Proof that lambdas don't work with listeners on weak references.

weak-refs-and-lambdas.scala

package foo

import java.lang.ref.WeakReference

class Listeners[T] {
  private var buffer = List[WeakReference[T => Unit]]()

  def fire(e: T): Unit = {
    buffer.map(_.get()).filter(_ ne null).foreach(_.apply(e))
    buffer = buffer.filter(_.get() ne null)
    println("listeners: " + buffer.size)
  }

  def add(f: T => Unit): Unit = {
    buffer = buffer :+ new WeakReference(f)
  }
}

object Test extends App {
  val listeners = new Listeners[Int]

  listeners.add(e => println(e))

  listeners.fire(42)
  
  System.gc()

  listeners.fire(42)
}