title: Swift UI update in callback is sometimes not executed
body: <p>When executing the code below, I see the callback execute (verified by "success!"), but the activityIndicator on the UI activityIndicator doesn't stop spinning.</p>
  
  <p>Sometimes it will stop after 10-20 seconds, and other times it will run forever. The callback should be running on the main thread, so I'm not sure what's causing it to delay / fail.</p>
  
  <pre><code>activityIndicator.startAnimating()
  loadData("data") {(success: Bool) in
      if success {
          // I see this being printed
          println("success!")
      }
  
      // This isn't updating on the UI! The wheel keeps spinning
      self.activityIndicator.stopAnimating()
  }
  </code></pre>
  
  <p>The loadData function is performing a block save using Core Data:</p>
  
  <pre><code>public func loadData(completionHandler: (success: Bool) -&gt; Void) {
      let appDelegate = UIApplication.sharedApplication().delegate as AppDelegate
      let appContext = appDelegate.managedObjectContext!
      let managedContext = NSManagedObjectContext(concurrencyType: .PrivateQueueConcurrencyType)
      managedContext.persistentStoreCoordinator = appContext.persistentStoreCoordinator
  
      managedContext.performBlock({
  
          //NSManagedObject inserts - working...
  
          if !managedContext.save(&amp;error) {
              completionHandler(success: false)
          }
      completionHandler(success: true)
      })
  }
  </code></pre>
  
  <p>Any help here is much appreciated!</p>