这个问题通常发生在没有正确地完成流处理时。为了解决这个问题，可以添加一个手动完成源的步骤。下面是一个使用Akka Streams的WebSocket示例，其中将手动完成源：

import akka.actor.ActorSystem
import akka.http.scaladsl.Http
import akka.http.scaladsl.model.{HttpRequest, HttpResponse}
import akka.http.scaladsl.model.headers.Accept
import akka.http.scaladsl.model.ws.{Message, UpgradeToWebSocket}
import akka.stream.ActorMaterializer
import akka.stream.scaladsl.{Sink, Source}
import akka.stream.stage.{GraphStage, GraphStageLogic, InHandler, OutHandler}
import akka.util.ByteString

import scala.concurrent.Future

object WebSocketExample extends App {
  implicit val system = ActorSystem()
  implicit val materializer = ActorMaterializer()

  val endpointUrl = "ws://localhost:8080"

  val request = HttpRequest().withUri(endpointUrl).withHeaders(Accept)

  val flow = Http().singleWebSocketRequest(UpgradeToWebSocket(request), outSubprotocol = None)
    .flatMap(webSocket => {
      val in = Sink.foreach[Message] {
        case message: Message.Text =>
          println(s"Incoming message: $message")
        case _ =>
          println("Unknown message received")
      }

      val out = Source.single(Message.Text("This is a message"))

      webSocket.handleMessagesWithSinkSource(in, out).andThen {
        case _ => println("WebSocket closed")
      }
    })

  flow.run()

  // Just to keep the example running
  Thread.sleep(10000)

  system.terminate()
}

在这个示例中，我们用一个计数器来计算向WebSocket发送的消息数。当达到指定数量时，我们会使用结束流的方式来手动完成源：

val out = Source(List(
    "Message 1",
    "Message 2",
    "Message 3"
  ))
  .map(s => Message.Text(s))
  .take(3)
  .watchTermination()((_, done) => {
    done.onComplete(_ => println("Flow completed"))
    Sink.ignore
  })

web