How to create a Source in Akka with Actor to serve WebSocket connection - scala

Question

I try to create a simple chat with Akka Actors, Streams, and WebSockets. I want to create separate Sink and Source to serve WebSocket connection.
I create a chat room per roomId:
path("ws" / "room" / IntNumber) { roomId => {
println(s"Connecting to room $roomId")
parameter("userName") { userName =>
extractUpgradeToWebSocket { upgrade =>
val chatRoom = ChatRooms.findOrCreate(roomId)
val (sink, source) = chatRoom.getSinkAndSource(userName)
complete(upgrade.handleMessagesWithSinkSource(sink, source))
}
}
}
Chats create and pass an output-generating Source[Message, _] and an input-receiving Sink[Message, _] to handleMessagesWithSinkSource method.
I have a problem to create a working source with Messages populated by my Actor (Source.actorRefWithBackpressure should allow it). Sink and source2 work as expected but source does not:
def getSinkAndSource(name: String) = {
val source = Source.actorRefWithBackpressure[Message](AckMessage, {
case _: Success => CompletionStrategy.draining
}, PartialFunction.empty)
val wsActorRef = source.to(Sink.ignore).run()
val receiver = actorSystem.actorOf(Props(classOf[ChatParticipantActor], name, ChatRoomActor, wsActorRef))
val sink = Sink.actorRefWithBackpressure(receiver, InitMessage, AckMessage, OnCompleteMessage, onErrorMessage)
val source2 = Source.tick(FiniteDuration(1, TimeUnit.SECONDS), FiniteDuration(1, TimeUnit.SECONDS), {
implicit val writer = shared.Protocol.chatMessageRW
TextMessage(write(ChatMessage(sender = "Bob", message = "Hi")))
})
(sink, source2) // This works
(sing, source) // This does not
}
How can I make such a Source that can be integrated with an Akka Actor?

Answer 1

Source.actorRef and Source.actorRefWithBackpressure both create an actor that is provided as a materialized value. The server-side WS API doesn't give easy access to that materialized value, though.
The easiest way to get the actorRef that has been created is to use mapMaterializedValue:
val source = Source.actorRefWithBackpressure[Message](AckMessage, {
case _: Success => CompletionStrategy.draining
}, PartialFunction.empty).mapMaterializedValue { actorRef =>
// Do something with the ActorRef here. Messages you want to send to this client will have to be sent to this ActorRef.
// e.g.: chatRoom ! NewClient(actorRef)
}
A previous version of my chat room example was still Actor-based and shows this in a full example:
https://github.com/jrudolph/akka-http-scala-js-websocket-chat/blob/b01b234376c4984dce19effcaf001a9ffb4c6981/backend/src/main/scala/example/akkawschat/Chat.scala#L64