Room with Flowable: initialize database if it's empty

问题

I have following @Dao, that provides Flowable<User> stream:

@Dao
interface UsersDao {
  @Query("SELECT * FROM users")
  fun loadUsers(): Flowable<List<User>>
}

I want the subscriber of the stream to receive updates of the database as soon as some change happens there. Subscribing to Room's Flowable I will get that feature out of the box.

What I want is following: if database is empty I want to perform a web request and save users into database. The subscriber will automatically receive new updates that had just happened.

Now I want the client of the repository not to be aware all of the initialization logics: all he does - he performs usersRepository.loadUsers(). And all of these magic should take place inside the repository class:

class UsersRepository @Inject constructor(
    private val api: Api,
    private val db: UsersDao
) {

  fun loadUsers(): Flowable<List<User>> {
    ...
  }
}

Of course I can use following approach:

fun loadUsers(): Flowable<List<User>> {
  return db.loadTables()
      .doOnSubscribe {
        if (db.getCount() == 0) {
          val list = api.getTables().blockingGet()
          db.insert(list)
        }
      }
}

But I would like to construct the stream without using side-effects (doOn... operators). I've tried composing() but that didn't help much. Been stuck on this for a while.

回答1:

You could apply some conditional flatMaps:

@Dao
interface UsersDao {
    @Query("SELECT * FROM users")
    fun loadUsers(): Flowable<List<User>>

    @Query("SELECT COUNT(1) FROM users")
    fun userCount() : Flowable<List<Integer>>

    @Insert // I don't know Room btw.
    fun insertUsers(List<User> users) : Flowable<Object>
}

interface RemoteUsers {
     fun getUsers() : Flowable<List<User>>
}

fun getUsers() : Flowable<List<User>> {
     return
         db.userCount()
         .take(1)
         .flatMap({ counts ->
             if (counts.isEmpty() || counts.get(0) == 0) {
                 return remote.getUsers()
                        .flatMap({ users -> db.insertUsers(users) })
                        .ignoreElements()
                        .andThen(db.loadUsers())
             }
             return db.loadUsers()
         })
}

Disclaimer: I don't know Room so please adapt the example above as the features of it allow.

回答2:

Assuming your insert() call is async and also handles updates, you could do something like this:

fun loadUsers(): Flowable<List<User>> = userDao.getAllUsers().switchIfEmpty { api.getAllUsers().doOnNext { userDao.insert(it) }  }

You could also use some:

fun loadUsers(): Flowable<List<User>> = userDao.getAllUsers().flatMap { it->  if (it.isEmpty()) api.getAllUsers().doOnNext { userDao.insert(it) } else Flowable.just(it)}

Advice:

You should consider the case when the data is stale, therefore you need to go another way around, do a network request and database call at the same time. Whichever observable finish first, take the result and display it. Updating database should be right after network call is done.

来源：https://stackoverflow.com/questions/48478090/room-with-flowable-initialize-database-if-its-empty

标签

android

kotlin

rx-java

rx-java2

android-room