Listener.scala

import scala.collection.mutable.{HashMap, MultiMap, LinkedHashSet, Set}
import scala.reflect.runtime.universe._

trait OrderedMultiMap[K, V] extends MultiMap[K, V] {
  override def makeSet: Set[V] = new LinkedHashSet[V]
}

object Listener {
  trait Manager {

    private val listeners = new HashMap[Type, Set[Any => Unit]] with OrderedMultiMap[Type, Any => Unit]

    def addListener[E: TypeTag](listener: E => Unit) {
      listeners.addBinding(typeOf[E], listener.asInstanceOf[Any => Unit])
    }

    def removeListener[E: TypeTag](listener: E => Unit) {
      listeners.removeBinding(typeOf[E], listener.asInstanceOf[Any => Unit])
    }

    protected def getListeners[E: TypeTag]() = listeners.get(typeOf[E]).asInstanceOf[List[E => Unit]]

    protected def notify[E: TypeTag](event: E) {
      getListeners[E].foreach(_.apply(event))
    }
  }

  object Test extends Manager {

    case class A(i: Int)
    case class B(s: String)
    case class C(i: Int, B: String)

    def printA(a: A) = {
      print("A"); print(a)
    }

    def printB(b: B) = {
      print("B"); print(b)
    }

    addListener(printA)

    val a = new A(1)
    notify(3) // does nothing
    notify(a) // calls printA once
    addListener(printA)
    notify(a) // calls printA twice
  }

}