title: Receive notification on adding subview
body: <p><br>
  Hello, <br>
  For example, I have a view, on that view there is a scrollview.
  Is it possible to know, when any subview has been added on that scrollview without subclassing the UIScrollView and overriding the method <code>didAddSubview:</code>? <br>
  Basically, is it possible to receive notifications when the number of subviews has been changed?<br>
  Thanks.</p>