<span style="font-size:18px;">时间控制器NSTimer可以实现定时器功能，即每隔一定时间执行具体函数，可以重复也可以只执行一次。</span>

class DownloadManagerTableTableViewController: UITableViewController {
    
    var timer : NSTimer?
    
    override func viewDidLoad() {
        super.viewDidLoad()

    }
    //页面显示之前设定定时器
    override func viewDidAppear(animated: Bool) {
        super.viewDidAppear(animated)
        self.timer = NSTimer.scheduledTimerWithTimeInterval(0.5,target: self,selector: "timerFired",userInfo: nil,repeats: true)
       
    }

    //页面消失后关闭定时器
    override func viewDidDisappear(animated: Bool) {
        super.viewDidDisappear(animated)
        guard let timer = self.timer
            else { return }
        timer.invalidate()
    }
    func timerFired () {
        //print("timer ticked")
        self.tableView.reloadData()
    }
}