fromAsyncStateAction.scala

  import monix.reactive._
  import monix.eval.{Callback, Task}
  import monix.execution.Ack.{Continue, Stop}
  import scala.util.control.NonFatal

  def fromAsyncStateAction[S,A](seed: => S)(f: S => Task[(A,S)]): Observable[A] =
    Observable.unsafeCreate[A] { (subscriber) =>
      import subscriber.scheduler

      def loop(state: S): Task[Unit] =
        try f(state).flatMap { case (a,s) =>
          Task.fromFuture(subscriber.onNext(a)).flatMap {
            case Continue => loop(s)
            case Stop => Task.unit
          }
        } catch {
          case NonFatal(ex) =>
            Task.raiseError(ex)
        }

      loop(seed).runAsync(Callback.empty)
    }