可以使用Akka Streams的fold和merge方法来高效地合并子流。

下面是一个使用fold和merge方法转换WebSocket帧为消息的示例代码：

import akka.actor.ActorSystem
import akka.stream._
import akka.stream.scaladsl._
import akka.util.ByteString

case class WebSocketFrame(data: ByteString)

object WebSocketFrame {
  val EndOfStream = WebSocketFrame(ByteString.empty)
}

case class Message(data: String)

object WebSocketFlow {
  def apply(): Flow[WebSocketFrame, Message, Any] = {
    val start = Flow[WebSocketFrame].fold(ByteString.empty)(_ ++ _.data)
    val complete = Flow[ByteString].map(_.utf8String).map(Message)

    Flow.fromGraph(GraphDSL.create() { implicit builder =>
      import GraphDSL.Implicits._

      val merge = builder.add(Merge[ByteString](2))
      val endOfStream = builder.add(Source.single(WebSocketFrame.EndOfStream))

      val bcast = builder.add(Broadcast[WebSocketFrame](2))

      bcast ~> start ~> merge
                  merge <~ endOfStream

      bcast ~> complete

      FlowShape(bcast.in, merge.out)
    })
  }
}

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

  val source = Source(List(
    WebSocketFrame(ByteString("Hello")),
    WebSocketFrame(ByteString("World")),
    WebSocketFrame(ByteString("!")),
    WebSocketFrame.EndOfStream
  ))

  val flow = WebSocketFlow()

  val sink = Sink.foreach[Message](println)

  source.via(flow).runWith(sink)
}

在这个示例代码中，通过fold方法将WebSocketFrame转换为ByteString，然后使用merge方法合并WebSocketFrame流和单个EndOfStream元素。最后，使用Broadcast将WebSocketFrame流拆分为两路，一路连接合并后的流并使用map将ByteString转换为Message，另一路直接将WebSocketFrame转换为Message并发送到下游。

当收到EndOfStream元素时，将流完成并输出拼接完成的消息