Accepted answer

The correct way of testing Sources, Flows and Sinks is by using test probes.

Given following source the logic of witch you want to test

val source = Source(Seq("TestValue")).collect {
  case s @ "TestValue" => Right[String, String](s)
  case _               => Left[String, String]("error")

(I know that Left will never trigger here but it's just an example)

Now, you can define a TestSink that is connected to the given source. The resulting graph is executed in following way

"assert correct value" in {
  implicit val system: ActorSystem = ???
  val probe = source.toMat(TestSink.probe)(Keep.right).run()

TestSink.probe is a sink that materializes into a TestSubscriber.Probe[T] and it provides controls over the stream. Since it's a sink, it needs to emit demand for an element. It does it via request(1) or request one element. Then it's followed by assert sink.expectNext(Right("TestValue")) that checks that correct value is received.

There is also a counterpart called TestSource.probe that allows testing of a Sink.

And by combing both of them, you can test a Flow.

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