如何在使用Akka Client Side Web Sockets时检测连接被拒绝

Question

我写了一个小样本程序（包括在下面），演示了我与Akka的问题。如果我试图在Akka尝试将连接升级到WebSocket的阶段之前尝试连接的服务器出现问题，这对代码是如何可见的？当我运行这个例子时，我可以看到一个调试语句，表明连接被拒绝但Future没有返回，最终Await.result会超时。

import akka.Done
import akka.actor.ActorSystem
import akka.http.scaladsl.Http
import akka.http.scaladsl.model.StatusCodes
import akka.http.scaladsl.model.ws.{Message, TextMessage, WebSocketRequest, WebSocketUpgradeResponse}
import akka.stream.ActorMaterializer
import akka.stream.scaladsl.{Flow, Keep, Sink, Source}
import com.typesafe.config.ConfigFactory

import scala.concurrent.duration._
import scala.concurrent.{Await, Future}
import scala.util.control.NonFatal

object WebSocketClientProblem {
  def main(args: Array[String]) : Unit = {
    try {
      implicit val sys = ActorSystem("websocket",ConfigFactory.parseString("akka.loglevel = \"DEBUG\""))
      implicit val mat = ActorMaterializer()
      import sys.dispatcher
      val incoming = Sink.foreach[Message](m => println(m))

      val outgoing = Source.single(TextMessage.Strict("Hello"))

      val flow: Flow[Message, Message, Future[WebSocketUpgradeResponse]] = Http().webSocketClientFlow(WebSocketRequest("ws://localhost:8081/echo"))

      val upgradeResponse = outgoing
        .viaMat(flow)(Keep.right)
        .toMat(incoming)(Keep.left)
        .run()

      val connected: Future[Done.type] = upgradeResponse.flatMap { upgrade =>
        if(upgrade.response.status == StatusCodes.OK || upgrade.response.status == StatusCodes.SwitchingProtocols) {
          Future.successful(Done)
        } else {
          println("switch to websocket was not successful")
          throw new RuntimeException(s"Connection failed: ${upgrade.response}")
        }
      }

      // When the server is not listening how does the Connection Refused error get made visible to the code?
      val r = Await.result(connected, 30 seconds)
      println(s"Result was [$r]")
    } catch {
      case NonFatal(e) =>
        println(s"Caught ${e.getMessage}")
    }
  }
}