Elapsed Timer not starting - ios

Question

I am building an elapsed timer and while the code gives no errors the timer does not start.
I am using two ViewControllers, one called Stopwatch which has the start stop function in it under the class Stopwatch() and then a regular ViewController with the rest in it.
View Controller Code:
import UIKit
class ViewController: UIViewController {
let watch = Stopwatch()
#IBOutlet weak var elapsedTimeLabel: UILabel!
#IBAction func startButton(_ sender: Any) {
Timer.scheduledTimer(timeInterval: 0.1, target: self, selector: #selector(self.updateElapsedTimeLabel), userInfo: nil, repeats: true)
watch.start()
}
#IBAction func stopButton(_ sender: Any) {
watch.stop()
}
#objc func updateElapsedTimeLabel (timer : Timer) {
if watch.isRunning {
let minutes = Int (watch.elapsedTime/60)
let seconds = watch.elapsedTime.truncatingRemainder(dividingBy: 60)
let tenOfSeconds = (watch.elapsedTime * 10).truncatingRemainder(dividingBy: 10)
elapsedTimeLabel.text = String (format: "%02d:%02d:%02d", minutes, seconds, tenOfSeconds)
} else {
timer.invalidate()
}
}
override func viewDidLoad() {
super.viewDidLoad()
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
}
override var prefersStatusBarHidden: Bool {
return true
}
}
The Stopwatch View Controller code:
import Foundation
class Stopwatch {
private var startTime : Date?
var elapsedTime: TimeInterval {
if let startTime = self.startTime {
return -startTime.timeIntervalSinceNow
} else {
return 0
}
}
var isRunning: Bool {
return startTime != nil
}
func start() {
startTime = Date()
}
func stop() {
startTime = nil
}
}
There is nothing at all coming in the debug window, so not sure what the issue is, I reconnected the buttons over and over so it's not that. I also get no other errors in the code as mentioned above.
Can anyone shed some light on this. Maybe I am using the wrong #selector or I am doing the updateElapsedTimeLabel minutes, seconds, tenOfSeconds calculations wrong. Not sure. Thanks for having a look.

Answer 1

If you Option-click on seconds and tenOfSeconds you will find that one is of type TimeInterval (i.e. Double) and the other is of type Double. So your format specifier of %02d was wrong. In C, a mismatch between the format specifier and the argument is undefined behavior. Swift doesn't say how it handles that but I guess it will ignore the argument.
To fix it, change your format specifier for the last 2 components to %02.f:
let minutes = Int(watch.elapsedTime/60)
let seconds = watch.elapsedTime.truncatingRemainder(dividingBy: 60)
let tenOfSeconds = (watch.elapsedTime * 100).truncatingRemainder(dividingBy: 100) // fixed the math here
elapsedTimeLabel.text = String(format: "%02d:%02.f:%02.f", minutes, seconds, tenOfSeconds)
But why not use a DateFormatter to make your life simpler:
class ViewController: UIViewController {
private let formatter: DateFormatter = {
let formatter = DateFormatter()
formatter.dateFormat = "mm:ss:SS"
return formatter
}()
#objc func updateElapsedTimeLabel (timer : Timer) {
if watch.isRunning {
elapsedTimeLabel.text = formatter.string(from: Date(timeIntervalSince1970: watch.elapsedTime))
} else {
timer.invalidate()
}
}
}