如何在swift 2.2中睡几毫秒？

请有人告诉我如何在swift 2.2中使用sleep()几毫秒？

while (true){
    print("sleep for 0.002 seconds.")
    sleep(0.002) // not working
}

但

while (true){
    print("sleep for 2 seconds.")
    sleep(2) // working
}

这是工作。

usleep()占用百万分之一秒

usleep(1000000) //will sleep for 1 second
usleep(2000) //will sleep for .002 seconds

要么

let ms: UInt32 = 1000
usleep(2 * ms) //will sleep for 2 milliseconds (.002 seconds)

要么

let second: UInt32 = 1000000
usleep(UInt32(0.002) * second) //will sleep for 2 milliseconds (.002 seconds)