Stopwatch not synced - ios

Question

Sorry if this is a newbie question, I am very new to iOS & Swift. I have a problem with the timer interval: I set 0.01 time interval but it doesn't correspond with the timer label, because 0.01 corresponds in one millisecond but it doesn't show it. So basically the timer is skewed.
timer = Timer.scheduledTimer(timeInterval: 0.01, target: self, selector: #selector(updateStopwatch) , userInfo: nil, repeats: true)
#IBAction func startStopButton(_ sender: Any) {
buttonTapped()
}
func updateStopwatch() {
milliseconds += 1
if milliseconds == 100 {
seconds += 1
milliseconds = 0
}
if seconds == 60 {
minutes += 1
seconds = 0
}
let millisecondsString = milliseconds > 9 ?"\(milliseconds)" : "0\(milliseconds)"
let secondsString = seconds > 9 ?"\(seconds)" : "0\(seconds)"
let minutesString = minutes > 9 ?"\(minutes)" : "0\(minutes)"
stopWatchString = "\(minutesString):\(secondsString).\(millisecondsString)"
labelTimer.text = stopWatchString
}
func buttonTapped() {
if isTimerRunning {
isTimerRunning = !isTimerRunning
timer = Timer.scheduledTimer(timeInterval: 0.01, target: self, selector: #selector(updateStopwatch) , userInfo: nil, repeats: true)
startStopButton.setTitle("Stop", for: .normal)
}else{
isTimerRunning = !isTimerRunning
timer.invalidate()
startStopButton.setTitle("Start", for: .normal)
}
}

Answer 1

Devices have maximum screen update rate (most are 60 fps), so there is no point in going faster than that. For maximum screen refresh rate, use a CADisplayLink rather than a Timer, which is coordinated perfectly for screen refreshes (not only in frequency, but also the timing within the screen refresh cycle).
Also don't try to keep track of the time elapsed by adding some value (because you are not guaranteed that it will be called with the desired frequency). Instead, before you start your timer/displaylink, save the start time and then when the timer/displaylink is called, display the elapsed time in the desired format.
For example:
var startTime: CFTimeInterval!
weak var displayLink: CADisplayLink?
func startDisplayLink() {
self.displayLink?.invalidate() // stop prior display link, if any
startTime = CACurrentMediaTime()
let displayLink = CADisplayLink(target: self, selector: #selector(handleDisplayLink(_:)))
displayLink.add(to: .current, forMode: .commonModes)
self.displayLink = displayLink
}
func handleDisplayLink(_ displayLink: CADisplayLink) {
let elapsed = CACurrentMediaTime() - startTime
let minutes = Int(elapsed / 60)
let seconds = elapsed - CFTimeInterval(minutes) * 60
let string = String(format: "%02d:%05.2f", minutes, seconds)
labelTimer.text = string
}
func stopDisplayLink() {
displayLink?.invalidate()
}
Note, CACurrentMediaTime() uses mach_time, like hotpaw2 correctly suggested, but does the conversion to seconds for you.

Answer 2

The time delay of a scheduledTimer is only approximate, and can differ from what is requested by many milliseconds, due to iOS overhead. A repeating Timer is even worse for timing, as any delay jitter errors will accumulate. So don't use a Timer for timing longer events.
A CADisplayLink is a more reliable timer, as it is synchronized to the 60 Hz display refresh (e.g. this is the maximum rate that any UILabel can be changed on devices other than the latest iPad Pros). There is no use trying to update a time display any faster (except possibly on the latest iPad Pros).
Also, do not use Date methods for timing, as they are not guaranteed to be monotonic when the device is connected to a network (as NTP can change the clock time right in the middle of your timing activity).
You should check any elapsed time measurement UI against one of the built-in timers such as mach_time. mach_absolute_time() is guaranteed to be monotonic, and not affected by NTP or other network activity.