akka-stream-如何在流/图中不同地对待流的最后一个元素 [英] akka-stream - How to treat the last element of a stream differently in a Flow/Graph

问题描述

我正在尝试实现Akka流Flow，它将把JSON对象流转换为单个JSON对象数组的流.我可以使用Concat在元素之间插入逗号，然后在Zip之前添加"["，但是不能弄清楚如何不插入最后一个逗号.

I'm trying to implement an Akka Streams Flow that will convert a stream of JSON objects to a stream of a single array of JSON objects. I can use Concat to add an "[" before and "]" after, as well as Zip to insert commas in between elements, but I can't figure out how to not insert the final comma.

到目前为止，我的代码是:

The code I have so far is:

trait JsonStreamSupport {

  protected def toJsonArrayString[T : Writes] = Flow[T].map(Json.toJson(_)).map(_.toString()).via(jsonArrayWrapper)


  private[this] val jsonArrayWrapper: Flow[String, String, NotUsed] = Flow.fromGraph(GraphDSL.create() { implicit b =>
    import GraphDSL.Implicits._
    val start = Source.single("[")
    val comma = Source.repeat(",")
    val end = Source.single("]")
    val concat = b.add(Concat[String](3))
    val zip = b.add(Zip[String,String])

    comma ~> zip.in1
    start ~> concat.in(0)
    zip.out.map({case (msg,delim) => msg + delim}) ~> concat.in(1)
    end ~> concat.in(2)
    FlowShape(zip.in0, concat.out)
  })
}

当前输出为:
[{"key":"value},{"key","value"},]
但我需要成为
[{"key":"value},{"key","value"}](不带最后一个逗号)，
其中数组的每个元素仍然是流的不同元素，因此可以例如分别通过分块HTTP发送.

Currently the output is:
[{"key":"value},{"key","value"},]
but I need it to be
[{"key":"value},{"key","value"}] (without final comma),
where each element of the array is still a distinct element of the stream so can be, for example, sent over chunked HTTP separately.

推荐答案

刚刚发现了有关intersperse的信息，这正是您所需要的，并且比我最初建议的要简单得多:

just found out about intersperse which is exactly what you need, and much simpler than what I suggested in the first place:

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