使用Swift 4在WatchOS中创建简单计时器

看起来很简单,但我正在努力。。。下面的代码在设置workoutTimer日期的行上崩溃。另外,我的WKInterfaceTimer没有连接到IBMoutlet,它需要连接吗?我想用它只是为了时间的目的。

class InterfaceController { 
            var workoutTimer: WKInterfaceTimer!
            var workoutStartTime: NSDate? = nil

    func startWorkOutTimer() {
   self.startWorkout()
            if let test = self.workoutSecondsElapsed() {
            print("timer seconds = \(test)")
            }
}


    func startWorkout() {
        // To count up use 0.0 or less, otherwise the timer counts down.
        workoutTimer.setDate(NSDate(timeIntervalSinceNow: 0.0) as Date)
        workoutTimer.start()
        self.workoutStartTime = NSDate()
    }

    func stopWorkout() {
        workoutTimer.stop()
    }

    func workoutSecondsElapsed() -> TimeInterval? {
        // If the timer hasn't been started then return nil
        guard let startTime = self.workoutStartTime else {
            return nil
        }
        // Time intervals from past dates are negative, so
        // multiply by -1 to get the elapsed time.
        return -1.0 * (self.workoutStartTime?.timeIntervalSinceNow)!
    }

}