stopWatch.swift

import UIKit

class ViewController: UIViewController {

    var countNum = 0
    var timerRunning = false
    var timer = Timer()
    
    @objc func updateDisplay(){
        countNum += 1
        let ms = countNum % 100
        let s = (countNum - ms) / 100 % 60
        let m = (countNum - s - ms) / 6000 % 3600
        timeDisplay.text = String(format: "%02d:%02d.%02d", m,s,ms)
    }
    
    @IBOutlet weak var timeDisplay: UILabel!
    @IBAction func startButton(_ sender: UIButton) {
        if timerRunning == false{
            timer = Timer.scheduledTimer(timeInterval: 0.01, target: self, selector: #selector(ViewController.updateDisplay), userInfo: nil, repeats: true)
            timerRunning = true
        }
    }
    @IBAction func stopButton(_ sender: UIButton) {
        if timerRunning == true{
            timer.invalidate()
            timerRunning = false
        }
    }
    @IBAction func resetButton(_ sender: UIButton) {
        countNum = 0
        timeDisplay.text = "00:00.00"
    }
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
}