Executing delete with room (rxjava)

问题:

In room the @Delete annotation doesn't emit anything. This is what the dao looks like

@Dao
public interface UserDao {
    @Delete
    void deleteUser(User user);
    //We can't use Maybe or Single or anything here

}

This makes it a problem while doing something like

userRepository.deleteUser().subscribeOn since we have no emission coming the dao. I use the following code to call deleteUser on a background thread.

Observable.just(appDatabase).
            subscribeOn(SchedulerProvider.getInstance().computation()).

            subscribe(db -> {
                userRepository.logoutUser(loggedUser.getLoggedInUser());
                loggedUser.setLoggedInUser(null);


            }, this::handleError);

This works fine. However, in the subscribe method I now need to access the Android UI to display a toast announcing a successful delete. Naturally, I get this exception (since the observeOn is missing from the chain)

java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()

However when I put the observeOn like this

Observable.just(appDatabase).
        subscribeOn(SchedulerProvider.getInstance().computation()).
        observeOn(SchedulerProvider.getInstance().ui()).
        subscribe(db -> {
            userRepository.logoutUser(loggedUser.getLoggedInUser());
            loggedUser.setLoggedInUser(null);

            Message message = new Message(R.string.user_logged_out_msg);
            message.setMessageType(Message.MessageType.SUCCESS_MESSAGE);
            view.showMessages(Arrays.asList(message)); //this leads to a taost

        }, this::handleError);

I strangely get this exception:

cannot access database on the main thread since it may potentially lock the UI for a long period of time.

回答1:

Based on info from this question: Run Void Method in Backgound (StackOverflow)

Using a Completable and subscribing on another thread like this: Completable.fromAction(this::clearCachedData).subscribeOn(Schedulers.single()).subscribe(); worked for me. clearCachedData method executes the query in Room that I'm calling.

My query is:

/**
 * Delete all data in the items table.
 */
@Query("DELETE FROM items")
void deleteWeatherItems();

回答2:

I know this is late. But I ran into the same problem and were able to solve it with following

In Dao class

@Query("DELETE FROM users")
    fun deleteAllUser()

Call it like this. This way you can subscribe and it will run in the background.

Single.fromCallable {
        user.deleteAllUser() //User Dao fun
    }
            .subscribeOn(Schedulers.io())
            .observeOn(AndroidSchedulers.mainThread())
            .subscribe(..do stuff..)

回答3:

Replace Observable just method to create.

  Observable.create(new ObservableOnSubscribe<Object>() {
            @Override
            public void subscribe(@io.reactivex.annotations.NonNull ObservableEmitter<Object> e) throws Exception {
                e.onNext(tileDatabase.getCategoryDeo().deleteCategory(category));

            }
        })
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(new Observer<Object>() {
                    @Override
                    public void onSubscribe(@io.reactivex.annotations.NonNull Disposable d) {
                    }

                    @Override
                    public void onNext(@io.reactivex.annotations.NonNull Object o) {
                        Toast.makeText(MainActivity.this, "Record deleted ", Toast.LENGTH_SHORT).show();
                    }

                    @Override
                    public void onError(@io.reactivex.annotations.NonNull Throwable e) {
                    }

                    @Override
                    public void onComplete() {
                    }
                });

回答4:

Try to pass a list of users to deletion function and return the count of the deleted users (the code is in Kotlin).

@Dao
public interface UserDao {
    ...

    @Delete
    fun deleteUsers(users: List<User>): Int
}

This would be the database class:

@Database(entities = [User::class], version = 1)
abstract class UsersDatabase : RoomDatabase() {

    abstract fun userDao(): UserDao

    companion object {
        private var INSTANCE: UsersDatabase? = null

        fun getInstance(context: Context): UsersDatabase? {
            if (INSTANCE == null) {
                INSTANCE = Room.databaseBuilder(context.applicationContext, UsersDatabase::class.java, "my_database.db").build()
            }
            return INSTANCE
        }

        ...
}

And these the repository and the viewModel (Room Android Architecture):

class UsersRepository internal constructor(val application: Application) {

   private val userDao: UserDao = UsersDatabase.getInstance(application)!!.userDao()

   ...

   fun deleteUsers(users: List<User>): Single<Int> {
        return Single.fromCallable { userDao.deleteUsers(users) }
    }
}


class UsersViewModel(val app: Application) : AndroidViewModel(app) {
    private val repo: UsersRepository = UsesRepository(application = app)
    ...

    fun deleteUsers(users: List<User>): Single<Int> {
        return repo.deleteUsers(users).subscribeOn(Schedulers.io())
    }
}

And then in some activity or fragment:

class UsersActivity : AppCompatActivity() {

    private lateinit var viewModel: UsersViewModel
    ...

    override fun onCreate(savedInstanceState: Bundle?) {
    ...
       viewModel = ViewModelProviders.of(this).get(UsersViewModel::class.java)
    }

    fun removeUsers(users: List<User>) {
       ...
       viewModel.deleteUsers(users).subscribe(this::toast)
    }
    ...

    fun toast() {
        Toast.make(this, "Users deleted", Toast.LENGTH_SHORT).show
    }
}

回答5:

@Query("DELETE FROM User.TABLE_NAME")
public void nukeTable();

@Query("DELETE FROM " + User.TABLE_NAME + " WHERE " + User.COLUMN_ID + " = :id")
int deleteById(long id);

These method could be useful.