Android LiveData和Room:getValue返回NULL
当使用ROOM从DB读取数据并返回LiveData时,getValue()方法返回null。一段时间以来,我一直无法理解到底出了什么问题。你能帮我解决这个问题吗?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:Android LiveData和Room:getValue返回NULL,android,architecture,Android,Architecture,当使用ROOM从DB读取数据并返回LiveData时,getValue()方法返回null。一段时间以来,我一直无法理解到底出了什么问题。你能帮我解决这个问题吗?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 exercise
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);
}
}
这显然是正确的方法://您试图在filterSelete中使用空值获取类似的搜索<代码>this.exerciseViewModel.setFilters(“,”)代码>。 这个过滤器基本上是个问题。你不能像搜索一样空。 安装此功能后,您可以使用
loadAll
query
@Query(“从练习中选择*)
LiveData loadAll()代码>
编辑:
public LiveData filterSelect(字符串musclegroup,字符串类型){
如果(肌肉群==“”&类型=“”){
返回这个.dao.loadAll()
}否则{
返回此.dao.filterSelect(“%”+muscleGroups+“%”,“%”+type+“%”);
}
希望它能解决您的问题。您应该查看MediaLiveData,而不是MutableLiveData,它将允许您从存储库中提出更改
这类似于下面,您将存储库数据作为数据源添加到mediatorLiveData。然后,当您从存储库接收数据时,将触发回调,然后可以通过调用setValue从mediatorLiveData发出回调
public class ExerciseViewModel extends AndroidViewModel {
ExercisesRepository repository;
MediatorLiveData<List<Exercise>> mediatorLiveData = new MediatorLiveData<>();
public ExerciseViewModel(Application application) {
super(application);
this.repository = new ExercisesRepository(application);
}
public void setFilters(String muscleGroups, String type) {
LiveData<List<Exercise>> repositoryLiveData = this.repository.filterSelect(muscleGroups, type);
mediatorLiveData.addSource(repositoryLiveData, exercisList -> {
mediatorLiveData.removeSource(repositoryLiveData);
mediatorLiveData.setValue(exercisList);
});
}
public LiveData<List<Exercise>> select() {
return mediatorLiveData;
}
public类ExerciseViewModel扩展了AndroidViewModel{
演习储备库;
MediatorLiveData MediatorLiveData=新的MediatorLiveData();
公共练习视图模型(应用程序){
超级(应用);
this.repository=newexerciserespository(应用程序);
}
公共void集合过滤器(字符串musclegroup,字符串类型){
LiveData repositoryLiveData=this.repository.filterSelect(MuscleGroup,类型);
mediatorLiveData.addSource(repositoryLiveData,exercisList->{
removeSource(repositoryLiveData);
mediatorLiveData.setValue(exerciselist);
});
}
公共LiveData选择(){
返回数据;
}
我通过使用MediaLiveData和转换解决了这个问题
现在我的ViewModel构造函数如下所示:
public ExerciseViewModel(Application application) {
super(application);
this.repository = new ExercisesRepository(application);
this.filterMutableLiveData = new MutableLiveData<>();
ExercisesFilter filter = new ExercisesFilter();
filter.muscleGroup = "";
filter.type = "";
this.filterMutableLiveData.setValue(filter);
LiveData<List<Exercise>> source = Transformations.switchMap(
this.filterMutableLiveData, value -> repository.filterSelect(value.muscleGroup, value.type)
);
this.data.addSource(source, val -> this.data.setValue(val));
}
使用这种方法,每次更新过滤器时,都会触发传递给Transformations.switchMap的函数以获取新数据。最后,新的LiveData对象被添加到MediatorLiveData对象,因此所有源都可以使用一个观察者
来源:谢谢你的回答。不幸的是,这就是问题所在。我已经测试了你的建议,但仍然存在相同的问题。之前我返回了一个新的LiveData对象,它获取了正确的数据,但随后我需要创建一个新的observable,这不是正确的方法。loadAll函数返回空对象?loadAll retuns LiveDat一个对象,但这是一个新的LiveData对象。为了使用相同的可观察对象,我需要将其数据放在现有的可变LiveData中。因此,我使用LiveData.getValue()这就是返回null的内容。除非有其他我不知道的方法。对于使用相同的引用,您可以使用可编辑的答案。谢谢Chris。这实际上会起作用,所以我会将您的答案标记为正确。我实际上选择了使用介体和转换。我将在另一个答案中解释。
public LiveData<List<Exercise>> filterSelect(String muscleGroups, String type) {
return this.dao.filterSelect("%" + muscleGroups + "%", "%" + type + "%");
}
@Override
public void onClick(View v) {
this.exerciseViewModel.setFilters("Biceps", "weight");
this.exerciseViewModel.select().observe(this, exercises -> {
if (exercises != null) {
this.recyclerViewerAdapter.updateDataset(exercises);
}
});
}
public LiveData<List<Exercise>> filterSelect(String muscleGroups, String type) {
if(muscleGroups == "" && type == "") {
return this.dao.loadAll()
}else {
return this.dao.filterSelect("%" + muscleGroups + "%", "%" + type + "%");
}
public class ExerciseViewModel extends AndroidViewModel {
ExercisesRepository repository;
MediatorLiveData<List<Exercise>> mediatorLiveData = new MediatorLiveData<>();
public ExerciseViewModel(Application application) {
super(application);
this.repository = new ExercisesRepository(application);
}
public void setFilters(String muscleGroups, String type) {
LiveData<List<Exercise>> repositoryLiveData = this.repository.filterSelect(muscleGroups, type);
mediatorLiveData.addSource(repositoryLiveData, exercisList -> {
mediatorLiveData.removeSource(repositoryLiveData);
mediatorLiveData.setValue(exercisList);
});
}
public LiveData<List<Exercise>> select() {
return mediatorLiveData;
}
public ExerciseViewModel(Application application) {
super(application);
this.repository = new ExercisesRepository(application);
this.filterMutableLiveData = new MutableLiveData<>();
ExercisesFilter filter = new ExercisesFilter();
filter.muscleGroup = "";
filter.type = "";
this.filterMutableLiveData.setValue(filter);
LiveData<List<Exercise>> source = Transformations.switchMap(
this.filterMutableLiveData, value -> repository.filterSelect(value.muscleGroup, value.type)
);
this.data.addSource(source, val -> this.data.setValue(val));
}
public void setFilterMutableLiveData(String muscleGroups, String type) {
ExercisesFilter filter1 = new ExercisesFilter();
filter1.muscleGroup = muscleGroups;
filter1.type = type;
this.filterMutableLiveData.setValue(filter1);
}