【发布时间】：2019-02-07 17:24:02
【问题描述】：
使用 ROOM 从 DB 读取数据并返回 LiveData 时，getValue() 方法返回 null。我已经有一段时间无法理解出了什么问题。你能帮忙解决这个问题吗？数据库中有数据，这似乎更像是我如何使用 LiveData 对象的问题。
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);
}
}
存储库：
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;
}
}
}
道：
@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 中我更改过滤器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 + "%");
}
但是在我的活动中，我需要创建一个新的观察者，因为现在数据由 LiveData 的新实例提供：
@Override
public void onClick(View v) {
this.exerciseViewModel.setFilters("Biceps", "weight");
this.exerciseViewModel.select().observe(this, exercises -> {
if (exercises != null) {
this.recyclerViewerAdapter.updateDataset(exercises);
}
});
}
现在这绝对是正确的做法：/
【问题讨论】：
标签： android architecture