scala - Scodec 组合器：标头包含用于区分类型的幻数

Question

我正在寻找一种方法来处理类似于以下示例的协议：

case class Request(bodyType: Int, foo: Int, bar: Int, body: RequestBody)

sealed trait RequestBody
case class Read(key: String) extends RequestBody
case class Write(key: String, value: Array[Byte]) extends RequestBody 

在这里，bodyType == 0将代表Read，bodyType != 0并将编码Write。请注意，有几个字段将鉴别器与鉴别值分开。

我看过一个 byte-ordering 的例子。但据我了解，这种“鱿鱼”编码的鉴别器不会往返。解决此类问题的正确方法是什么？

Answer 1

有几种方法可以做到这一点，但这是我用过的一种：

import scodec._
import scodec.codecs._
import scodec.bits._

case class Request(bodyType: Int, foo: Int, bar: Int, body: RequestBody)

sealed trait RequestBody
case class Read(key: String) extends RequestBody
object Read {
  implicit val codec: Codec[Read] = ("key" | utf8).as[Read]
  implicit val discriminator: Discriminator[RequestBody, Read, Int] = Discriminator(0)
}
case class Write(key: String, value: ByteVector) extends RequestBody
object Write {
  implicit val codec: Codec[Write] = {
    ("key"   | utf8  ) ::
    ("value" | bytes )
  }.as[Write]
  implicit val discriminator: Discriminator[RequestBody, Write, Int] = Discriminator(1)
}

object Request {
  implicit val codec: Codec[Request] = {
    ("bodyType" | uint16 ).flatPrepend { bodyType =>
    ("foo"      | uint16 ) ::
    ("bar"      | uint16 ) ::
    ("body"     | Codec.coproduct[RequestBody].discriminatedBy(provide(bodyType)).auto)
  }}.as[Request]
}