CancelDispatchAfter.swift

  var block: dispatch_block_t?
  
  override func viewDidLoad() {
    super.viewDidLoad()    
  }
  
  override func viewDidAppear(animated: Bool) {
    block = dispatch_block_create(DISPATCH_BLOCK_INHERIT_QOS_CLASS) {
      print("I executed")
    }
    let time: dispatch_time_t = dispatch_time(DISPATCH_TIME_NOW, Int64(5 * NSEC_PER_SEC))
    dispatch_after(time, dispatch_get_main_queue(), block!)
  }
  
  @IBAction func didTapButton(sender: UIButton) {
    dispatch_block_cancel(block!)
  }