Scala play Json-使用类型参数读取转换器

Question

我想用这样的类型参数创建一个JSON reads转换器：

case class AClass(name: String, age: Int)
case class AClass(name: String)

object GeneralReadsType extends Enumeration {
  type GeneralReadsType = Value
  val AReadType = Value("atype")
  val BReadType = Value("btype")
}

implicit val repoReads: Reads[GeneralReadsType.GeneralReadsType] = {

  GeneralReadsType match {
    case GeneralReadsType.AReadType => (
      (JsPath \ "name").read[String] and
        (JsPath \ "age").read[Int]
      ) (GeneralReadsType.AReadType.apply _)


    case GeneralReadsType.BReadType => (
      (JsPath \ "name").read[String]
      ) (GeneralReadsType.BReadType.apply _)

  }
}

但是当我尝试使用它时

def getResponse[GeneralReadsType](request: WSRequest): Future[List[GeneralReadsType]] = {
      request.get().map {
        case response if response.status == 200 => {
          (response.json).validate[List[GeneralReadsType]] match {
            case JsSuccess(items: List[GeneralReadsType], _) => items
            case _: JsError => List[GeneralReadsType]()
          }
        }
        case _ => List[GeneralReadsType]()
      }
    }


getResponse[AReadType](request)

我收到编译错误

找不到类型为List [GeneralReadsT​​ype]的Json解串器。尝试为此类型实现隐式的Reads或Format。

所以，应该如何实现？

谢谢！

Answer 1

我发现最好将格式对象作为隐式字段放置在对象本身（或case类的伴随对象中）中。同样，当对Enumeration对象使用Enumeration复数形式，而对值使用单数形式时，则使其读起来更好。因此：

object GeneralReadsTypes extends Enumeration {
  type GeneralReadsType = Value
  val AReadType = Value("atype")
  val BReadType = Value("btype")

  implicit val repoReads: Reads[GeneralReadsType] = {
    ((JsPath \ "name").read[String] and
      (JsPath \ "age").readNullable[Int]).apply { (_, maybeAge) =>
      maybeAge.fold(BReadType)( _ => AReadType)
    }
  }  
}

或者如果您发现功能样式太陌生：
object GeneralReadsTypes extends Enumeration {
  type GeneralReadsType = Value
  val AReadType = Value("atype")
  val BReadType = Value("btype")

  implicit val repoReads: Reads[GeneralReadsType] = {
    ((JsPath \ "name").read[String] and
      (JsPath \ "age").readNullable[Int]).apply { (_, maybeAge) =>
      if (maybeAge.isDefined) AReadType
      else BReadType
    }
  }
}