Akka: scheduling self-cancelling tasks?

[Haoyi Li]

I'm trying to schedule tasks which are unique to an object: I want the tasks to happen every so often, but only as long as that object is "active". In order to do this in a generic way, i'm trying to define a thing.schedule, which looks exactly like scheduler.schedule, but kills the task after the object is no longer active:

class thing{

  def schedule(start: Duration, interval: Duration)(task: => Any): Any = {

      val cancellable: Cancellable = Akka.system.scheduler.schedule(start, interval) {

      if (!this.isActive)){

        cancellable.cancel()

      }else{

        task

      }       

    }

  }

}

where this.isActive is some method which returns whether the thing is active. However, I'm getting a 

forward reference extends over definition of value cancellable

Presumable because I'm trying to cancel cancellable, but it hasn't been fully defined within it's own scope.

I would have expected the `task` passed into the scheduler to be able to return some sort of flag (e.g. making it return true/false) to indicate that it should stop, but it doesn't.

I could just place the whole task inside an `if` statement, and the task won't get executed, but then the # of scheduled things will build up unbounded.

Is there a canonical way of doing this? I would think this is a pretty common requirement: scheduling a task contingent until some condition occurs, then killing it forever.

Thanks!

-Haoyi

[Alec Zorab]

The akka mailing list is probably a better place to ask this.

However, my normal solution to this problem is something like this:

class thing{

  def schedule(start: Duration, interval: Duration)(task: => Any): Any = {

      val promise = Promise[Unit] 

      val cancellable: Cancellable = Akka.system.scheduler.schedule(start, interval) {

        task()

        if (!this.isActive)

          promise.success(())

      }

      promise.foreach(cancellable.cancel())