Swift如何实现倒计时5秒功能

import UIKit
 
class GAPublishViewController: GABaseViewController {
 
 var jumpBut = UIButton(frame: CGRect(x: 15, y: 64, width: 80, height: 40));
 var limitTime: Int = 5+1;
 
 override func viewDidLoad() {
  super.viewDidLoad()
  setupJumpButton();
  startCountDown();
 }
 
 func setupJumpButton() {
  view.addSubview(jumpBut);
  jumpBut.setTitle("跳过(5S)", for: .normal);
  jumpBut.setTitleColor(UIColor.red, for: .normal);
  jumpBut.addTarget(self, action: #selector(tapJumpAction(sender:)), for: .touchUpInside);
 }
 
 @objc func tapJumpAction(sender: Any) {
  let but = sender as! UIButton;
  let text = but.titleLabel?.text ?? "";
  if (text == "跳过") {
   print("点击“跳过”");
  }
 }
 
 // MARK: 定时方式一
 
 func startCountDown() {
  performSelector(inBackground: #selector(countDownThread), with: nil)
 }
 
 @objc func countDownThread() {
  let timeCount = limitTime;
  for _ in 0..<timeCount {
   limitTime = limitTime - 1;
   self.performSelector(onMainThread: #selector(updateJumpBtn), with: self, waitUntilDone: true)
   sleep(1);
  }
 }
 
 @objc func updateJumpBtn() {
  if (limitTime <= 0) {
   jumpBut.setTitle("跳过", for: .normal);
  } else {
   jumpBut.setTitle("跳过" + "(\(limitTime)S)", for: .normal);
  }
 }
 
}

 // MARK: 定时方式二
 
 func startCountDown() {
  // 将任务添加到队列，以异步的方式执行
  DispatchQueue.global().async { [weak self] in
   self?.countDownThread();
  }
 }
 
 func countDownThread() {
  let timeCount = limitTime;
  for _ in 0..<timeCount {
   limitTime = limitTime - 1;
   // 主线程刷新UI
   DispatchQueue.main.async { [weak self] in
    self?.updateJumpBtn();
   }
   sleep(1);
  }
 }
 
 func updateJumpBtn() {
  if (limitTime <= 0) {
   jumpBut.setTitle("跳过", for: .normal);
  } else {
   jumpBut.setTitle("跳过" + "(\(limitTime)S)", for: .normal);
  }
 }

// MARK: 定时方式三
 
 var limitTime: Int = 5;
 var timer: Timer?;
 
 func startCountDown() {
  // 初始化定时器
  timer = Timer.scheduledTimer(timeInterval: 1.0, target: self, selector: #selector(updateJumpBtn), userInfo: nil, repeats: true);
  
  /*
  // 避免timer在列表时，滑动时timer会暂停。 将timer放在另外一个线程中，然后开启这个线程的runloop。
  DispatchQueue.global().async { [weak self] in
   self?.timer = Timer.scheduledTimer(timeInterval: 1.0, target: self as Any, selector: #selector(self?.countDownThread), userInfo: nil, repeats: true);
   RunLoop.current.run();
  }
   */
 }
 
 @objc func countDownThread() {
  // 主线程刷新UI
  DispatchQueue.main.async { [weak self] in
   self?.updateJumpBtn();
  }
 }
 
 @objc func updateJumpBtn() {
  limitTime = limitTime - 1;
  if (limitTime <= 0) {
   jumpBut.setTitle("跳过", for: .normal);
   /*
   // 暂停定时器
   timer?.fireDate = Date.distantFuture;
   // 继续定时
   timer?.fireDate = NSDate.init() as Date;
   // 暂停定时器3秒
   timer?.fireDate = Date.init(timeIntervalSinceNow: 3.0);
    */
   // 停止定时器
   timer?.invalidate();
  } else {
   jumpBut.setTitle("跳过" + "(\(limitTime)S)", for: .normal);
  }
 }

// MARK: 定时方式四
 
 var limitTime: Int = 5+1;
 // 在global线程里创建一个时间源
 let codeTimer = DispatchSource.makeTimerSource(queue: DispatchQueue.global());
 
 func startCountDown() {
  // 设定这个时间源是每秒循环一次，立即开始
  codeTimer.schedule(deadline: .now(), repeating: .seconds(1));
  // 设定时间源的触发事件
  codeTimer.setEventHandler(handler: {
   // 主线程刷新UI
   DispatchQueue.main.async { [weak self] in
    self?.updateJumpBtn();
   }
  })
  // 判断是否取消，如果已经取消了避免调用resume()方法导致的崩溃
  if codeTimer.isCancelled {
   return;
  }
  // 启动时间源
  codeTimer.resume();
 }
 
 func updateJumpBtn() {
  limitTime = limitTime - 1;
  if (limitTime <= 0) {
   jumpBut.setTitle("跳过", for: .normal);
   // 暂停计时。暂停之后，再次开始计时（startCountDown()）接着上次暂停进行计时
   codeTimer.suspend();
   // 取消计时。取消之后，再次开始计时（startCountDown()）不会再计时
   //codeTimer.cancel();
  } else {
   jumpBut.setTitle("跳过" + "(\(limitTime)S)", for: .normal);
  }
 }