watchTermination is not triggered in an akka-http flow - scala

Question

I am currently trying to build an akka-http websocket connection that can:
broadcast to all connected clients
answer to certain requests of the clients
This is how I create my flow so far:
// keeps a list of all actors so I can broadcast to them
var actors: List[ActorRef] = Nil
private def wsFlow(implicit materializer: ActorMaterializer): Flow[ws.Message, ws.Message, NotUsed] = {
val (actor, source) = Source.actorRef[String](10, akka.stream.OverflowStrategy.dropTail)
.toMat(BroadcastHub.sink[String])(Keep.both)
.run()
// this never triggers
source.watchTermination() { (m, f) =>
f.onComplete(r => println("TERMINATION: " + r.toString))
actors = actors diff actor :: Nil
m
}
actors = actor :: actors
val wsHandler: Flow[ws.Message, ws.Message, NotUsed] =
Flow[ws.Message]
.merge(source)
.map {
case TextMessage.Strict(tm) => handleMessage(actor, tm)
case _ => TextMessage.Strict("Ignored message!")
}
wsHandler
}
def broadcast(msg: String): Unit = {
actors.foreach(_ ! TextMessage.Strict(msg))
}
The - hopefully - last problem I am encountering is that the watchTermination callback never triggers (I am never getting a "TERMINATION: ..." message on my console). Why is that? And how is it possible to detect when a client is leaving (so I can remove him from my actors list)?

Answer 1

I figured out how to do it:
val wsHandler: Flow[ws.Message, ws.Message, NotUsed] = Flow[ws.Message]
.watchTermination() { (m, f) =>
f.onComplete(r => {
println("Client left: " + r.toString)
actors = actors diff actor :: Nil
}
)
m
}
.merge(source)
.map {
case TextMessage.Strict(tm) => handleMessage(actor, tm)
case _ => TextMessage.Strict("Ignored message!")
}