Android 房间数据库中的列表始终为空

以下类用于在房间数据库中存储调查列表。LiveData用于更新

数据库的实体类：

@Entity
public class Survey {

@PrimaryKey
@NonNull
private String surveyID;
private String surveyName;
private String url;
private double lat;
private double lng;
private  double radius;

public Survey(String surveyID, String surveyName, double lat, double lng, double radius, String url) {
    this.surveyID = surveyID;
    this.surveyName = surveyName;
    this.lat = lat;
    this.lng = lng;
    this.radius = radius;
    this.url = url;
}

各自的Dao接口：

@Dao
public interface SurveyDao {

@Query("SELECT * FROM Survey")
LiveData<List<Survey>> getListOfSurveys();

/* Rest omitted */
}

各自的存储库类

public class SurveyRepository {

private final SurveyDao surveyDao;

@Inject
public SurveyRepository (SurveyDao surveyDao){
    this.surveyDao = surveyDao;
}

public LiveData<List<Survey>> getListOfSurveys(){
    return surveyDao.getListOfSurveys();
}
/* Rest omitted */
}

视图模型：

public class SurveyCollectionViewModule extends ViewModel {

private SurveyRepository repository;

public SurveyCollectionViewModule(SurveyRepository repository) {
    this.repository = repository;
}

public LiveData<List<Survey>> getSurveys(){
    return repository.getListOfSurveys();
}
/* Rest Omitted */
}

使用所有这些，我设置了一个片段，其中有一个显示调查列表的RecyclerView。该列表通过以下方式获得：

public void onActivityCreated(Bundle savedInstanceState){
    super.onActivityCreated(savedInstanceState);

    surveyCollectionViewModule = ViewModelProviders.of(this, viewModelFactory)
            .get(SurveyCollectionViewModule.class);

    surveyCollectionViewModule.getSurveys().observe(this, new Observer<List<Survey>>() {
        @Override
        public void onChanged(@Nullable List<Survey> surveys) {
            if(SurveyListFragment.this.listOfSurveys == null) {
                setSurveyData(listOfSurveys);
            }
        }
    });
}

---

但是，我遇到了一个问题，由于该列表为NULL，适配器中对getItemCount的调用失败。数据库不包含任何条目，但我仍然不明白为什么列表总是返回空值

您的onChanged实际上没有使用给定的surveys参数，请将其更改为调用setSurveyDatasurveys以实际填写数据。

在ViewModel类中进行一些更改，如下所示

    private final LiveData<List<Survey>> SurveyList;

private AppDatabase appDatabase;

public SurveyCollectionViewModule(Application application) {
    appDatabase = AppDatabase.getDatabase(this.getApplication());

    SurveyList = appDatabase.surveyDao().getAllSurvey();
}
public LiveData<List<Survey>> getSurveys() {
    return SurveyList;
}