How to get the next element from monix observable?

Question

I have a stream of data that I casually log, but at a certain state of my program I need the data from the stream, not the latest observed up to that point(I can do that), but the newest one after:

val dataStream : Observable[Data] = ...
dataStream
 .doOnNext(logger.trace(_))
 .subscribe()

// ...
// at some state of program later:
// ...
def awaitOneElement(Observable[Data]): Data = ???
val expecting: Data = awaitOneElement(dataStream.filter(...))

How do I implement def awaitOneElement(Observable[Data]): Data = ????

I understand it's probably idiomatically incorrect, but that dirty synchronous waiting is exactly what I need. I'm fine with Observable[Data] => Future[Data] too, will wrap with Await on the next step.

Iva Kam · Answer 1 · 2020-02-22T11:56:17.333

0

If you need exactly one element, you can use something like Consumer.head or Consumer.headOption:

val first: Task[Option[Int]] =
  Observable.empty[Int].consumeWith(Consumer.headOption)]

And now you can convert task to future. If you need all elements you can use foldLeft with accumulator:

Consumer.foldLeft[T,Vector[T]](Vector.empty[T])((vec, t) => vec :+ t )

Or implement custom consumer and fire callback when you need. Taken from docs: https://monix.io/docs/3x/reactive/consumer.html

UPD: for last element you can use such consumer:

Consumer.foldLeft[T,Option[T]](Option.empty[T])((opt, t) => Some(t))

edited Feb 22 '20 at 11:56

answered Feb 21 '20 at 11:51

Iva Kam

932
4
13

`first` returns first element from the observable, not the last – Ilya Smagin Feb 21 '20 at 16:27
@IlyaSmagin , you can then use foldLeft without Vector but with Option. – Iva Kam Feb 22 '20 at 11:57

Ilya Smagin · Accepted Answer · 2020-02-22T08:29:00.870

My solution:

  implicit val scheduler                     = mx
  val pendingPromise: AtomicReference[Option[Promise[A]]] = new AtomicReference(None)
  val lastSubject: ConcurrentSubject[A, A]   = ConcurrentSubject.publish[A]

  o.doOnNext(a => if (pendingPromise.get().nonEmpty) lastSubject.onNext(a))
    .subscribe()

  lastSubject
    .filter(filter)
    .doOnNext(a => pendingPromise.getAndSet(None).map(_.success(a)))
    .subscribe()

  def getNext(duration: Duration): Try[A] = {
    val promise = Promise[A]()
    pendingPromise.set(Some(promise))
    Try(Await.result(promise.future, duration))
  }
}

When getNext is called, a Promise is created and future is returned. When the desired event occures in Observable, the promise is filled and removed.

How to get the next element from monix observable?

2 Answers2