Play / Scala JSON 格式

我有一个接受的值类Either，我想生成 Play for Scala v2.5.6 JSONFormat for:

import org.joda.time.{DateTime, Duration}

case class When(when: Either[DateTime, Duration]) extends AnyVal

我想我有writes方法想出来了；我遇到的问题是reads方法。我尝试了两种方法，但由于不同的原因都失败了。

尝试#1，显示reads and writes方法：

import play.api.libs.json._
import play.api.libs.json.Json.obj

object When {    
  def apply(dateTime: DateTime): When = When(Left(dateTime))

  def apply(duration: Duration): When = When(Right(duration))

  implicit val whenFormat = new Format[When] {
    def reads(json: JsValue): JsResult[When] = {
      val reads = (__ \ "dateTime").read[Long] { (millis: Long) =>
        When(Left(new DateTime(millis)))
      } | (__ \ "duration").read[Long] { (millis: Long) =>
        When(Right(new Duration(millis)))
      }
      reads.reads(json)
    }

    def writes(o: When): JsValue = obj(
      o.when.fold(
        duration => "duration" -> duration.getMillis,
        dateTime => "dateTime" -> dateTime.getMillis
      )
    )
  }
}

错误消息是：

overloaded method value read with alternatives:
[error]   (t: Long)play.api.libs.json.Reads[Long] <and>
[error]   (implicit r: play.api.libs.json.Reads[Long])play.api.libs.json.Reads[Long]
[error]  cannot be applied to (Long => When)
[error]       val reads = (__ \ "dateTime").read[Long] { (millis: Long) =>

[error] overloaded method value read with alternatives:
[error]   (t: Long)play.api.libs.json.Reads[Long] <and>
[error]   (implicit r: play.api.libs.json.Reads[Long])play.api.libs.json.Reads[Long]
[error]  cannot be applied to (Long => When)
[error]       } | (__ \ "duration").read[Long] { (millis: Long) =>

尝试#2，仅显示reads method:

def reads(json: JsValue): JsResult[When] =
  JsSuccess(
    When(Left(new DateTime((__ \ "dateTime").read[Long]))) ||
    When(Right(new Duration((__ \ "duration").read[Long])))
  )

错误信息是：

value || is not a member of When
[error]  Note: implicit value whenFormat is not applicable here because it comes after the application point and it lacks an explicit result type
[error] Error occurred in an application involving default arguments.

我只想要一些有用的东西，我不在乎使用什么方法（即使是我没有展示的方法），只要它是可维护和高效的。了解每种方法的问题所在也会很有帮助。

以下是如何执行此操作的工作示例：

import org.joda.time.{DateTime, Duration}
import play.api.libs.functional.syntax._
import play.api.libs.json.Reads._
import play.api.libs.json._

object When {
  def apply(dateTime: DateTime): When = When(Left(dateTime))

  def apply(duration: Duration): When = When(Right(duration))

  val reads: Reads[When] = 
    (__ \ "dateTime").read[Long].map(millis => When(Left(new DateTime(millis)))) |
    (__ \ "duration").read[Long].map(millis => When(Right(new Duration(millis))))

  val writes: Writes[When] = new Writes[When] {
    override def writes(o: When): JsValue = Json.obj(
      o.when.fold(
        duration => "duration" -> duration.getMillis,
        dateTime => "dateTime" -> dateTime.getMillis
      )
    )
  }

  implicit val format = Format(reads, writes)
}

基本上你应该映射读数

(__ \ "dateTime").read[Long]

给你Reads[Long]，然后您可以将结果映射到When。你只是传递参数。该参数可以是Long，忽略读取的内容并返回该值，或者implicit读了很久你可能不想改变并且应该让它保持隐式。

因此，以类似的方式，您可以创建另一个持续时间的读取并将它们与替代（|）完成后，您已阅读完毕。

你的第二种方法没有意义。要么使用读取并组合它们，要么只是手动检查是否存在某些内容，如果没有则返回不同的结果，但不值得这样做，只需使用默认方法即可。