Swift Timer() trouble - ios

Question

I've done a simple timer in Swift. All is well apart from when the seconds reach 59 seconds. Instead of going back to zero they just carry on going. Would someone would be able to point out where I'm going wrong and why this is happening?
#IBAction func startButtonDidTouch(_ sender: Any) {
if !timerIsRunning{
timer = Timer.scheduledTimer(timeInterval: 0.01, target: self, selector: #selector(self.updateTimer), userInfo: nil, repeats: true)
timerIsRunning = true
}
}
#objc func updateTimer() {
totalSeconds += 0.01
let totalSecondsTimes100: Int = Int(totalSeconds*100)
let minutes = Int(totalSeconds/60)
let timerChoice = Double(minutes)
let minStr = (minutes == 0) ? "00" : "0\(minutes)"
let secStr = (totalSeconds < 9) ? "0\(Float(totalSecondsTimes100)/100)" : "\(Float(totalSecondsTimes100)/100)"
switch Int(timerChoice) {
case Int(timerCountdownLabel.text!)!:
timerLabel.text = "\(minStr):\(secStr)"
audioPlayer.play()
timer.invalidate()
timerIsRunning = false
default:
timerLabel.text = "\(minStr):\(secStr)"
}
}

Answer 1

You should calculate the seconds as:
let seconds = totalSeconds % 60
and then use seconds in your calculation of secStr instead of using totalSeconds.
There are better ways to write your code:
#objc func updateTimer() {
totalSeconds += 0.01
let minutes = Int(totalSeconds) / 60
let seconds = totalSeconds.remainder(dividingBy: 60)
let timeStr = String(format: "%02d:%06.3f", minutes, seconds)
timerLabel.text = timeStr
if Int(timerCountdonwLabel.text!)! == minutes {
audioPlayer.play()
timer.invalidate()
timerIsRunning = false
}
}
And you really shouldn't keep track of time simply by adding 0.01 to totalSeconds. A Timer is not accurate. Your clock will drift over time. It's best to save a timestamp (Date()) when you start the timer and get the current timestamp (Date()) inside updateTimer and get the difference between the two.

Answer 2

Here is a timer function that outputs format minutes:seconds:milliseconds, compare with your code and you'll find what's wrong with your code.
private weak var timer: Timer?
private var startTime: Double = 0
private var elapsed: Double = 0
private var time: Double = 0
private func startTimer(){
startTime = Date().timeIntervalSinceReferenceDate - elapsed
timer = Timer.scheduledTimer(timeInterval: (0.01), target: self, selector: #selector(updateTimeLabel), userInfo: nil, repeats: true)
}
private func stopTimer(){
elapsed = Date().timeIntervalSinceReferenceDate - startTime
timer?.invalidate()
}
#objc func updateTimeLabel(){
time = Date().timeIntervalSinceReferenceDate - startTime
let minutes = UInt8(time / 60.0)
let timeNoMin = time - (TimeInterval(minutes) * 60)
let seconds = UInt8(timeNoMin)
let timeNoSec = timeNoMin - (TimeInterval(seconds))
let milliseconds = UInt16(timeNoSec * 100)
let strMinutes = String(minutes)
var strSeconds = ""
if strMinutes == "0" {
strSeconds = String(seconds)
}
else {
strSeconds = String(format: "%02d", seconds)
}
let strMilliseconds = String(format: "%02d"), milliseconds)
if strMinutes != "0" {
timerLabel.text = "\(strMinutes):\(strSeconds).\(strMilliseconds)"
}
else {
timerLabel.text = "\(strSeconds).\(strMilliseconds)"
}
}

Answer 3

To get minutes and seconds from a floating point total number of seconds elapsed, elapsed you can:
To get minutes, divide by 60.0 and truncate to the nearest integer:
let minutes = Int(elapsed / 60)
To get seconds, get the remainder, either via:
let seconds = elapsed - Double(minutes) * 60
Or
let seconds = elapsed.truncatingRemainder(dividingBy: 60)
A couple of other observations:
There's no point in running a timer every 0.01 seconds when the screen refresh rate is usually capped at 60 frames per second. If you want to update it with the greatest frequency, use a CADisplayLink which is timed not only for maximum screen refresh rate, but also fires optimally to allow the update to happen before the next frame is to be rendered.
You should not use timer to increment the elapsed time by 0.01 (or any fixed interval) because you have no assurances that it will actually fire with that frequency. If something, for example, momentarily blocks the main thread by 200 milliseconds, you don't want this to affect your calculation of the amount of time that has elapsed.
Instead, save the start time when the timer starts, and every time the timer fires recalculate the elapsed time and format the results accordingly.
To complicate this further, you should not even be comparing Date() instances (or CFAbsoluteTimeGetCurrent() values) because, as the documentation warns us:
Repeated calls to this function do not guarantee monotonically increasing results. The system time may decrease due to synchronization with external time references or due to an explicit user change of the clock.
Instead, you should use a mach_absolute_time based calculation (such as returned by CACurrentMediaTime()), for which repeated calls are assured to return accurately elapsed time calculations.
The only time you should use Date() or CFAbsoluteTimeGetCurrent() if your app is saving the start time in persistent storage, to be retrieved later when the app is restarted (possibly after a device reboot) to render the effect of the elapsed time between starts of an app. But this is a pretty narrow edge case.
Anyway, this yields:
var start: CFTimeInterval?
weak var displayLink: CADisplayLink?
func startTimer() {
self.displayLink?.invalidate() // just in case timer had already been started
start = CACurrentMediaTime()
let displayLink = CADisplayLink(target: self, selector: #selector(handleDisplayLink(_:)))
displayLink.preferredFramesPerSecond = 100 // in case you're using a device that can render more than 60 fps
displayLink.add(to: .main, forMode: .commonModes)
self.displayLink = displayLink
}
#objc func handleDisplayLink(_ displayLink: CADisplayLink) {
let elapsed = CACurrentMediaTime() - start!
let minutes = Int(elapsed / 60)
let seconds = elapsed - Double(minutes) * 60
let string = String(format: "%02d:%05.2f", minutes, seconds)
label.text = string
}
func stopTimer() {
displayLink?.invalidate()
}