Swift：我可以加减 `dispatch_time_t` 变量吗？

Question

我需要在 iOS 和 Swift 中做一些时间计算。

我必须使用 dispatch_walltime。我希望这可以被视为公理化。在时间数学方面，我想我可能会得到 "just use NSDate," 的回应，但请相信：我一定会 dispatch_walltime.

现在，很明显为什么有人会建议 NSDate，因为当您使用 NSTimeInterval 和 NSDate 等好东西时，制作自定义时间戳并进行比较非常容易他们并做各种时间数学。

但我必须使用 dispatch_time_t，具体来说 dispatch_walltime 是这样创建的：

//Get the timeInterval of now.
let nowInterval = NSDate().timeIntervalSince1970

//Make a timespec from it.
var nowStruct = timespec(tv_sec: Int(nowInterval), tv_nsec: 0)

//Make a walltime definition from that.
let referenceWalltime = dispatch_walltime(&nowStruct, 0)

稍后我需要以各种方式使用该参考时间。例如，我需要获取参考时间和它恰好是的任何时间之间的时间间隔。

我正在尝试以与 NSTimeInterval 相同的方式进行此操作，换句话说，制作一个新的并从中减去旧的：

//Repeat everything from before to make a new wall time.
let newNowInterval = NSDate().timeIntervalSince1970
var newNowStruct = timespec(tv_sec: Int(newNowInterval), tv_nsec: 0)
let newWalltime = dispatch_walltime(& newNowStruct, 0)

//Time math a la NSTimeInterval to find the interval:
let walltimeInterval = newWalltime - referenceWalltime

这行得通吗？

Answer 1

简短的回答是：没有。该代码将崩溃。

更好的答案是：不，但可以做到，而且到头来也没什么不同。

我自己在 Playground 上做了一些调查，学到了一些有趣的东西，我相信我找到了正确的方法。

我将我的整个 Playground 粘贴到这里，以便其他人可以复制粘贴它并弄清楚如何进行他们自己的 dispatch_time 数学运算。

代码中标有 //********* 的注释表示我学到的关键内容。

import UIKit
import XCPlayground
XCPSetExecutionShouldContinueIndefinitely(continueIndefinitely: true)

public class IntervalMaker {

    var referenceWalltime: dispatch_time_t = 0
    var newWalltime: dispatch_time_t = 0
    var walltimeInterval: dispatch_time_t = 0

    func scheduleWalltimeSequence () {
         let threeSeconds = Int64(NSEC_PER_SEC) * 3
         let now = walltimeNow()
         let dispatchTimeInThree = dispatch_time(now, threeSeconds)
         let dispatchTimeInSix = dispatch_time(now,
             2 * threeSeconds)
         setReferenceWalltimeToNow()
         dispatch_after(dispatchTimeInThree, dispatch_get_main_queue(),
             setNewWalltimeToNow)
         dispatch_after(dispatchTimeInSix,
             dispatch_get_main_queue(), dispatchBasedOnDispatchMath)
    }

    func walltimeNow()->dispatch_time_t{
         let nowInterval = NSDate().timeIntervalSince1970
         var nowStruct = timespec(tv_sec: Int(nowInterval), tv_nsec: 0)
         return dispatch_walltime(&nowStruct, 0)
    }

    func setReferenceWalltimeToNow () {
         referenceWalltime = walltimeNow()
    }

    func setNewWalltimeToNow (){
         newWalltime = walltimeNow()
    }

    func dispatchBasedOnDispatchMath () {
         computeInterval() //Should be three seconds
         let nineTheWrongWay = referenceWalltime + (walltimeInterval * 3)
         let nineTheRightWay = dispatch_time(referenceWalltime,
             Int64(walltimeInterval) * 3)
         dispatch_after(nineTheWrongWay,
             dispatch_get_main_queue(), finalPrintln)
//********** THE ABOVE DOES NOT WORK CORRECTLY - prints 6 seconds later
         dispatch_after(nineTheRightWay,
             dispatch_get_main_queue(), finalPrintln)
//********** THE ABOVE WORKS CORRECTLY - prints 9 seconds later
    }

    func finalPrintln () {
        let now = walltimeNow()
        println("I should be printing nine seconds from reference time.")
        println("It's actually \(referenceWalltime - now) nanoseconds after")
    }

    func computeInterval () {
        walltimeInterval = referenceWalltime - newWalltime
//********** dispatch_walltimes actually count backwards, and *CANNOT* be
//********** negative: writing `newWalltime - referenceWalltime` will crash
    }
}

 let intervaller = IntervalMaker()
 intervaller.scheduleWalltimeSequence()

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