Swift2 dispatch_get_main_queue（）不'；无法顺利运行新的异步作业 let dispatchGroup=dispatch\u group\u create（） 让我们现在就开始行动 对于0中的i..

问题不是

dispatch\u get\u main\u queue（）

。（如果使用不同的队列，您会注意到相同的行为。）问题在于

dispatch\u after（）

当您在之后使用

dispatch\u时，它会创建一个调度计时器，当

启动时，该计时器的剩余时间为

启动时间的10%。看看苹果github。净效应是，当这些计时器（
开始
±10%
回旋余地
）重叠时，可能会开始合并它们。当它们结合在一起时，它们会以“聚集”的方式发射，一束接着一束地发射，然后在到达下一束之前稍有延迟

有两种解决方案，都需要在调用后退出一系列
dispatch\u：

您可以手动构建计时器，强制
DispatchSource.TimerFlag.strict
禁用合并：

let dispatchGroup = dispatch_group_create()
let now = DISPATCH_TIME_NOW

for i in 0..<1000 {
    dispatch_group_enter(dispatchGroup)

    // Do some async tasks
    let delay = dispatch_time(now, Int64(Double(i) * 0.1 * Double(NSEC_PER_SEC)))

    dispatch_after(delay, dispatch_get_main_queue(), {
        print(i)
        dispatch_group_leave(dispatchGroup)
    })
}


这不仅解决了合并问题，而且效率更高

有关Swift 2.3格式副本，请参见

 let group = DispatchGroup()
 let queue = DispatchQueue.main

 let start = CACurrentMediaTime()

 os_log("start")

 for i in 0 ..< 1000 {
     group.enter()

     let timer = DispatchSource.makeTimerSource(flags: .strict, queue: queue) // use `.strict` to avoid coalescing
     timer.setEventHandler {
         timer.cancel()       // reference timer so it has strong reference until the handler is called
         os_log("%d", i)
         group.leave()
     }
     timer.schedule(deadline: .now() + Double(i) * 0.1)
     timer.resume()
 }

 group.notify(queue: .main) {
     let elapsed = CACurrentMediaTime() - start
     os_log("all done %.1f", elapsed)
 }

 var timer: DispatchSourceTimer?    // note this is property to make sure we keep strong reference 

 func startTimer() {
     let queue = DispatchQueue.main

     let start = CACurrentMediaTime()

     var counter = 0

     // Do some async tasks

     timer = DispatchSource.makeTimerSource(flags: .strict, queue: queue)
     timer!.setEventHandler { [weak self] in
         guard counter < 1000 else {
             self?.timer?.cancel()
             self?.timer = nil
             let elapsed = CACurrentMediaTime() - start
             os_log("all done %.1f", elapsed)
             return
         }
         os_log("%d", counter)
         counter += 1
     }
     timer!.schedule(deadline: .now(), repeating: 0.05)
     timer!.resume()
 }