LiftOffViewController.swift

public typealias CompletionHandler = () -> Void

class LiftOffViewController : UIViewController {
    
    var countDownTimer: CountDown?
    
    @IBOutlet weak var labelTimeLeft: UILabel!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        countDownTimer = CountDownTimer(endsAfter: TimeInterval(10000), repeatingTask: countDownTimerRepeatingTask, completion: countDownTimerCompletion)
    }
    
    private var countDownTimerRepeatingTask: CompletionHandler? {
        let handler: CompletionHandler = { [weak self] in
            guard let `self` = self,
                let countDownTimer = self.countDownTimer
                else {return}
            
            var timeRemaining: TimeInterval = (countDownTimer.timeLimit - countDownTimer.timeElapsed)
            timeRemaining.round(.toNearestOrEven)
            
            self.labelTimeLeft.text = "\(timeRemaining)"
        }
        return handler
    }
    
    private var countDownTimerCompletion: CompletionHandler? {
        let handler: CompletionHandler = { [weak self] in
            guard let `self` = self else {return}
            
            let alert = UIAlertController(title: "Lift Off!", message: "Launch confirmed. Successful lift off!", preferredStyle:
                .alert)
            let okAction = UIAlertAction(title: "OK", style: .default) { (_ : UIAlertAction) -> Void in
            }
            alert.addAction(okAction)
            self.present(alert, animated: true, completion: nil)
        }
        return handler
    }
  
    @IBAction func didTapRedButton(_ sender: UIButton) {
        guard let timer = countDownTimer else {return}
      
        if timer.isRunning {
            timer.stop()  // Stop if already running
        } else {
            timer.start() // Otherwise start the timer
        }
    }
  
    deinit {
        countDownTimer?.stop()
        countDownTimer = nil
    }
}