Given the idea of the flow:
infinite
stream
|
|
v
trigger --> runnable graph (ActorRef, ActorRef) --> ...
|
v
join | <-- onComplete <--
|
v
The first idea how to implement the missing block of handling the trigger and join stage in a stream was to use AsyncStage and actor ask pattern. Using https://github.com/akka/akka/blob/releasing-akka-stream-and-http-experimental-1.0/akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowMapAsyncSpec.scala as a reference.
Not sure if this is the ideal way of implementing the stage though. It lacks type safety and introduced annoying type casting.