title: Is a dispatch_group_notify block persistant for the lifespan of a dispatch_group_t?
body: <p>If I have a <code>dispatch_group</code> class property:</p>
  
  <pre><code>@property (nonatomic, readonly) dispatch_group_t _serialGroup;
  </code></pre>
  
  <p>and I have a block that I <em>always</em> want called whenever the group completes:</p>
  
  <pre><code>dispatch_group_notify(self._serialGroup, self._serialQueue, ^{
      dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
          //...do some stuff...
      });
  });
  </code></pre>
  
  <p>Can I just define the <code>dispatch_group_notify</code> upon initilization of the <code>dispatch_group</code> once and it'll be called whenever that group completes or do I need to redefine it every time I add items to the group?</p>