快速计时器（以毫秒为单位）

正如Martin在评论中所说，计时器的分辨率为50-100毫秒（0.05至0.1秒）。尝试以短于该间隔的时间运行计时器将无法获得可靠的结果。另外，计时器不是实时的。它们取决于它们所附加的运行循环，并且如果运行循环变得繁忙，则计时器的启动会延迟。

不要在每次触发计时器时尝试增加计数器，而是在启动计时器时记录开始时间，然后做一些数学运算以弄清楚发生了多少时间：

var startTime: NSTimeInterval//Sart the timerstartTime = NSDate.timeIntervalSinceReferenceDate()NSTimer.scheduledTimerWithTimeInterval(0.02,   target: self,   selector: Selector("advanceTimer:"),   userInfo: nil,  repeats: true)//And your  timer method...func advanceTimer(timer: NSTimer){  //Total time since timer started, in seconds  self.time = NSDate.timeIntervalSinceReferenceDate() - startTime  //The rest of your pre goes here }

此代码的Swift 3版本如下所示：

（写为测试项目中的视图控制器）

class ViewController: UIViewController {  weak var timer: Timer?  var startTime: Double = 0  var time: Double = 0  @IBOutlet weak var timevalueLabel: UILabel!    override func viewDidAppear(_ animated: Bool) {    startTime = Date().timeIntervalSinceReferenceDate    timer = Timer.scheduledTimer(timeInterval: 0.05,target: self,selector: #selector(advanceTimer(timer:)),userInfo: nil,repeats: true)  }  //When the view controller is about to disappear, invalidate the timer  override func viewWillDisappear(_ animated: Bool) {    timer?.invalidate()  }  func advanceTimer(timer: Timer) {    //Total time since timer started, in seconds    time = Date().timeIntervalSinceReferenceDate - startTime    //The rest of your pre goes here    //Convert the time to a string with 2 decimal places    let timeString = String(format: "%.2f", time)    //Display the time string to a label in our view controller    timevalueLabel.text = timeString  }}