Swift 3: Programming a simple stopwatch

For the last while, I have been extending my programming skill set by learning Swift. Swift is a programming language for MacOS, iOS, watchOS and tvOS applications. As both a resource for myself and hopefully a resource for fellow programmers, I thought I might start publishing some of my early scratchings. I emphasise early! Don’t expect the entire source code to an Uber app… yet.

The following code creates a simple stopwatch/timer. It could be easily extended to include advanced functionality like reverse timers, alarms, notifications, etc. I learnt a lot from this so I hope it may be useful to your Swift journey. I would love any feedback so feel free to comment.

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I used the following very helpful sources when researching how I should approach this Swift snippet:

I separated the three time blocks (min/sec/ms) into separate labels as I found that putting them all into one label made for a shaky viewing experience. I suspect by having them separate will allow for easy design enhancements.

//  ViewController.swift
//  Simple Stopwatch
//  Created by Mike Little on 2/03/17.
//  Copyright © 2017 Creative Digital. All rights reserved.

import UIKit

class ViewController: UIViewController {

weak var timer: Timer?
var startTime: Double = 0
var time: Double = 0
var elapsed: Double = 0
var status: Bool = false

@IBOutlet weak var labelMinute: UILabel!
@IBOutlet weak var labelSecond: UILabel!
@IBOutlet weak var labelMillisecond: UILabel!
@IBOutlet weak var resetBtn: UIButton!

override func viewDidLoad() {
// Do any additional setup after loading the view, typically from a nib.

// No point enabling reset until stopwatch actually started
resetBtn.isEnabled = false


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

@IBAction func toggleStartStop(_ sender: UIButton!) {

// If button status is true use stop function, relabel button and enable reset button
if (status) {
sender.setTitle("START", for: .normal)
resetBtn.isEnabled = true

// If button status is false use start function, relabel button and disable reset button
} else {
sender.setTitle("STOP", for: .normal)
resetBtn.isEnabled = false


@IBAction func resetBtn(_ sender: Any) {

// Invalidate timer

// Reset timer variables
startTime = 0
time = 0
elapsed = 0
status = false

// Reset all three labels to 00
let strReset = String("00")
labelMinute.text = strReset
labelSecond.text = strReset
labelMillisecond.text = strReset


func start() {

startTime = Date().timeIntervalSinceReferenceDate - elapsed
timer = Timer.scheduledTimer(timeInterval: 0.01, target: self, selector: #selector(updateCounter), userInfo: nil, repeats: true)

// Set Start/Stop button to true
status = true


func stop() {

elapsed = Date().timeIntervalSinceReferenceDate - startTime

// Set Start/Stop button to false
status = false


func updateCounter() {

// Calculate total time since timer started in seconds
time = Date().timeIntervalSinceReferenceDate - startTime

// Calculate minutes
let minutes = UInt8(time / 60.0)
time -= (TimeInterval(minutes) * 60)

// Calculate seconds
let seconds = UInt8(time)
time -= TimeInterval(seconds)

// Calculate milliseconds
let milliseconds = UInt8(time * 100)

// Format time vars with leading zero
let strMinutes = String(format: "%02d", minutes)
let strSeconds = String(format: "%02d", seconds)
let strMilliseconds = String(format: "%02d", milliseconds)

// Add time vars to relevant labels
labelMinute.text = strMinutes
labelSecond.text = strSeconds
labelMillisecond.text = strMilliseconds


4 responses to “Swift 3: Programming a simple stopwatch

  1. hi,
    if I setup the timer for 10 seconds, then I need to start another timer for 5 seconds after the first timer stopped before the first timer running back?
    I stopped the first one after 10 seconds, but the second timer started but stop counting!

    appreciated your help,

  2. Hi,

    Thanks for the tutorial, I am new to coding and trying to get my head around UInt8..

    I was wondering how you would calculate hours. If someone could assist that would be greatly appreciated.

    for example
    let hours = UInt8 ?

    I have tried quite a few different ways but I can’t seem to get it to work. I have also tried finding other stopwatch tutorials but everyone seems to have a different method.


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