果在我的viewModel中我更改了filter函数以返回一个新的LiveData对象，则正在获取正确的数据：

  public LiveData<List<Exercise>> filterSelect(String muscleGroups, String type) {
    return this.dao.filterSelect("%" + muscleGroups + "%", "%" + type + "%");
  }

但是在我的Activity中我需要创建一个新的观察者，因为现在数据是由LiveData的新实例提供的：

 @Override
  public void onClick(View v) {
    this.exerciseViewModel.setFilters("Biceps", "weight");
    this.exerciseViewModel.select().observe(this, exercises -> {
      if (exercises != null) {
        this.recyclerViewerAdapter.updateDataset(exercises);
      }
    });
  }

现在这是正确的方法：/

Answer 1

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Answer 2

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