scala - How can I use and return Source queue to caller without materializing it? -

i'm trying use new akka streams , wonder how can use , return source queue caller without materializing in code ?

imagine have library makes number of async calls , returns results via source. function looks this

def findarticlesbytitle(text: string): source[string, sourcequeue[string]] = {    val source = source.queue[string](100, backpressure)    source.mapmaterializedvalue { case queue =>      val url = s"http://.....&term=$text"     httpclient.get(url).map(httpresponsetosprayjson[searchresponse]).map { v =>       v.idlist.foreach { id =>         queue.offer(id)       }        queue.complete()     }   }    source } 

and caller might use this

// there implicit actormaterializer somewhere val stream = plugin.findarticlesbytitle(title) val results = stream.runfold(list[string]())((result, article) => article :: result) 

when run code within mapmaterializedvalue never executed.

i can't understand why don't have access instance of sourcequeue if should caller decide how materialize source.

how should implement ?

in code example you're returning source instead of return value of source.mapmaterializedvalue (the method call doesn't mutate source object).