[英]Argonaut: decoding a polymorphic array
我正在嘗試編寫DecodeJson[T]
的JSON對象包含一個不同“類型”的數組(意味着其元素的JSON結構不同)。 唯一的共同特征是type
字段,可用於區分類型。 所有其他領域都不同。 例:
{
...,
array: [
{ type: "a", a1: ..., a2: ...},
{ type: "b", b1: ...},
{ type: "c", c1: ..., c2: ..., c3: ...},
{ type: "a", a1: ..., a2: ...},
...
],
...
}
使用argonaut,是否可以將JSON數組映射到Scala Seq[Element]
,其中Element
是ElementA
, ElementB
等類型的合適案例類的超類型?
我用play-json
做了同樣的事情,這很容易(基本上是一個Reads[Element]
,用於評估type
字段並相應地轉發到更具體的Reads
)。 但是,我找不到與argonaut這樣做的方法。
編輯:示例
Scala類型(我希望使用):
case class Container(id: Int, events: List[Event])
sealed trait Event
case class Birthday(name: String, age: Int) extends Event
case class Appointment(start: Long, participants: List[String]) extends Event
case class ... extends Event
JSON實例(不在我的控制之下):
{
"id":1674,
"events": {
"data": [
{
"type": "birthday",
"name": "Jones Clinton",
"age": 34
},
{
"type": "appointment",
"start": 1675156665555,
"participants": [
"John Doe",
"Jane Doe",
"Foo Bar"
]
}
]
}
}
您可以創建一個小函數來幫助您構建處理此格式的解碼器。
請參閱下面的示例。
import argonaut._, Argonaut._
def decodeByType[T](encoders: (String, DecodeJson[_ <: T])*) = {
val encMap = encoders.toMap
def decoder(h: CursorHistory, s: String) =
encMap.get(s).fold(DecodeResult.fail[DecodeJson[_ <: T]](s"Unknown type: $s", h))(d => DecodeResult.ok(d))
DecodeJson[T] { c: HCursor =>
val tf = c.downField("type")
for {
tv <- tf.as[String]
dec <- decoder(tf.history, tv)
data <- dec(c).map[T](identity)
} yield data
}
}
case class Container(id: Int, events: ContainerData)
case class ContainerData(data: List[Event])
sealed trait Event
case class Birthday(name: String, age: Int) extends Event
case class Appointment(start: Long, participants: List[String]) extends Event
implicit val eventDecoder: DecodeJson[Event] = decodeByType[Event](
"birthday" -> DecodeJson.derive[Birthday],
"appointment" -> DecodeJson.derive[Appointment]
)
implicit val containerDataDecoder: DecodeJson[ContainerData] = DecodeJson.derive[ContainerData]
implicit val containerDecoder: DecodeJson[Container] = DecodeJson.derive[Container]
val goodJsonStr =
"""
{
"id":1674,
"events": {
"data": [
{
"type": "birthday",
"name": "Jones Clinton",
"age": 34
},
{
"type": "appointment",
"start": 1675156665555,
"participants": [
"John Doe",
"Jane Doe",
"Foo Bar"
]
}
]
}
}
"""
def main(args: Array[String]) = {
println(goodJsonStr.decode[Container])
// \/-(Container(1674,ContainerData(List(Birthday(Jones Clinton,34), Appointment(1675156665555,List(John Doe, Jane Doe, Foo Bar))))))
}
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