title: Is there a way to specify granularity on senders/receivers of NSNotifications?
body: <p>Consider something like the following example:</p>
  
  <ul>
  <li>I have a library with books, books have a difficulty and genre.</li>
  <li>Each book is an instance of some object and could be presented by a view controller (I.E. a collection view, where each cell is a book, of perhaps a detail view where only one book is displayed). </li>
  <li>The data of these books can be updated in the background by some kind of synchronisation method. It is possible only one book is updated, or perhaps one genre. </li>
  </ul>
  
  <p>I would like the classes (mostly the views) to receive a notification of updates. I would like these notifications to be pretty clear. So when all non-fiction books are updated this is what should be notified. 
  I could of course use separate notification names for each kind, but if we are talking about an entire library, a big collection view containing thousands of objects would mean registering too many observers. In this case the observer would perhaps choose to receive any notification on books, of maybe any of a genre. </p>
  
  <p>What I am missing (or can't seem to figure out) in NSNotification is some kind of granularity to specify this need. 
  So in short:</p>
  
  <ul>
  <li>Is there a way to tell NSNotification more specifically what kind of notifications I would like to receive/who to send it to?</li>
  <li>Alternatively, can I attach an object to a notification? If so, I could model scope (like meta-data) of the notification in this object and let the receiver check this data. </li>
  </ul>