Ref.scala

sealed trait Ref[T]{
  def get:T
  def registWatcher(f:()=>Unit):Unit
}

case class ElemRef[T](var v:T) extends Ref[T]{
  override def get: T = v
  var callbacks:Vector[()=>Unit] =Vector()
  def set(x:T) = {
    v = x
    callbacks.foreach(x=>x())
  }

  def map[B](f:T=>B) = new RefF(this,f)

  def registWatcher(f:()=>Unit) = callbacks = callbacks :+ f
}

case class RefF[A,B](ref:Ref[A],f:A=>B) extends  Ref[B]{
  var v = f(ref.get)


  override def get = v

  registWatcher(()=>v = f(ref.get))

  def registWatcher(f:()=>Unit) =
    ref.registWatcher(f)
}

object Test extends App{
  val a = ElemRef(List(1,2,3))
  val b = a.map(x=>{
    println("b func execute")
    x.filter(x=>x<2)
  })
  println(b.get)
  println(b.get)
  a.set(List(1,1,2,3,4))
  println(b.get)
}

b func execute
List(1)
List(1)
b func execute
List(1, 1)

Process finished with exit code 0