如何使用Architecture Components观察数据库删除事件

Question

I have a DAO as follows.

@Dao
public interface PostDAO {
    @Query("SELECT * FROM posts order by time DESC")
    LiveData<List<Post>> getPosts();

    @Insert(onConflict = OnConflictStrategy.REPLACE)
    @NonNull
    void insert(Post... posts);

    @Delete
    void delete(Post post);

    @Query("SELECT * FROM posts WHERE id = :id")
    Post getPost(String id);

    @Query("SELECT * FROM posts WHERE id = :id")
    LiveData<Post> getPostLiveData(String id);
}

I understand that I can listen for new added data with something like the following.

postDAO().getPosts().observe(builder.getLifecycleOwner(), data::postValue)

But how can I know when a post has been deleted?

Answer 1

LiveData<List<Post>> getPosts(); returns a LiveData that is updated by any insert , update , or delete done to the Post table.

So if you are observing the list initially, then you are already observing for inserts/updates/deletes.

Answer 2

You observe for all the items ( LiveData<List<Post>> ) and get notified with the new data as soon as something changes.

You may want to use the new DiffUtil which can show you the affected changes.

Another way is to set/update the value of your LiveData after deleting. Returning Long means that it returns the count of the deleted rows.

@Delete void delete(Post post) : Long

LiveData data = new MutableLiveData<Post>()
if (yourDao.delete(yourItem) > 0) data.setValue( yourItem ) 

I wrote an answer why Delete and Insert is not using LiveData at Android Room : LiveData callback of update insert?