title: kill items in a dispatch_async queue in iOS
body: <p>I am running a bunch of items in the background using dispatch_async and sometimes I want to kill what I have in the queue - is this possible?  For instance this code is run on a view, and then the user goes back a screen.  All of these fired actions keep running regardless of the back navigation.  Ideally I would like to kill these items from running:</p>
  
  <pre><code>dispatch_async(dispatch_get_global_queue(2, 0), ^{
          for (int i=0; i&lt;[self.manufacturers count]; i++) {
              NSString *manufacturerID = [[[self.manufacturers objectAtIndex:i] ManufacturerID] stringValue];
              [self doSync:manufacturerID withIndex:i setTimer:YES];
          }
      });
  </code></pre>
  
  <p>If I create a queue and name it and then release it on the dealloc of the view this is called in they still continue to run.</p>