Play Framework 2.5 Serializing JSON Traits - json

Question

I have a sealed trait that is like below:
sealed trait MyMessages
object MyMessages {
case object Init extends MyMessages
case object Destroy extends MyMessages
case class Tick(elem: Long) extends MyMessages
}
I have to now write a formatter for serializing and de-serializing this into to and from a JSON. This is what I came up with:
implicit object MyMessagesWrites extends Writes[MyMessages] {
def writes(myMessages: MyMessages): JsValue = myMessages match {
case Init => Json.toJson("INIT")
case Destroy => Json.toJson("DESTROY")
case tick: Tick => Json.toJson(Tick)
}
def reads(json: JsValue): MyMessages = {
// How do I get from JSValue to a MyMessages type???
}
}
Implementing the writes was easy, but how do I implement the reads?

Answer 1

Assuming you serialize the Tick instance as a bare JSON number, I would do it like so:
implicit object MyMessageReads extends Reads[MyMessages] {
def reads(json: JsValue) = json match {
case JsString("INIT") => JsSuccess(MyMessages.Init)
case JsString("DESTROY") => JsSuccess(MyMessages.Destroy)
case JsNumber(n) => JsSuccess(Tick(n.toLongExact))
case e => JsError(s"Invalid message: $e")
}
}
Note that you can also make the reads/writes a bit more succinct by using the more functional style:
implicit val myMessagesWrites = Writes[MyMessages] {
case Init => JsString("INIT")
case Destroy => JsString("DESTROY")
case Tick(n) => JsNumber(n)
}
implicit val myMessageReads = Reads[MyMessages] {
case JsString("INIT") => JsSuccess(MyMessages.Init)
case JsString("DESTROY") => JsSuccess(MyMessages.Destroy)
case JsNumber(n) => JsSuccess(Tick(n.toLongExact))
case e => JsError(s"Invalid message: $e")
}