sclass DoubleVarWithNotify extends DoubleVar {
  event fireChange;
  
  *() {}
  *(double value) { super(value); }
  
  @Override
  public synchronized void set(double a) {
    if (get() != a) {
      super.set(a);
      fireChange();
    }
  }
  
  selfType onChangeAndNow(Runnable r) {
    onChange(r);
    r.run();
    this;
  }
}

Began life as a copy of #1034280