//截止日期let endDate = datePicker.date//開始日期let startDate = Date()//時間間隔let timeInterval:TimeInterval = endDate.timeIntervalSince(startDate)if timer == nil { //剩余時間 var timeout = timeInterval if timeout != 0 { //創建全局隊列 let queue = DispatchQueue.global() //在全局隊列下創建一個時間源 timer = DispatchSource.makeTimerSource(flags: [], queue: queue) //設定循環的間隔是一秒,并且立即開始 timer?.scheduleRepeating(wallDeadline: DispatchWallTime.now(), interval: .seconds(1)) //時間源出發事件 timer?.setEventHandler(handler: {  //必須是當前日期往后的日期,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 {  //計算剩余時間  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  //主隊列中刷新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  } }) //啟動時間源 timer?.resume() }}