不调用LiveData观察器onChanged

Question

我正在尝试在测验应用程序中使用Room数据库。 我无法从数据库中获取问题列表。这是我正在使用的代码：

 @Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {

    final View rootView = inflater.inflate(R.layout.new_exam, container, false);

    queViewModel=ViewModelProviders.of(this).get(QueViewModel.class);

    queViewModel.getQuizList(examType).observe(this, new Observer<List<QueEnt>>() {
        @Override
        public void onChanged(List<QueEnt> queEnts) {

            Log.e(TAG, "onChanged is called!"); // this line never shows in the Logcat 
            questionsList2 = queEnts;
        }
    });

这是ViewModel：

public class QueViewModel extends AndroidViewModel {
private QueRepository repository;
private LiveData<List<QueEnt>> quesList;

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

    repository=new QueRepository(application);
    quesList=repository.getQues();
}

public void insert(QueEnt queEnt){
    repository.insert(queEnt);
}
public void delete(QueEnt queEnt){
    repository.delete(queEnt);
}

public LiveData<List<QueEnt>> getQuizList(int examtype){

    Log.e(TAG,"returning a list"); // this appears in the Logcat

    return quesList;
}
}

我得到的错误是

  

无效的索引0，大小为0

关于questionsList2，当然是因为从未填充过该列表。从Logcat看来，未调用onChanged方法。 我怎样才能解决这个问题 ？

编辑： QueRepository文件：

public class QueRepository {
private QueDao queDao;
private LiveData<List<QueEnt>> ques;

public QueRepository(Application application) {
    QueDatabase database = QueDatabase.getInstance(application);
    queDao = database.queDao();
    ques = queDao.getQuesAll();
}

public void insert(QueEnt queEnt) {
    new InsertQAsync(queDao).execute(queEnt);
}

public void update(QueEnt queEnt) {
    new UpdateQAsync(queDao).execute(queEnt);
}

public void delete(QueEnt queEnt) {
    new DeleteQAsync(queDao).execute(queEnt);
}


public LiveData<List<QueEnt>> getQues() {

    return ques;
}

private static class InsertQAsync extends AsyncTask<QueEnt, Void, Void> {
    private QueDao queDao;

    private InsertQAsync(QueDao queDao) {
        this.queDao = queDao;
    }

    @Override
    protected Void doInBackground(QueEnt... queEnts) {
        queDao.insert(queEnts[0]);
        return null;
    }
}

private static class UpdateQAsync extends AsyncTask<QueEnt, Void, Void> {
    private QueDao queDao;

    private UpdateQAsync(QueDao queDao) {
        this.queDao = queDao;
    }

    @Override
    protected Void doInBackground(QueEnt... queEnts) {
        queDao.update(queEnts[0]);
        return null;
    }
}

private static class DeleteQAsync extends AsyncTask<QueEnt, Void, Void> {
    private QueDao queDao;

    private DeleteQAsync(QueDao queDao) {
        this.queDao = queDao;
    }

    @Override
    protected Void doInBackground(QueEnt... queEnts) {
        queDao.delete(queEnts[0]);
        return null;
    }
}
}

还有QueDao：

@Dao
public interface QueDao {

@Insert
void insert(QueEnt queEnt);

@Update
void update(QueEnt queEnt);

@Delete
void delete(QueEnt queEnt);

@Query("SELECT * FROM questions LIMIT 30" )
LiveData<List<QueEnt>> getQuesAll();

}

和数据库：

@Database(entities = {QueEnt.class}, version = 1)
public abstract class QueDatabase extends RoomDatabase {

public static QueDatabase instance;


public static synchronized QueDatabase getInstance(Context context) {

    if (instance == null) {
        instance = Room.databaseBuilder(context.getApplicationContext(), QueDatabase.class, "que_database")
                .fallbackToDestructiveMigration()
                .addCallback(roomCallback)
                .build();
    }
    return instance;
}

public abstract QueDao queDao();


private static RoomDatabase.Callback roomCallback = new RoomDatabase.Callback() {
    @Override
    public void onCreate(@NonNull SupportSQLiteDatabase db) {
        super.onCreate(db);
        new PopulateDbAsync(instance).execute();
    }
};

private static class PopulateDbAsync extends AsyncTask<Void, Void, Void> {
    private QueDao queDao;

    private PopulateDbAsync(QueDatabase db) {
        queDao = db.queDao();
    }

    @Override
    protected Void doInBackground(Void... voids) {
        queDao.insert(new QueEnt(91, 93, "Question 1?", "   Answer 1", "Answer2 ", "Answer 3 ", " Answer4   ", 2, 2, 1,0,0,0,0,0,"","","","",""));

        queDao.insert(new QueEnt(93, 95, "Question 2?", "   Answer 1", "Answer2 ", "Answer 3 ", " Answer4   ", 3, 2, 4,0,0,0,0,0,"","","","",""));
             return null;
    }
}
}