the best way to change headers and content

Hi all,I have the following definition of path:
  path("_uuids") {    transformResponse { response =>      val pragma = HttpHeader("Pragma", "no-cache")      val expires = HttpHeader("Expires", "Fri, 01 Jan 1990 00:00:00 GMT")      val etag = HttpHeader("ETag", """"5XE1EL506AN07CVT0MQHBVN6P"""") //TODO ETag must be dynamic      val cache = HttpHeaders.`Cache-Control`(List(CacheDirectives.`must-revalidate`, CacheDirectives.`no-cache`))      response.copy(headers = cache :: expires :: etag :: pragma :: response.headers)    } {      parameter("count".as[Int]?) { count =>        get { ctx =>          val n = count getOrElse 1          val list = List.fill(n)(Uuids.generateNew.id)          val result = Map("uuids" -> list)          val json = PrettyPrinter(JsValue(result))          ctx.complete(HttpContent(`application/json`, json))        }      }    }  }
The problem is that I would like to make the 'ETag' header dynamic, depending on the content.What is the recommended way to do it ?
-Andrey