问题描述
我正在使用来自websocket的消息。它们有以下两种格式之一:
{"typ": "subscription","channel": "BTC-USD"}
或
{"typ": "ticker","price": 10213.42}
因此,我可以使用Circe的解码器使用三个案例类:
case class GenericMessage(typ: String)
case class SubscriptionMessage(channel: String)
case class TickerMessage(price: Double)
我如何让Circe找出消息的类型,而又不嵌套很多解码尝试?
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
}
}
解决方法
您可以尝试使用我的Circe ADT扩展程序: https://github.com/abdolence/circe-tagged-adt-codec
因此,您的情况应该是:
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,我不建议将Double
用于货币/价格类型。
感谢@LuisMiguelMejiaSuarez。
我使用circe-generic-extras提出了以下解决方案:
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.
})