svoid <A> addWeakChangeListener(IHasChangeListeners src, A target, IVF1<A> action) {
  if (src == null || action == null || target == null) ret;
  
  var ref = weakRef(target);
  src.onChange(r {
    var target = ref!;
    if (target == null)
      var.removeChangeListener(this);
    else
      action.get(target);
  });
}