gistfile1.scala

trait EventListener[EventType] {
  type Listener = (EventType) => Unit
  var listeners = List.empty[Listener]
  def +(f: Listener ) = f :: listeners
  def -(f: Listener) = ...
  def notify(event: EventType) = listeners.foreach{ l => l(event) }
  def removeAll = listeners = Nil
}

class Button {
  val click = new EventListener[ClickEvent]
  
  def update() {
     ....
	 // был клик по кнопке
	 click.notify()
  }
}

val b = new Button
def printEvent(ev: Event) = println(ev)
b.click += (printEvent _)