Question

Play Framework的iteratee库定义了一个方法Enumerator.fromCallback，它允许根据Future的结果生成元素：

http://www.playframework.com/documentation/2.2.x/Enumerators

def fromCallback[E](
  retriever: () => Future[Option[E]],
  onComplete: () => Unit = () => (),
  onError: (String, Input[E]) => Unit = (_: String, _: Input[E]) => ()
): Enumerator[E]

您可以在此处看到用于从Web服务提供分页结果的一个很好的示例：

http://engineering.klout.com/2013/01/iteratees-in-big-data-at-klout/

def pagingEnumerator(url:String):Enumerator[JsValue]={
  var maybeNextUrl = Some(url) //Next url to fetch
  Enumerator.fromCallback[JsValue] ( retriever = {
    val maybeResponsePromise =
      maybeNextUrl map { nextUrl=>  
        WS.url(nextUrl).get.map { reponse =>
          val json = response.json
          maybeNextUrl = (json \ "next_url").asOpt[String]
          val code = response.status //Potential error handling here
          json
        }   
      }

    /* maybeResponsePromise will be an Option[Promise[JsValue]].
     * Need to 'flip' it, to make it a Promise[Option[JsValue]] to
     * conform to the fromCallback constraints */
    maybeResponsePromise match {
      case Some(responsePromise) => responsePromise map Some.apply
      case None => PlayPromise pure None
    }
  })
}

用于执行相同操作的等效scalaz-stream代码是什么？我非常确定可以使用Process.emit或Process.await或Process.eval来完成，但我很乐意看到一个成功的例子。这可能还需要将scala Future提升为scalaz任务，这里有一个答案：

Convert scala 2.10 future to scalaz.concurrent.Future // Task

如果它使事情变得更简单，我们可以忽略scala Future vs scalaz Task位并假设我们有一个Task。

Answer 1

要从scala.concurrent.Future获取scalaz.concurrent.Task，您可以使用Task.async，当您手头有任务时，您可以这样做：

  import java.util.concurrent.atomic.AtomicInteger
  import scalaz.concurrent.Task
  import scalaz.stream.Process.End
  import scalaz.stream._

  val cnt = new AtomicInteger(0)

  val task: Task[String] = Task {
    if (cnt.incrementAndGet() <= 10) s"Task ${cnt.get}" else throw End
  }

  Process.repeatEval(task).runLog.run.foreach(println)

什么是与Play Framework的Enumerator.fromCallback等效的scalaz-stream

1 个答案: