//截止日期
let endDate = datePicker.date
//开始日期
let startDate = Date()
//时间间隔
let timeInterval:TimeInterval = endDate.timeIntervalSince(startDate)
if timer == nil {
 //剩余时间
 var timeout = timeInterval
 if timeout != 0 {
 //创建全局队列
 let queue = DispatchQueue.global()
 //在全局队列下创建一个时间源
 timer = DispatchSource.makeTimerSource(flags: [], queue: queue)
 //设定循环的间隔是一秒,并且立即开始
 timer?.scheduleRepeating(wallDeadline: DispatchWallTime.now(), interval: .seconds(1))
 //时间源出发事件
 timer?.setEventHandler(handler: {
  //必须是当前日期往后的日期,在datePicker上也做了限制
  if timeout <= 0 {
  self.timer?.cancel()
  self.timer = nil
  DispatchQueue.main.async(execute: {
   self.day.text = "00"
   self.hour.text = "00"
   self.minute.text = "00"
   self.second.text = "00"
  })
  } else {
  //计算剩余时间
  let days = Int(timeout) / (3600 * 24)
  if days == 0 {
   self.day.text = ""
  }
  let hours = (Int(timeout) - Int(days) * 24 * 3600) / 3600
  let minutes = (Int(timeout) - Int(days) * 24 * 3600 - Int(hours) * 3600) / 60
  let seconds = Int(timeout) - Int(days) * 24 * 3600 - Int(hours) * 3600 - Int(minutes) * 60
  //主队列中刷新UI
  DispatchQueue.main.async(execute: {
   if days == 0 {
   self.day.text = "0"
   } else {
   self.day.text = "\(days)"
   }
   if hours < 10 {
   self.hour.text = "0" + "\(hours)"
   } else {
   self.hour.text = "\(hours)"
   }
   if minutes < 10 {
   self.minute.text = "0" + "\(minutes)"
   } else {
   self.minute.text = "\(minutes)"
   }
   if seconds < 10 {
   self.second.text = "0" + "\(seconds)"
   } else {
   self.second.text = "\(seconds)"
   }
  })
  timeout -= 1
  }
 })
 //启动时间源
 timer?.resume()
 }
}