为特征播放 2.1 Json 序列化？

Question

我有这个：

package models

import play.api.libs.json._
import play.api.libs.functional.syntax._

object ModelWrites {
    implicit val tmoWrites= Json.writes[TestModelObject]
    implicit val ihWrites = Json.writes[IntHolder]
}

case class TestModelObject(s1:String, s2:String)

case class IntHolder(i1:Int, i2:Int)

trait HasInts {
    val ints: List[IntHolder]
}

当我这样做时：

scala> val tmo = new TestModelObject("hello", "world") with HasInts {
  val ints = List(IntHolder(1,2), IntHolder(3,4))
}

scala> Json.toJson(tmo)
res0: play.api.libs.json.JsValue = {"s1":"hello","s2":"world"}

如何隐式序列化混合的 val 'ints'？喜欢：

scala> val someInts = List(IntHolder(8,9), IntHolder(10,11))
someInts: List[models.IntHolder] = List(IntHolder(8,9), IntHolder(10,11))

scala> Json.toJson(someInts)
res1: play.api.libs.json.JsValue = [{"i1":8,"i2":9},{"i1":10,"i2":11}]

注意：如果我尝试：implicit val hasIntsWrites = Json.writes[HasInts] 我（预期？）得到：

[error] Models.scala:10: No unapply function found
[error]     implicit val hasIntsWrites = Json.writes[HasInts]
[error]                                             ^

Answer 1

您将无法在此处直接使用the experimental "Inception" feature (Json.writes[...])，因为这仅适用于案例类。但是，您可以在 Inception 可以 提供的 Writes 实例的基础上构建，只需很少的样板即可完成您想要的工作。

请注意，我忽略了这样的问题：在实例化这样的案例类时混合特征是否是一个好主意——它可能不是——但我在这里给出的方法也适用于更一般的情况.

首先是类和导入（这里没有变化）：

case class TestModelObject(s1: String, s2: String)
case class IntHolder(i1: Int, i2: Int)
trait HasInts { val ints: List[IntHolder] }

import play.api.libs.json._
import play.api.libs.functional.syntax._

现在我们需要将所有优先级较低的实例放入一个特征中，以确保编译器会选择正确的实例，因为TestModelObject with HasInts 是TestModelObject 和HasInts 的子类型：

trait LowPriorityWritesInstances {
  implicit val tmoWrites = Json.writes[TestModelObject]
  implicit val ihWrites = Json.writes[IntHolder]

  implicit object hiWrites extends OWrites[HasInts] {
    def writes(hi: HasInts) = Json.obj("ints" -> hi.ints)
  }
}

现在是重头戏：

object WritesInstances extends LowPriorityWritesInstances {
  implicit val tmowhiWrites = new Writes[TestModelObject with HasInts] {
    def writes(o: TestModelObject with HasInts) =
      tmoWrites.writes(o) ++ implicitly[OWrites[HasInts]].writes(o)
  }
}

我们完成了：

scala> import WritesInstances._
import WritesInstances._

scala> val tmo = new TestModelObject("hello", "world") with HasInts {
     |   val ints = List(IntHolder(1, 2), IntHolder(3, 4))
     | }

scala> println(Json.toJson(tmo))
{"s1":"hello","s2":"world","ints":[{"i1":1,"i2":2},{"i1":3,"i2":4}]}

根据需要。