Manually managed Lagom readside

HelloreadSide.scala

class HelloreadSide extends ReadSideProcessor[HelloWorldEvent] {
  override def aggregateTags: Set[AggregateEventTag[HelloWorldEvent]] =
    Set(HelloWorldEvent.Tag.tag)
  override def buildHandler()
    : ReadSideProcessor.ReadSideHandler[HelloWorldEvent] = {
    new ReadSideHandler[HelloWorldEvent] {
      override def globalPrepare(): Future[Done] = Future.successful(Done)

      // Returns the last Offset processed by this readside processor.
      override def prepare(
        tag: AggregateEventTag[HelloWorldEvent]
      ): Future[Offset] = ???

      // processes the event and updates the offset
      override def handle()
        : Flow[EventStreamElement[HelloWorldEvent], Done, NotUsed] = {
        Flow[EventStreamElement[HelloWorldEvent]]
          .mapAsync(1) { eventElement =>
            ???
          }
      }
    }
  }
}