Scala Circe推论类

Question

我在消耗来自websocket的信息。它们有两种形式之一：

{"typ": "subscription", "channel": "BTC-USD"}

或

{"typ": "ticker", "price": 10213.42}

所以我有三个案例类和Circe的解码器一起使用：

case class GenericMessage(typ: String)

case class SubscriptionMessage(channel: String)

case class TickerMessage(price: Double) 

我怎样才能让Circe知道消息的类型，而不嵌套大量的解码尝试？

val message: String = ??? // From websocket
decode[GenericMessage](message) match {
    case Left(e) => ??? // Handle error
    case Right(decoded) => decoded.typ match {
        case "subscription" => decode[SubscriptionMessage](message) match {
            case Left(e) => ??? // Handle error
            case Right(subscriptionMessage) => ??? // Handle subscription message
         }
        case "ticker" =>  decode[TickerMessage](message) match {
            case Left(e) => ??? // Handle error
            case Right(tickerMessage) => ??? // Handle ticker message
         }
        case other => ??? // Handle unrecognized message
    }
}

Answer 1

多亏了@LuisMiguelMejiaSuarez。

我使用做了以下解决方案

sealed trait WebSocketMessage
final case class Subscriptions(typ: String, channel: String) extends WebSocketMessage
final case class Ticker(typ: String, price: Double) extends WebSocketMessage

implicit val config: Configuration = Configuration.default
  .withDiscriminator("type") // Tell circe that the 'type' attribute specifies which case class to decode to
  .copy(transformConstructorNames = _.toLowerCase) // Map 'ticker' string to 'Ticker' class name. Could also pass a more complex mapping function here to use arbitrary class names
  .copy(transformMemberNames = {
      case "typ" => "type" // The word 'type' is a reserved Scala, so class members are renamed to'typ' instead
      case other => Configuration.snakeCaseTransformation(other) // Tell circe to map product_ids field to productIds class member, for example.
  })

Answer 2

您可以尝试一下我的Circe ADT扩展：https://github.com/abdolence/circe-tagged-adt-codec

因此，就你的情况而言，应该是：

sealed trait Message

object Message {

  @JsonAdt("ticker")
  case class Ticker(price : BigDecimal) extends Message

  @JsonAdt("subscription")
  case class Subscription(channel: Channel) extends Message


implicit val encoder : Encoder[Message] = JsonTaggedAdtCodec.createEncoder[Message]("typ")

implicit val decoder : Decoder[Message] = JsonTaggedAdtCodec.createDecoder[Message]("typ")
}

PS。由于浮点舍入问题，我不建议在货币/价格类型中使用Double。