Sample snippet for a canonical problem resolved using RxJava

RxApp.kt

interface DataSource {
  fun allTasksByUser(user: User): Observable<List<Task>>
}

class LocalDataSource : DataSource {
  private val localCache: Map<User, List<Task>> =
    mapOf(User(UserId("user1")) to listOf(Task("LocalTask assigned to user1")))

  override fun allTasksByUser(user: User): Observable<List<Task>> = Observable.create { emitter ->
    val cachedUser = localCache[user]
    if (cachedUser != null) {
      emitter.onNext(cachedUser)
    } else {
      emitter.onError(UserNotInLocalStorage(user))
    }
  }
}

class RemoteDataSource : DataSource {
  private val internetStorage: Map<User, List<Task>> =
    mapOf(User(UserId("user2")) to listOf(Task("Remote Task assigned to user2")))

  override fun allTasksByUser(user: User): Observable<List<Task>> = Observable.create { emitter ->
    val networkUser = internetStorage[user]
    if (networkUser != null) {
      emitter.onNext(networkUser)
    } else {
      emitter.onError(UserNotInRemoteStorage(user))
    }
  }
}

class TaskRepository(private val localDS: DataSource, private val remoteDS: RemoteDataSource) {

  fun allTasksByUser(user: User): Observable<List<Task>> =
    localDS.allTasksByUser(user)
      .subscribeOn(Schedulers.io())
      .observeOn(Schedulers.computation())
      .onErrorResumeNext { _: Throwable -> remoteDS.allTasksByUser(user) }
}

class Module {
  private val localDataSource: LocalDataSource = LocalDataSource()
  private val remoteDataSource: RemoteDataSource = RemoteDataSource()
  val repository: TaskRepository = TaskRepository(localDataSource, remoteDataSource)
}

object test {

  @JvmStatic
  fun main(args: Array<String>): Unit {
    val user1 = User(UserId("user1"))
    val user2 = User(UserId("user2"))
    val user3 = User(UserId("unknown user"))

    val dependenciesModule = Module()
    dependenciesModule.run {
      repository.allTasksByUser(user1).subscribe({ println(it) }, { println(it) })
      repository.allTasksByUser(user2).subscribe({ println(it) }, { println(it) })
      repository.allTasksByUser(user3).subscribe({ println(it) }, { println(it) })
    }
  }
}