def main(args: Array[String]): Unit = {
    implicit val system: ActorSystem = ActorSystem("scratch")
    implicit val materializer: ActorMaterializer = ActorMaterializer()
    implicit val executionContext: ExecutionContextExecutor = system.dispatcher

    val start = Instant.now()
    def elapsed = time.Duration.between(start, Instant.now()).toMillis
    val intSource = Source.queue[Int](2, OverflowStrategy.dropHead)
    val intSink = Sink foreach { ii: Int =>
      Thread.sleep(1000)
      println(s"processing $ii at $elapsed")
    }
    val intChannel = intSource.to(intSink).run()
    (1 to 4) map { ii =>
      println(s"offer invocation for $ii at $elapsed")
      (ii, intChannel.offer(ii))
    } foreach { intFutureOfferResultPair =>
      val (ii, futureOfferResult) = intFutureOfferResultPair
      futureOfferResult onComplete { offerResult =>
        println(s"offer result for $ii: $offerResult at $elapsed")
      }
    }
    intChannel.complete()

    intChannel.watchCompletion.onComplete { _ => system.terminate() }
  }

offer invocation for 1 at 72
offer invocation for 2 at 77
offer invocation for 3 at 77
offer invocation for 4 at 77
offer result for 1: Success(Enqueued) at 90
processing 1 at 1084
offer result for 2: Success(Enqueued) at 1084
processing 2 at 2084
offer result for 3: Success(Enqueued) at 2084
processing 3 at 3084
offer result for 4: Success(Enqueued) at 3084
processing 4 at 4084