2 files changed, 130 insertions, 0 deletions
diff --git a/shared/src/main/scala/DerivedFormats.scala b/shared/src/main/scala/DerivedFormats.scala
new file mode 100644
index 0000000..d0cac38
--- /dev/null
+++ b/shared/src/main/scala/DerivedFormats.scala
@@ -0,0 +1,108 @@
+package spray.json
+
+import magnolia._
+
+import scala.language.experimental.macros
+
+/** Mixin that enables derivation of JSON formats for any product
+  * (case classes) or coproduct (sealed traits) types. */
+trait DerivedFormats { self: BasicFormats =>
+  type Typeclass[T] = JsonFormat[T]
+
+  def combine[T](ctx: CaseClass[JsonFormat, T]): JsonFormat[T] =
+    new JsonFormat[T] {
+      override def write(value: T): JsValue = {
+        val fields: Seq[(String, JsValue)] = ctx.parameters.map { param =>
+          param.label -> param.typeclass.write(param.dereference(value))
+        }
+        JsObject(fields: _*)
+      }
+
+      override def read(value: JsValue): T = value match {
+        case obj: JsObject =>
+          if (ctx.isObject) {
+            ctx.rawConstruct(Seq.empty)
+          } else {
+            ctx.construct { param =>
+              param.typeclass.read(obj.fields(param.label))
+            }
+          }
+        case js =>
+          deserializationError(
+            s"Cannot read JSON '$js' as a ${ctx.typeName.full}")
+      }
+    }
+
+  def dispatch[T](ctx: SealedTrait[JsonFormat, T]): JsonFormat[T] = {
+    val typeFieldName = ctx.annotations
+      .collectFirst {
+        case g: gadt => g.typeFieldName
+      }
+      .getOrElse("type")
+
+    new JsonFormat[T] {
+      override def write(value: T): JsValue = ctx.dispatch(value) { sub =>
+        sub.typeclass.write(sub.cast(value)) match {
+          case obj: JsObject =>
+            JsObject(
+              (Map(typeFieldName -> JsString(sub.typeName.short)) ++
+                obj.fields).toSeq: _*)
+          case js => js
+        }
+      }
+
+      override def read(js: JsValue): T = {
+        val typeName: String = js match {
+          case obj: JsObject if obj.fields.contains(typeFieldName) =>
+            obj.fields(typeFieldName).convertTo[String]
+          case JsString(str) =>
+            str
+          case _ =>
+            deserializationError(
+              s"Cannot deserialize JSON to ${ctx.typeName.full} " +
+                "because serialized type cannot be determined.")
+        }
+
+        ctx.subtypes.find(_.typeName.short == typeName) match {
+          case Some(tpe) => tpe.typeclass.read(js)
+          case None =>
+            deserializationError(
+              s"Cannot deserialize JSON to ${ctx.typeName.full} " +
+                s"because type '${typeName}' is unsupported.")
+        }
+      }
+    }
+  }
+
+  def jsonFormat[T]: RootJsonFormat[T] =
+    macro DerivedFormatMacros.derivedFormat[T]
+
+}
+object DerivedFormats extends DerivedFormats with DefaultJsonProtocol
+
+trait ImplicitDerivedFormats extends DerivedFormats { self: BasicFormats =>
+  implicit def implicitJsonFormat[T]: RootJsonFormat[T] =
+    macro DerivedFormatMacros.derivedFormat[T]
+}
+object ImplicitDerivedFormats
+    extends ImplicitDerivedFormats
+    with DefaultJsonProtocol
+
+object DerivedFormatMacros {
+  import scala.reflect.macros.whitebox._
+
+  /** Utility that converts a magnolia-generated JsonFormat to a RootJsonFormat. */
+  def derivedFormat[T: c.WeakTypeTag](c: Context): c.Tree = {
+    import c.universe._
+    val tpe = weakTypeOf[T].typeSymbol.asType
+    val sprayPkg = c.mirror.staticPackage("spray.json")
+    val valName = TermName(c.freshName("format"))
+    q"""{
+      val $valName = ${Magnolia.gen[T](c)}
+      new $sprayPkg.RootJsonFormat[$tpe] {
+        def write(value: $tpe) = $valName.write(value)
+        def read(value: $sprayPkg.JsValue) = $valName.read(value)
+      }
+    }"""
+  }
+}
diff --git a/shared/src/main/scala/annotations.scala b/shared/src/main/scala/annotations.scala
new file mode 100644
index 0000000..9d35d16
--- /dev/null
+++ b/shared/src/main/scala/annotations.scala
@@ -0,0 +1,22 @@
+package spray.json
+
+import scala.annotation.StaticAnnotation
+
+/** An annotation that designates that a sealed trait is a generalized algebraic
+  * datatype (GADT), and that a type field containing the serialized childrens'
+  * types should be added to the final JSON objects.
+  *
+  * Note that by default all sealed traits are treated as GADTs, with a type
+  * field called `type`. This annotation enables overriding the name of that
+  * field and is really only useful if a child itself has a field called `type`
+  * that would result in a conflict.
+  *
+  * Example:
+  * {{{
+  * // the JSON field "kind" will contain the actual type of the serialized child
+  * @gadt("kind") sealed abstract class Keyword(`type`: String)
+  * case class If(`type`: String) extends Keyword(`type`)
+  * }}}
+  * @param typeFieldName the name of the field to inject into a serialized JSON
+  *        object */
+final class gadt(val typeFieldName: String = "type") extends StaticAnnotation