ios – 如何在Swift中实现非常精确的计时？

我在iPhone 7上测试了Timer和dispatchSourceTimer(又名GCD计时器)的1000个数据点,间隔为0.05秒.我期待GCD计时器明显更准确(假设它有一个专用队列),但我发现它们是可比较的,我的各种试验的标准偏差范围从0.2-0.8毫秒,最大偏差平均值约为2 -8毫秒.

当按照Technical Note TN2169: High Precision Timers in iOS / OS X中的概述尝试mach_wait_until时,我实现的计时器大约是我用Timer或GCD计时器获得的计数器的4倍.

话虽如此,我并不完全相信mach_wait_until是最好的方法,因为确定thread_policy_set的具体策略值似乎记录不清.但是下面的代码反映了我在测试中使用的值,使用了从How to set realtime thread in Swift?和TN2169改编的代码：

var timebaseInfo = mach_timebase_info_data_t()

func configureThread() {
    mach_timebase_info(&timebaseInfo)
    let clock2abs = Double(timebaseInfo.denom) / Double(timebaseInfo.numer) * Double(NSEC_PER_SEC)

    let period      = UInt32(0.00 * clock2abs)
    let computation = UInt32(0.03 * clock2abs) // 30 ms of work
    let constraint  = UInt32(0.05 * clock2abs)

    let THREAD_TIME_CONSTRAINT_POLICY_COUNT = mach_msg_type_number_t(MemoryLayout<thread_time_constraint_policy>.size / MemoryLayout<integer_t>.size)

    var policy = thread_time_constraint_policy()
    var ret: Int32
    let thread: thread_port_t = pthread_mach_thread_np(pthread_self())

    policy.period = period
    policy.computation = computation
    policy.constraint = constraint
    policy.preemptible = 0

    ret = withUnsafeMutablePointer(to: &policy) {
        $0.withMemoryRebound(to: integer_t.self,capacity: Int(THREAD_TIME_CONSTRAINT_POLICY_COUNT)) {
            thread_policy_set(thread,UInt32(THREAD_TIME_CONSTRAINT_POLICY),$0,THREAD_TIME_CONSTRAINT_POLICY_COUNT)
        }
    }

    if ret != KERN_SUCCESS {
        mach_error("thread_policy_set:",ret)
        exit(1)
    }
}

然后我可以这样做：

private func nanosToAbs(_ nanos: UInt64) -> UInt64 {
    return nanos * UInt64(timebaseInfo.denom) / UInt64(timebaseInfo.numer)
}

private func startMachTimer() {
    Thread.detachNewThread {
        autoreleasepool {
            self.configureThread()

            var when = mach_absolute_time()
            for _ in 0 ..< maxCount {
                when += self.nanosToAbs(UInt64(0.05 * Double(NSEC_PER_SEC)))
                mach_wait_until(when)

                // do something
            }
        }
    }
}

注意,你可能想知道什么时候还没有通过(如果你的处理无法在规定的时间内完成,你想确保你的计时器没有积压),但希望这说明了这个想法.

无论如何,使用mach_wait_until,我获得了比定时器或GCD定时器更高的保真度,代价是cpu /功耗,如What are the do’s and dont’s of code running with high precision timers?所述

我很欣赏你对这一最后一点的怀疑,但我怀疑潜入CoreAudio并看看它是否可以提供更强大的解决方案是明智的.