title: What is parameter `object` in NSNotification addObserver:?
body: <p>One of my class named Message.m is posting a notification with an object <code>sentObject</code> as below</p>
  
  <pre><code>NSDictionary *sentObject = [NSDictionary dictionaryWithObjectsAndKeys:draftData.arr,@"data", nil];
  
  //Post notification to inform a receiver to reload data     
  [[NSNotificationCenter defaultCenter] postNotificationName:@"reloadDuringSave" object:self userInfo:sentObject];
  </code></pre>
  
  <p>DraftData.m will be be the receiver to catch the notification as follow</p>
  
  <pre><code>[[NSNotificationCenter defaultCenter] addObserver:self 
                                           selector:@selector(dataReloaded:) 
                                               name:@"reloadDuringSave" 
                                             object:nil];
  </code></pre>
  
  <p>For posting notification, <code>userInfo</code> can be <code>nil</code>  or can be an <code>object</code> (like <code>sentObject</code> as type of <code>NSDictionary</code> in this example). </p>
  
  <h3>Question:</h3>
  
  <p>What are other params for <code>object</code> in <code>addObserver</code> method? Can they be 
   anything other than <code>nil</code>, and if so what?</p>