监听room数据库更新

要监听room数据库更新，可以使用Room的LiveData和Observer。具体步骤如下：

1. 在DAO中定义一个返回LiveData的查询方法，例如：

   @Query("SELECT * FROM room_table WHERE id = :id")
   LiveData<RoomEntity> getRoomById(int id);

2. 在ViewModel中获取LiveData对象，并在构造函数中初始化Observer，例如：

   public class RoomViewModel extends ViewModel {
       private LiveData<RoomEntity> mRoom;
   
       public RoomViewModel(RoomDatabase database, int roomId) {
           mRoom = database.roomDao().getRoomById(roomId);
           mRoom.observeForever(new Observer<RoomEntity>() {
               @Override
               public void onChanged(RoomEntity roomEntity) {
                   // 数据库更新时会回调该方法
               }
           });
       }
   
       public LiveData<RoomEntity> getRoom() {
           return mRoom;
       }
   }

3. 在Activity或Fragment中获取ViewModel对象，并观察LiveData对象，例如：

   public class RoomActivity extends AppCompatActivity {
       private RoomViewModel mViewModel;
   
       @Override
       protected void onCreate(Bundle savedInstanceState) {
           super.onCreate(savedInstanceState);
           setContentView(R.layout.activity_room);
   
           int roomId = getIntent().getIntExtra("room_id", 0);
           RoomDatabase database = Room.databaseBuilder(getApplicationContext(),
                   RoomDatabase.class, "room_database").build();
           mViewModel = new ViewModelProvider(this, new ViewModelProvider.NewInstanceFactory() {
               @NonNull
               @Override
               public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
                   return (T) new RoomViewModel(database, roomId);
               }
           }).get(RoomViewModel.class);
   
           mViewModel.getRoom().observe(this, new Observer<RoomEntity>() {
               @Override
               public void onChanged(RoomEntity roomEntity) {
                   // 数据库更新时会回调该方法
               }
           });
       }
   }