jmonkey-test/src/main/scala/wow/doge/mygame/subsystems/events/EventBus.scala

package wow.doge.mygame.subsystems.events

import scala.reflect.ClassTag

import akka.actor.typed.ActorRef
import akka.actor.typed.Behavior
import akka.actor.typed.PostStop
import akka.actor.typed.Scheduler
import akka.actor.typed.SpawnProtocol
import akka.actor.typed.scaladsl.AskPattern._
import akka.actor.typed.scaladsl.Behaviors
import akka.event.EventStream
import akka.util.Timeout
import monix.bio.UIO
import monix.execution.Ack
import monix.execution.Cancelable
import monix.execution.cancelables.SingleAssignCancelable
import monix.reactive.Observable
import monix.reactive.OverflowStrategy
import wow.doge.mygame.implicits._
import wow.doge.mygame.subsystems.events.EventBus.ObservableSubscription
import wow.doge.mygame.utils.AkkaUtils

/**
  * A (typed) event bus
  * Copied (and repurposed) from Akka's EventStream
  */
object EventBus {
  sealed trait Command[A]
  final case class Publish[A, E <: A](
      event: E,
      publisherName: String
  ) extends Command[A]

  final case class Subscribe[A, E <: A](subscriber: ActorRef[E])(implicit
      classTag: ClassTag[E]
  ) extends Command[A] {
    def topic: Class[_] = classTag.runtimeClass
  }

  final case class Unsubscribe[A, E <: A](subscriber: ActorRef[E])
      extends Command[A]

  final case class ObservableSubscription[A, E <: A](
      replyTo: ActorRef[Observable[E]]
  )(implicit classTag: ClassTag[E])
      extends Command[A] {
    def ct = classTag
  }

  def apply[A: ClassTag]()(implicit
      timeout: Timeout,
      spawnProtocol: ActorRef[SpawnProtocol.Command]
  ): Behavior[EventBus.Command[A]] =
    Behaviors.setup { ctx =>
      val eventStream = new EventStream(ctx.system.classicSystem)
      implicit val scheduler = ctx.system.scheduler
      new EventBus().eventStreamBehavior(eventStream)
    }

  def observable[A, B <: A](eventBus: ActorRef[EventBus.Command[A]])(implicit
      timeout: Timeout,
      scheduler: Scheduler,
      spawnProtocol: ActorRef[SpawnProtocol.Command],
      ct: ClassTag[A],
      ct2: ClassTag[B]
  ) =
    Observable.create[B](OverflowStrategy.DropOld(50)) { sub =>
      implicit val s = sub.scheduler
      val c = SingleAssignCancelable()
      val behavior = Behaviors
        .receive[B] { (ctx, msg) =>
          ctx.log.traceP(s"Emitted $msg")
          if (sub.onNext(msg) == Ack.Stop) {
            c.cancel()
            Behaviors.stopped
          } else Behaviors.same
        }
        .receiveSignal {
          case (_, PostStop) =>
            sub.onComplete()
            Behaviors.same
        }
      val actor =
        AkkaUtils
          .spawnActorL(
            behavior,
            actorName =
              Some(s"eventBusObservable-${ct.toString.split("""\.""").last}")
          )
          .mapError(err => new Throwable(err.toString))
          .tapError {
            case ex => UIO(sub.onError(ex))
          }

      (for {
        a <- actor
        _ <- eventBus !! Subscribe(a)
        _ <- UIO(c := Cancelable(() => eventBus ! Unsubscribe(a)))
      } yield ()).runToFuture
      c
    }

}

class EventBus[A] {
  import akka.actor.typed.scaladsl.adapter._

  private def eventStreamBehavior(
      eventStream: EventStream
  )(implicit
      timeout: Timeout,
      scheduler: Scheduler,
      spawnProtocol: ActorRef[SpawnProtocol.Command],
      ct: ClassTag[A]
  ): Behavior[EventBus.Command[A]] =
    Behaviors.setup { ctx =>
      Behaviors.receiveMessage {
        case EventBus.Publish(event, name) =>
          eventStream.publish(event)
          Behaviors.same
        case s @ EventBus.Subscribe(subscriber) =>
          eventStream.subscribe(subscriber.toClassic, s.topic)
          Behaviors.same
        case EventBus.Unsubscribe(subscriber) =>
          eventStream.unsubscribe(subscriber.toClassic)
          Behaviors.same
        case s @ ObservableSubscription(replyTo) =>
          val obs = EventBus.observable(
            ctx.self
          )(timeout, scheduler, spawnProtocol, ct, s.ct)
          replyTo ! obs
          Behaviors.same
      }
    }
}