provide convenient user-api to manually create codecs

[domdorn]

Quite often I find myself in the need to manually create a codec for a certain data type or multiple different codecs for a single type.
For example, in one of our services we have a java.lang.enum that holds physical units from timeseries databases. This unit gets delivered by a 3rd-party system as e.g. "EUR/MWh" and we convert it to our UnitsEnum.EUR_PRO_MEGAWATTSTUNDE value

The enum type is used in the data classes that interact with the 3rd-party system, as well in the responses given out by our services. We do not want to have two enums containing the same information and map between them.

In Jsoniter-Scala this is straightforward, we simply created two codecs by hand with minimal boilerplate. Something similar should be possible with zio-json

  val unitWebCodec: JsonValueCodec[UnitEnum] = new JsonValueCodec[UnitEnum] {
    override def decodeValue(in: JsonReader, default: UnitEnum): UnitEnum =
      UnitEnum.valueOf(in.readString("KEINE"))

    override def encodeValue(x: UnitEnum, out: JsonWriter): Unit = out.writeVal(x.name())

    override def nullValue: UnitEnum = UnitEnum.KEINE
  }

  val unitClientCodec: JsonValueCodec[UnitEnum] = new JsonValueCodec[UnitEnum] {
    override def decodeValue(in: JsonReader, default: UnitEnum): UnitEnum =
      UnitEnum.getFromCode(in.readString("none"))

    override def encodeValue(x: UnitEnum, out: JsonWriter): Unit = out.writeVal(x.getCode)

    override def nullValue: UnitEnum = UnitEnum.KEINE
  }

Dependening if we're in the layer interacting with the 3rd-party service or in the layer serving our web-responses, we simply define one of these two codecs as implicit and the macros generating the responses pick them up, e.g.

object ThirdPartyClientJsoniterSupport {
... 
  implicit val UnitCodec: JsonValueCodec[UnitEnum] = unitClientCodec
}

object MyWebController { 
 case class Response(..., unit: UnitEnum, ...) 

implicit val UnitCodec: JsonValueCodec[UnitEnum]              = unitWebCodec
implicit val RequestJsonCodec: JsonValueCodec[Request] = JsonCodecMaker.make[Request]
implicit val ResponseJsonCodec: JsonValueCodec[Response] = JsonCodecMaker.make[Response](
  CodecMakerConfig
    .withTransientEmpty(false)
    .withTransientNone(false)
    .withTransientDefault(false))
}