无法识别的选择器发送到实例 NSTimer Swift

我正在尝试开发一个包含简单秒表功能的应用程序。我正在使用 Xcode 6 和 Swift 语言。这是 FirstViewController 中的代码

@IBAction func Stopwatch (Sender:UIButton) { 

var startTime = NSTimeInterval()

    func updateTime(timer:NSTimer) {

        //Find Current TIme

        var currentTime = NSDate.timeIntervalSinceReferenceDate()

        //Find the difference between current time and start time

        var elapsedTime :NSTimeInterval = currentTime - startTime

        //calculate the minutes in elapsed time.
        let minutes = UInt(elapsedTime / 60.0)
        elapsedTime -= (NSTimeInterval(minutes) * 60)

        //calculate the seconds in elapsed time.
        let seconds = UInt(elapsedTime)
        elapsedTime -= NSTimeInterval(seconds)

        //find out the fraction of milliseconds to be displayed.
        let hours = UInt(elapsedTime * 100)

        let strMinutes = minutes > 9 ? String(minutes):String(minutes)
        let strSeconds = seconds > 9 ? String(seconds):String(seconds)
        let strHours = hours > 9 ? String(hours):String(hours)

        //assign text to Label
        elapsedTimeLabel.text = "You have slept for \(strHours)"
    }

    //Timer 
    var timer = NSTimer.scheduledTimerWithTimeInterval(0.5, target: self, selector: Selector("updateTime:"), userInfo: nil, repeats: true)
    startTime = NSDate.timeIntervalSinceReferenceDate()

}
}

每次我运行应用程序（在 iOS 模拟器中）时，当我按下按钮（执行秒表功能）时，应用程序就会崩溃。错误总是显示

2014-11-23 11:44:53.617 Sleep[9630:644637] -[Sleep.FirstViewController updateTime:]: 
unrecognized selector sent to instance 0x7ff6d9676e80
2014-11-23 11:44:53.622 Sleep[9630:644637] *** Terminating app due to uncaught exception 
'NSInvalidArgumentException', reason: '-[Sleep.FirstViewController updateTime:]: unrecognized 
selector sent to instance 0x7ff6d9676e80'
*** First throw call stack:

我不知道为什么会发生这种情况，而且我对 iOS 编程还很陌生，所以请帮忙！

让 updateTime 成为一个类方法。
如果它是在纯 Swift 类中，则需要在该方法的声明前面加上 @objc，如下所示：

class A {
     @objc func updateTime(timer:NSTimer) {

     }
}