Measuring Time Accurately in Swift for Comparison Across Devices

Question

I need to be able to record reaction time, from when the screen loads or the question label refreshes until the user taps a number button. I\'m not finding documentation fro

Answer 1

As already said, the precision of NSDate() is probably good enough for your purpose. Just for the sake of completeness, mach_absolute_time() from the referenced Technical Q&A QA1398 works in Swift as well:

let t1 = mach_absolute_time()
// do something
let t2 = mach_absolute_time()

let elapsed = t2 - t1
var timeBaseInfo = mach_timebase_info_data_t()
mach_timebase_info(&timeBaseInfo)
let elapsedNano = elapsed * UInt64(timeBaseInfo.numer) / UInt64(timeBaseInfo.denom);
print(elapsedNano)

Possible advantages of this method:

• Calling mach_absolute_time() seems to be much faster than calling NSDate().timeIntervalSinceReferenceDate, so this method might be better suited to measure extremely short intervals.
• The value of NSDate() changes if the clock is adjusted, mach_absolute_time() does not have this problem.

Answer 2

You can use NSTimeInterval to measure time (much better than a timer). You just need to store two dates (two points in time) and subtract endTime - StartTime as follow:

import UIKit

class ViewController: UIViewController {

    var startTime: TimeInterval = 0
    var endTime: TimeInterval = 0

    override func viewDidLoad() {
        super.viewDidLoad()
        startTime = Date().timeIntervalSinceReferenceDate
    }

    @IBAction func stopTimeAction(_ sender: Any) {
        endTime = Date().timeIntervalSinceReferenceDate
        print((endTime-startTime).time)
    }

}

---

extension TimeInterval {
    var time: String { .init(format: "%d:%02d:%02d.%03d", Int(self/3600),
                        Int((self/60).truncatingRemainder(dividingBy: 60)),
                        Int(truncatingRemainder(dividingBy: 60)),
                        Int((self*1000).truncatingRemainder(dividingBy: 1000))) }
}

Answer 3

NSDate is tied to the realtime clock on iOS and Mac devices, and has sub-millisecond accuracy.

Use NSDate's timeIntervalSinceReferenceDate method to convert an NSDate (or the current time) to a double that is the number of seconds since January 1, 2001, including fractional seconds. Once you do that you're free to do math on the doubles that you get back.