Understanding Spray: Demistification - scala

Question

Can someone help me get a hold on the spray code to better understand how to spray ?
I'm the context of sending a file as multipart data.
The code that was suggested to me is :
import akka.actor.ActorSystem
import spray.client.pipelining._
import spray.http.{MediaTypes, BodyPart, MultipartFormData}
object UploadFileExample extends App {
implicit val system = ActorSystem("simple-spray-client")
import system.dispatcher // execution context for futures below
val pipeline = sendReceive
val payload = MultipartFormData(Seq(BodyPart(new File("/tmp/test.pdf"), "datafile", MediaTypes.`application/pdf`)))
val request =
Post("http://localhost:8080/file-upload", payload)
pipeline(request).onComplete { res =>
println(res)
system.shutdown()
}
}
Which is fine and works of course.
However i want to understand what is under the hood so i can do things by myself:
Here are the confusion coming from this code:
BodyPart(new File("/tmp/test.pdf"), "datafile", MediaTypes.`application/pdf`)
is the first issue, indeed, BodyPart only has one apply method that is closely match:
def apply(file: File, fieldName: String, contentType: ContentType): BodyPart =
apply(HttpEntity(contentType, HttpData(file)), fieldName, Map.empty.updated("filename", file.getName))
it takes a contentType and not a MediaType.
However i found in contentType
object ContentType {
private[http] case object `; charset=` extends SingletonValueRenderable
def apply(mediaType: MediaType, charset: HttpCharset): ContentType = apply(mediaType, Some(chars et))
implicit def apply(mediaType: MediaType): ContentType = apply(mediaType, None) }
But ContentType is not in the scope of the Main (see first code at the top.) Hence i don't know where that implicit conversion comes from?
Then the last thing that i do not understand here is
val payload = MultipartFormData(Seq(BodyPart(new File("/tmp/test.pdf"),
"datafile", MediaTypes.`application/pdf`)))
Post("http://localhost:8080/file-upload", payload)
The problem here is that, it is based on the RequestBuilding (as can be found in the RequestBuilding Source)
val Post = new RequestBuilder(POST)
with an object that contain the apply method:
def apply[T: Marshaller](uri: String, content: Option[T]): HttpRequest = apply(Uri(uri), content)
....
....
def apply[T: Marshaller](uri: Uri, content: Option[T]): HttpRequest = {
val ctx = new CollectingMarshallingContext {
override def startChunkedMessage(entity: HttpEntity, ack: Option[Any],
headers: Seq[HttpHeader])(implicit sender: ActorRef) =
sys.error("RequestBuilding with marshallers producing chunked requests is not supported")
}
content match {
case None ⇒ HttpRequest(method, uri)
case Some(value) ⇒ marshalToEntityAndHeaders(value, ctx) match {
case Right((entity, headers)) ⇒ HttpRequest(method, uri, headers.toList, entity)
case Left(error) ⇒ throw error
}
}
}
MultiPartFormData is not a Marshaler , hence i do not understand how does it works.
Can someone help me figure that out ?
Many thanks,
M