Code for NSTimer Tutorial

ViewController.swift

//
//  ViewController.swift
//  NSTimerTutorial
//
//  Created by Kc on 22/02/2016.
//  Copyright © 2016 Kenechi Okolo. All rights reserved.
//

import UIKit

class ViewController: UIViewController {
    
    var timer: NSTimer?
    var finishedWithTimer = false
    var timerStartedAt: NSDate?
    
    // MARK: Actions & Outlets
    @IBOutlet weak var timeElapsed: UILabel!
    @IBAction func startTimer(sender: UIButton) {
        timerStartedAt = NSDate()
        finishedWithTimer = false
        timer = NSTimer.scheduledTimerWithTimeInterval(5, target: self, selector: "fire", userInfo: nil, repeats: false)
    }
    @IBAction func stopTimer(sender: UIButton) {
        finishedWithTimer = true
    }
    
    func fire() {
        
        timeElapsed.text = "Time since function last called: \(Int(NSDate().timeIntervalSinceDate(timerStartedAt!)))"
        
        timerStartedAt = NSDate()
        
        let number = randomNumberInRange(1, upper: 10)
        
        timer = NSTimer.scheduledTimerWithTimeInterval(Double(number), target: self, selector: "fire", userInfo: nil, repeats: false)
        
        
        timer?.tolerance = Double(number) / 10
        
        if finishedWithTimer {
            timer?.invalidate()
        }
        
    }
    
    func randomNumberInRange(lower: Int, upper: Int) -> Int {
        return lower + Int(arc4random_uniform(UInt32(upper - lower + 1)))
    }
    
    
    
    
}