JSON 在 Play 应用程序中写入转换器不接受类型

Question

我有一个 Play 框架应用程序，想要创建一个 Writes 转换器，以便将某些模型转换为 JSON。我目前拥有的：

implicit val locationWrites = new Writes[Board] {
  def writes(board: Board) = Json.obj(
    "gameSize" -> board.n,
    "fields" -> Seq(FieldWithCoords((1,2,3), "field", true)) // error for this line
  )
}

implicit val fieldWrites = new Writes[FieldWithCoords] {
  def writes(field: FieldWithCoords) = Json.obj(
    "coords" -> field.coords.toString,
    "player" -> field.p, 
    "isHighlighted" -> field.highlighted
  )
}

不幸的是，我得到了错误：

type mismatch; found: Seq[util.FieldWithCoords] required: play.api.libs.json.Json.JsValueWrapper Note: implicit value locationWrites is not applicable here because it comes after the application point and it lacks an explicit result type

如您所见，FieldWithCoords 有一个转换。我该如何解决这个问题？感谢任何帮助，谢谢。

编辑：

"fields" 稍后应该包含多个项目，因此 Seq

Answer 1

编译器需要将您的 Seq[FieldWithCoords] 隐式转换为 JsValueWrapper。我能够通过提供显式结果类型来编译它，从而为它提供隐含函数启动所需的额外推动力。

implicit val fieldWrites: Writes[FieldWithCoords] = new Writes[FieldWithCoords] {
  def writes(field: FieldWithCoords) = Json.obj(
    "coords" -> field.coords.toString,
    "player" -> field.p, 
    "isHighlighted" -> field.highlighted
  )
}

implicit val locationWrites: Writes[Board] = new Writes[Board] {
  def writes(board: Board) = Json.obj(
    "gameSize" -> board.n,
   "fields" -> Seq(FieldWithCoords((1,2,3), "field", true)) // error for this line
  )
}

Answer 2

使用写入组合器被认为是比直接创建 JSON 更好的做法：

implicit lazy val fieldWrites: Writes[FieldWithCoords] = (
  (__ \ "coords").write[String] and
  (__ \ "player").write[String] and
  (__ \ "isHighlighted").write[Boolean]
).apply(field => (field.coords.toString, field.p, field.highlighted))

implicit lazy val locationWrites: Writes[Board] = (
  (__ \ "gameSize").write[Int] and
  (__ \ "fields").write[Seq[FieldWithCoords]]
).apply(board => (board.n, Seq(FieldWithCoords((1,2,3), "field", true))

JSON 在 Play 应用程序中写入转换器不接受类型

JSON writes converter in Play application doesn't accept type

json

scala

playframework

playframework-2.0