Chronometer-1.swift

class ChronometerVC: UIViewController {
    private var displayLink:DisplayLink?
     
    private let lblCounter:UILabel = {
        let lbl = UILabel()
        lbl.font = UIFont(name: "Verdana", size: 28)
        lbl.textAlignment = NSTextAlignment.center
        lbl.textColor = UIColor.black
        return lbl
    }()
    
    
    override func viewDidDisappear(_ animated: Bool) {
        super.viewDidDisappear(animated)
        displayLink?.releaseDisplayLink()
        self.displayLink = nil
    }
    
    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        setUpDisplayLink()
        setUpUI()
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
    }

    private func setUpDisplayLink() {
        guard displayLink == nil else { return }
        displayLink = DisplayLink()
        displayLink?.displayLinkUpdated = { timeInterval in
           self.updateCounter(timeInterval: timeInterval)
        }
        displayLink?.startDisplayLink()
    }
  
    private func updateCounter(timeInterval:CFTimeInterval) {
        let minutes:Int = Int(timeInterval/60)
        let seconds:Int = Int(timeInterval) - minutes*60
        let miliseconds:Int = Int((timeInterval - Double(minutes)*60 - Double(seconds))*1000)
        self.lblCounter.text = minutes.timeString(digits: 2)+":"+seconds.timeString(digits: 2)+":"+miliseconds.timeString(digits:3)
    }
    
    private func setUpUI() {
        self.view.addSubview(lblCounter)
        lblCounter.text = ""
        lblCounter.frame = CGRect(x: 0, y: 0, width: 200, height: 200)
        lblCounter.center = self.view.center
    }
}

// Converting integer to human readable string
extension Int {
    func timeString(digits:Int)->String {
        var str:String = String(describing:self)
        if str.count >= digits {
            return str
        }
        for _ in 0..<digits - str.count {
            str = "0"+str
        }
        return str
    }
}