title: iOS notification receive: some can and the other can't
body: <p>I use a <code>UINavigationController</code> to control the view controller change.Supposing I push A1 and A2 one by one. Both A1 and A2 is the instance of my custom View Controller A.
  A has registered a notification, and it will refresh the UI when it receive the notification.</p>
  
  <p>My problem is :<br>
  when I post the notification in A2， and A2 can receive it.<br>
  However, when I pop to A1, there is no change in A1.   </p>
  
  <p>So could any one help me how to fix it?</p>
  
  <p>BTW，the reason why I use notification is that I may push several instances of A,  and if any one posts the notification, the other instances in the stack also need to update.</p>