如何使用Continuations拆分和分派异步控制流？

Question

我有一个异步控制流，如下所示：

ActorA ! DoA(dataA, callback1, callbackOnErrorA)

def callback1() = {
  ...
  ActorB ! DoB(dataB, callback2, callbackOnErrorB)
}

def callback2() = {
  ActorC ! DoC(dataC, callback3, callbackOnErrorC)
} 

...

我如何将这个流程分成几个部分(延续)，并在维护整体状态的同时，将这些部分顺序地分派给不同的参与者(或线程/任务)？

任何提示都很感谢，谢谢

Answer 1

这非常简单，但展示了如何在三个参与者之间拆分单个控制流，将状态传递给每个参与者：

package blevins.example

import scala.continuations._
import scala.continuations.ControlContext._
import scala.actors.Actor._
import scala.actors._

object App extends Application {

  val actorA, actorB, actorC = actor {
    receive {
      case f: Function1[Unit,Unit] => { f() }
    }
  }

  def handle(a: Actor) = shift { k: (Unit=>Unit) =>
    a ! k
  }

  // Control flow to split up
  reset {
      // this is not handled by any actor
      var x = 1
      println("a: " + x)

      handle(actorA)  // actorA handles the below
      x += 4
      println("b: " + x)

      handle(actorB) // then, actorB handles the rest
      var y = 2
      x += 2
      println("c: " + x)

      handle(actorC) // and so on...
      y += 1
      println("d: " + x + ":" + y)
  }

}

Answer 2

我喜欢使用scalaz.concurrent.Promise。这个例子与你问题中的例子并不完全一样，但它给了你一些启发。

object Async extends Application {
  import scalaz._
  import Scalaz._
  import concurrent._
  import concurrent.strategy._
  import java.util.concurrent.{ExecutorService, Executors}

  case class ResultA(resultb: ResultB, resulta: ResultC)
  case class ResultB()
  case class ResultC()

  run

  def run {
    implicit val executor: ExecutorService = Executors.newFixedThreadPool(8)
    import Executor.strategy

    val promiseA = doA
    println("waiting for results")
    val a: ResultA = promiseA.get
    println("got " + a)
    executor.shutdown    
  }

  def doA(implicit s: Strategy[Unit]): Promise[ResultA] = {
    println("triggered A")
    val b = doB
    val c = doC
    for {bb <- b; cc <- c} yield ResultA(bb, cc)
  }

  def doB(implicit s: Strategy[Unit]): Promise[ResultB] = {
    println("triggered B")
    promise { Thread.sleep(1000); println("returning B"); ResultB() }
  }

  def doC(implicit s: Strategy[Unit]): Promise[ResultC] = {
    println("triggered C")
    promise { Thread.sleep(1000); println("returning C"); ResultC() }
  }
}

输出：

triggered A
triggered B
triggered C
waiting for results
returning B
returning C
got ResultA(ResultB(),ResultC())

您可以在Runar的这个presentation中找到对Scalaz并发性的介绍。

这种方法不像Actors那样灵活，但组合得更好，不会死锁。

Answer 3

看Akka's Futures and how to compose them或scalaz's Promises，它们几乎是一样的，只有细微的差别。