How can I make a countdown with NSTimer?

Question

How can I make a countdown with an NSTimer using Swift?

Answer 1

Swift 5 another way. Resistant to interaction with UI

I would like to show a solution that is resistant to user interaction with other UI elements during countdown. In the comments I explained what each line of code means.

 var timeToSet = 0
 var timer: Timer?

 ...

 @IBAction func btnWasPressed(_ sender: UIButton) {

      //Setting the countdown time
      timeLeft = timeToSet
      //Disabling any previous timers.
      timer?.invalidate()
      //Initialization of the Timer with interval every 1 second with the function call.
      timer = Timer(timeInterval: 1.0, target: self, selector: #selector(countDown), userInfo: nil, repeats: true)
      //Adding Timer to the current loop
      RunLoop.current.add(timer!, forMode: .common)

  }

 ...

 @objc func countDown() {

     if timeLeft > 0 {
         print(timeLeft)
         timeLeft -= 1
     } else {
         // Timer stopping
         timer?.invalidate()
     }
 }