房间数据库更改时未触发java Observer

Question

当房间数据库更改时，我正在尝试更新RecyclerView。但是，当数据库发生更改时，不会调用MainActivity中定义的观察者的onChanged方法。如果我让DAO返回LiveData而不是列表<&燃气轮机；并在我的ViewModel中使用LiveData—它可以按预期工作。但是，我需要MutableLiveData，因为我想在运行时将查询更改为数据库

我的设置：

主要活动。java

BorrowedListViewModel viewModel = ViewModelProviders.of(this).get(BorrowedListViewModel.class);

    viewModel.getItemAndPersonList().observe(MainActivity.this, new Observer<List<BorrowModel>>() {
        @Override
        public void onChanged(@Nullable List<BorrowModel> itemAndPeople) {
            recyclerViewAdapter.addItems(itemAndPeople);
        }
    });

借用ListViewModel。java

public class BorrowedListViewModel extends AndroidViewModel {

    private final MutableLiveData<List<BorrowModel>> itemAndPersonList;

    private AppDatabase appDatabase;

    public BorrowedListViewModel(Application application) {
        super(application);

        appDatabase = AppDatabase.getDatabase(this.getApplication());

        itemAndPersonList = new MutableLiveData<>();

        itemAndPersonList.setValue(appDatabase.itemAndPersonModel().getAllBorrowedItems());
    }

    public MutableLiveData<List<BorrowModel>> getItemAndPersonList() {
        return itemAndPersonList;
    }


    //These two methods should trigger the onChanged method
    public void deleteItem(BorrowModel bm) {
        appDatabase.itemAndPersonModel().deleteBorrow(bm);
    }

    public void addItem(BorrowModel bm){
        appDatabase.itemAndPersonModel().addBorrow(bm);
    }

}

借用了DAO。java

public interface BorrowModelDao {

    @Query("select * from BorrowModel")
    //Using LiveData<List<BorrowModel>> here solves the problem
    List<BorrowModel> getAllBorrowedItems();

    @Query("select * from BorrowModel where id = :id")
    BorrowModel getItembyId(String id);

    @Insert(onConflict = REPLACE)
    void addBorrow(BorrowModel borrowModel);

    @Delete
    void deleteBorrow(BorrowModel borrowModel);

}

AppDatabase。java

public abstract class AppDatabase extends RoomDatabase {

    private static AppDatabase INSTANCE;

    public static AppDatabase getDatabase(Context context) {
        if (INSTANCE == null) {
            INSTANCE =
                    Room.databaseBuilder(context.getApplicationContext(), AppDatabase.class, "borrow_db")
                            .allowMainThreadQueries()
                            .build();
            }
            return INSTANCE;
    }

    public static void destroyInstance() {
        INSTANCE = null;
    }

    public abstract BorrowModelDao itemAndPersonModel();

}