Observable defer in Akka Streams

Question

I´m coming from ReactiveX and there we have the operator defer, in order to create an Observable and get the emission value once we have a subscriber.

Here in Akka Streams I was wondering if something like that exists:

 @Test def defer(): Unit = {
    var range = 0 to 10
    val graphs = Source(range)
      .to(Sink.foreach(println))
    range = 10 to 20
    graphs.run()
    Thread.sleep(2000)
  }

Having this code, even before we execute run(), changing the value of the range, the value is not changed since the blueprint is already created, and emits 0 to 10.

Is anything like Observable.defer in Akka Streams?

SOLUTION:

I found the solution, the solution is using lazy keyword, where we provide a function which to be executed once we run the stream.

I will keep the question just in case there´s a better way or someone else has the same question

  @Test def defer(): Unit = {
    var range = 0 to 10
    val graphs = Source.lazily(() => Source(range))
      .to(Sink.foreach(println))
    range = 10 to 20
    graphs.run()
    Thread.sleep(2000)
  }

Regards.

Answer 1

The simplest way would probably be Source.fromIterator(() => List(1).iterator) or something similar. In the Akka Streams API we opted to try to keep the minimal set of operators, so sometimes you may get into situations where the same is achievable in an one-liner, but would not have a direct counterpart with a name like in defer's case here. If you think it's a common enough thing please let us know on github.com/akka/akka and we could consider adding it as an API.

Note that there's also fromFuture and other ones, which while not directly related may be useful depending on your actual use-case (esp. when combined with a Promise etc).