1-2 seconds delay in NStimer countdown - swift2

Question

after looking online for a countDown implementation in swift 2, i couldn't find anyones that works in a countdown way. so i made my own however when it reaches second 01 it takes 2 seconds to become 59. for instance if the timer is on 05:01 it takes 2 seconds of lag or timer freeze, then it becomes 4:59.
it looks weird, i'm a complete beginner so my code is a disaster, here it is:
#IBOutlet var countDown: UILabel!
var currentSeconds = 59
var currentMins = 5
var timer = NSTimer()
#IBAction func start(sender: UIButton) {
timer = NSTimer.scheduledTimerWithTimeInterval(1.0, target: self, selector: #selector(ViewController.updateTime), userInfo: nil, repeats: true)
}
func updateTime() {
if (currentSeconds > 9) {
countDown.text = "0\(currentMins):\(currentSeconds)"
currentSeconds -= 1
} else if ( currentSeconds > 0) && (currentSeconds <= 9) {
countDown.text = "0\(currentMins):0\(currentSeconds)"
currentSeconds -= 1
} else {
currentMins -= 1
currentSeconds = 59
}
if (currentSeconds == 0) && (currentMins == 0) {
countDown.text = "time is up!"
timer.invalidate()
}
}
#IBAction func stop(sender: AnyObject) {
timer.invalidate()
}

Answer 1

Because you forgot to update the label:
if (currentSeconds > 9) {
countDown.text = "0\(currentMins):\(currentSeconds)"
currentSeconds -= 1
} else if ( currentSeconds > 0) && (currentSeconds <= 9) {
countDown.text = "0\(currentMins):0\(currentSeconds)"
currentSeconds -= 1
} else {
countDown.text = "0\(currentMins):00" // <-- missed this
currentMins -= 1
currentSeconds = 59
}
However, it'd be better if you use NSDateFormatter to format the number of seconds left instead of managing 2 separate variables:
class ViewController: UIViewController, UITextFieldDelegate {
var secondsLeft: NSTimeInterval = 359
var formatter = NSDateFormatter()
override func viewDidLoad() {
super.viewDidLoad()
timer = NSTimer.scheduledTimerWithTimeInterval(1.0, target: self, selector: #selector(ViewController.updateTime), userInfo: nil, repeats: true)
formatter.dateFormat = "mm:ss"
formatter.timeZone = NSTimeZone(abbreviation: "UTC")!
}
func updateTime()
{
countDown.text = formatter.stringFromDate(NSDate(timeIntervalSince1970: secondsLeft))
secondsLeft -= 1
if secondsLeft == 0 {
countDown.text = "time is up!"
timer.invalidate()
}
}
}