关于 scala：复合流是否会产生循环？

Does composite flow make loop?

我想了解以下代码片段的工作原理：

1 2	val flow: Flow[Message, Message, Future[Done]] = Flow.fromSinkAndSourceMat(printSink, helloSource)(Keep.left)

两个人在这个线程上给出了非常精彩的解释。我了解复合流的概念，但它是如何在 websocket 客户端上工作的。

考虑以下代码：

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import akka.actor.ActorSystem
import akka.{ Done, NotUsed }
import akka.http.scaladsl.Http
import akka.stream.ActorMaterializer
import akka.stream.scaladsl._
import akka.http.scaladsl.model._
import akka.http.scaladsl.model.ws._

import scala.concurrent.Future

object SingleWebSocketRequest {
def main(args: Array[String]) = {
implicit val system = ActorSystem()
implicit val materializer = ActorMaterializer()
import system.dispatcher

// print each incoming strict text message
val printSink: Sink[Message, Future[Done]] =
Sink.foreach {
case message: TextMessage.Strict =>
println(message.text)
}

val helloSource: Source[Message, NotUsed] =
Source.single(TextMessage(“hello world!”))

// the Future[Done] is the materialized value of Sink.foreach
// and it is completed when the stream completes
val flow: Flow[Message, Message, Future[Done]] =
Flow.fromSinkAndSourceMat(printSink, helloSource)(Keep.left)

// upgradeResponse is a Future[WebSocketUpgradeResponse] that
// completes or fails when the connection succeeds or fails
// and closed is a Future[Done] representing the stream completion from above
val (upgradeResponse, closed) =
Http().singleWebSocketRequest(WebSocketRequest(“ws://echo.websocket.org”), flow)

val connected = upgradeResponse.map { upgrade =>
// just like a regular http request we can access response status which is available via upgrade.response.status
// status code 101 (Switching Protocols) indicates that server support WebSockets
if (upgrade.response.status == StatusCodes.SwitchingProtocols) {
Done
} else {
throw new RuntimeException(s“Connection failed: ${upgrade.response.status}”)
}
}

// in a real application you would not side effect here
// and handle errors more carefully
connected.onComplete(println)
closed.foreach(_ => println(“closed”))
}
}