Flow.fromSinkAndSource提供了一种方便的方法来组装一个由flow
作为输入组成的sink
和一个source
作为其未连接的输出,这可以通过下图(API链接中提供)得到最好的说明:
+----------------------------------------------+
| Resulting Flow[I, O, NotUsed] |
| |
| +---------+ +-----------+ |
| | | | | |
I ~~>| Sink[I] | [no-connection!] | Source[O] | ~~> O
| | | | | |
| +---------+ +-----------+ |
+----------------------------------------------+
如@ gabrielgiussi的回答所示,它通常用于需要将现有source
(或flow
)的输出“切换”到某个不同输出的情况 - 用于测试目的或什么不是。这是一个简单的例子:
import akka.actor.ActorSystem
import akka.stream.scaladsl._
implicit val system = ActorSystem("system")
implicit val materializer = ActorMaterializer()
val switchFlow = Flow.fromSinkAndSource( Sink.ignore, Source(List("a", "b", "c")) )
Source(1 to 5).via(switchFlow).runForeach(println)
// res1: scala.concurrent.Future[akka.Done] = Future(Success(Done))
// a
// b
// c
值得注意的是,该方法的“Mat”版本fromSinkAndSourceMat有一些有趣的用例。一个例子是使用它来保持half-closed
WebSockets打开,使用Source.maybe[T]
维护一个Promise[Option[T]]
作为物化值,当想要关闭连接时将完成。以下是Akka-http WebSockets client support文档中相关部分的示例代码:
// using Source.maybe materializes into a promise
// which will allow us to complete the source later
val flow: Flow[Message, Message, Promise[Option[Message]]] =
Flow.fromSinkAndSourceMat(
Sink.foreach[Message](println),
Source.maybe[Message])(Keep.right)
val (upgradeResponse, promise) =
Http().singleWebSocketRequest(
WebSocketRequest("ws://example.com:8080/some/path"),
flow)
// at some later time we want to disconnect
promise.success(None)
也许在某些情况下,您只需要提供Flow,在某些情况下,您需要NoOp Flow。然后你可以做到
Flow.fromSinkAndSource(Sink.ignore,Source.empty)
或者忽略Source中的每个元素并使用另一个元素
Flow.fromSinkAndSource(Sink.ignore,Source.tick(1.second,1.second,"something"))