播放框架：JSON 阅读单属性案例 class

Question

我正在尝试为包含单个属性的案例 class 创建 JSON 隐式读取，但我收到错误 "Reads[Nothing] doesn't conform to expected type"。这是代码：

import play.api.libs.functional.syntax._
import play.api.libs.json.Reads._
import play.api.libs.json.{JsPath, Json, Reads}

case class Feedback(message: String)
object Feedback {
  implicit val reads: Reads[Feedback] = (
      (JsPath \ "message").read[String](maxLength[String](2000))
    )(Feedback.apply _)
}

为什么这不起作用？如果我将额外的属性添加到案例 class 和多个 .read 调用与 and 连接它工作......

Answer 1

Json 组合器不适用于单字段情况 class.

您可以执行以下操作：

import play.api.libs.json.Reads._
import play.api.libs.json.{__, Reads}

case class Feedback(message: String)
object Feedback {
  implicit val reads: Reads[Feedback] = (__ \ "message")
    .read[String](maxLength[String](2000)).map {message => Feedback(message)}
}

这是因为当前宏实现的限制。您可以在这里阅读更多相关信息：Pacal is the writer of this API

播放框架：JSON 阅读单属性案例 class

Play framework: JSON Reads for a single-attribute case class

json

scala

playframework

play-json