超类变量不适用于CFAbsoluteTime

Question

我遇到了在超类中定义的函数无法正常工作的问题。经过一些调查后发现，超类中定义的变量在子类中不起作用，尽管代码提示识别它并且代码正在运行，并且在经过大量的摆弄之后我完全陷入困境：

class Super {

    //records absolute time of last frame
    var timeThen: Double = 0

    func getElapsedTime () -> CGFloat {
        //records current time
        let timeNow = CFAbsoluteTimeGetCurrent()
        //finds duration since last update
        let duration = timeNow-timeThen
        //resets timeThen to current time for next run
        timeThen = timeNow
        //casts Double value to CGFloat and returns
        return CGFloat(duration)
    }

    init () {
        timeThen = CFAbsoluteTimeGetCurrent()
    }

}

子类如下：

class Sub: Super {

    //gets time from function but doesn't work and seems to run incredibly slowly
    let timeElapsed = getElapsedTime()
    //run code based on interval
}

所以我尝试手动完成：

class Sub: Super {

    //still doesn't work
    let timeNow = CFAbsoluteTimeGetCurrent()
    let timeElapsed = CGFloat(timeNow-timeThen)
    timeThen = timeNow
    //run code based on interval

}  

但是，如果我使用子类中定义的变量，它的完美运作：

class Sub: Super {

    //new variable to replace 'faulty' one 
    var timeThen2: Double = 0

    // Now it works
    let timeNow = CFAbsoluteTimeGetCurrent()
    let timeElapsed = CGFloat(timeNow-timeThen2)
    timeThen2 = timeNow
    // run code based on interval
} 

有什么想法吗？

非常感谢， 千瓦