Keep组合的含义？

Question

我试图在 akka 流中的 Keep 组合下创建以下示例：

import java.nio.file.Paths

import akka.NotUsed
import akka.actor.ActorSystem
import akka.stream.{ActorMaterializer, IOResult}
import akka.stream.scaladsl.{FileIO, Flow, Keep, Sink, Source}
import akka.util.ByteString

import scala.concurrent.Future
import scala.util.{Failure, Success}

object FileConsumer extends App {

  implicit val system = ActorSystem("reactive-tweets")
  implicit val materializer = ActorMaterializer()

  val source: Source[Int, NotUsed] = Source(1 to 100)
  val factorials = source.scan(BigInt(1))((acc, next) => acc * next)

  val result: Future[IOResult] =
    factorials.map(_.toString).runWith(lineSink("factorial2.txt"))

  implicit val ec = system.dispatcher
  result.onComplete {
    case Success(v) => println(s"Fileinfo ${ v.count }")
    case Failure(e) => println(e)
  }

  def lineSink(filename: String): Sink[String, Future[IOResult]] =
    Flow[String].map(s => ByteString(s + "\n")).toMat(FileIO.toPath(Paths.get(filename)))(Keep.right)


}

在 akka streams website 上面写着：

The resulting blueprint is a Sink[String, Future[IOResult]], which means that it accepts strings as its input and when materialized it will create auxiliary information of type Future[IOResult] (when chaining operations on a Source or Flow the type of the auxiliary information—called the “materialized value”—is given by the leftmost starting point; since we want to retain what the FileIO.toPath sink has to offer, we need to say Keep.right).

但是，当我想将 ByteString 保留在左侧时，我试过：

  def lineSink2(filename: String): Sink[String, Future[ByteString]] =
Flow[String].map(s => ByteString(s + "\n")).toMat(Sink.foreach(println))(Keep.left)

但它根本无法编译。

我也不懂：

is given by the leftmost starting point

最左边的起点是Flow吗？

我想，我还不明白Keep的想法。

Answer 1

Sink.foreach的定义如下：

def foreach[T](f: T ⇒ Unit): Sink[T, Future[Done]]

这意味着物化价值是未来[完成]

在流量的情况下你有：

 val value: Flow[String, ByteString, NotUsed] = Flow[String].map(s => ByteString(s + "\n"))

其物化值为NotUsed

在这种情况下：

Keep.left - 未使用 - 源或流的物化值

Keep.right - 未来[完成] - 水槽的物化价值

Keep.both -（未使用，未来[完成]）

重要的事实是，在许多情况下物化值不是流经流的元素的值，而是

诊断信息
流状态
其他关于流的信息

Keep组合的含义？

The meaning of the Keep combination?

scala

akka

akka-stream