JsonSerializerGenerator.scala

import com.lightbend.lagom.scaladsl.playjson.JsonSerializer

import reflect.macros._

object JsonSerializerImpl {
  /**
    * Generates a list of JsonSerializers for all direct subtypes of type T. Every subtype needs an implicit Format[T]
    * in scope at the place of expansion.
    * @param c
    * @tparam T
    * @return
    */
  def generateJsonSerializersFor[T: c.WeakTypeTag](c: blackbox.Context): c.Expr[List[JsonSerializer[_]]] = {
    import c.universe._

    val rootSymbol = symbolOf[T]

    if (!rootSymbol.isClass)
      c.abort(c.enclosingPosition, "error: not a class")
    else if (!rootSymbol.asClass.isSealed)
      c.abort(c.enclosingPosition, "error: not sealed")
    else {
      val jsonSerializers = rootSymbol.asClass.knownDirectSubclasses.map{ subclassSymbol =>
        val format = q"implicitly[_root_.play.api.libs.json.Format[$subclassSymbol]]"
        val classTag = q"_root_.scala.reflect.ClassTag(classOf[$subclassSymbol])"
        q"_root_.com.lightbend.lagom.scaladsl.playjson.JsonSerializer($classTag, $format)"
      }

      c.Expr[List[JsonSerializer[_]]](q"_root_.scala.collection.immutable.List(..$jsonSerializers)")
    }
  }
}


// Compile in a separate compilation unit (another project basically)
// 
// Usage:
// override val serializers = 
//   generateJsonSerializersFor[EventsRootTrait] ++ generateJsonSerializersFor[CommandsRootTrait]