title: DispatchQueue main isn't called
body: <p>I have an async task o perform the calculation and the animation, as the animation begins, the queue suspends, when the animation finishes, the queue is resumed. But for some reason the DispatchQueue.main.sync is never called.</p>
  
  <pre><code>self.someQueue = DispatchQueue(label: "Animation", qos: .background, attributes: DispatchQueue.Attributes.concurrent)
      for index in stride(from: volumeObjects.count, to: -1, by: -1) {
          mainAsyncQueue.sync{
              self.dynamicViewModel.currentPercentile = Float(index)/Float(volumeObjects.count)
              self.currentObjectIndex = index
              print(index)
  
              DispatchQueue.main.sync {
                  UIView.animate(withDuration: 1, animations: {
                      self.updateViewModelDynamicViewModel()
                  }, completion: { (finished) in
                      self.someQueue.resume()
                  })
              }
              self.someQueue.suspend()
          }
      }
  </code></pre>
  
  <p>This is just small calculation, I'll use that to solve tasks more complex than that</p>