专门处理Akka流的第一个元素 [英] Handle Akka stream's first element specially

问题描述

是否有一种特殊的方式来以特殊的方式处理Akka流的 Source 第一个元素？我现在所拥有的是：

Is there an idiomatic way of handling Akka stream's Source first element in a special way? What I have now is:

    var firstHandled = false
    source.map { elem =>
      if(!firstHandled) {
        //handle specially
        firstHandled = true
      } else {
        //handle normally
      }
    }

谢谢

推荐答案

虽然我通常会接受Ramon的回答，但是您也可以使用 prefixAndTail ，前缀为1，以及 flatMapConcat 来实现类似的效果：

While I would generally go with Ramon's answer, you could also use prefixAndTail, with a prefix of 1, together with flatMapConcat to achieve something similar:

val src = Source(List(1, 2, 3, 4, 5))
val fst = Flow[Int].map(i => s"First: $i")
val rst = Flow[Int].map(i => s"Rest:  $i")

val together = src.prefixAndTail(1).flatMapConcat { case (head, tail) =>
  // `head` is a Seq of the prefix elements, which in our case is
  // just the first one. We can convert it to a source of just
  // the first element, processed via our fst flow, and then
  // concatenate `tail`, which is the remainder...
  Source(head).via(fst).concat(tail.via(rst))
}

Await.result(together.runForeach(println), 10.seconds)
// First: 1
// Rest:  2
// Rest:  3
// Rest:  4
// Rest:  5

这当然不仅适用于第一个项目，而且适用于第一个 N 个项目，但前提是这些项目项目将被严格收集。

This of course works not just for the first item, but for the first N items, with the proviso that those items will be taken up as a strict collection.

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