Usage of NSTimer in Swift

timer.swift

import Foundation

class Timer {
    var timer: NSTimer?
    var elapsedTime = 0.0

    func start() {
        elapsedTime = 0.0
        timer = NSTimer(timeInterval: 1.0, target: self, selector: "timerFired:", userInfo: nil, repeats: true)
        NSRunLoop.currentRunLoop().addTimer(timer, forMode: NSRunLoopCommonModes)
    }

    func stop() {
        timer?.invalidate()
        timer = nil
    }

    @objc func timerFired(timer: NSTimer) {
        elapsedTime = elapsedTime + 1
    }
}