先写代码，之后写讲解，最近有点忙

 //发送验证码
    @IBAction func sendCodeBtn(_ sender: Any) {
        
        var codeNum = codeNumTF.text
        
        //调用方法
        self.countDown(timeOut: 5)
          
    }
    
    //验证码倒计时
    func countDown(timeOut:Int){
        
        //倒计时时间
        var timeout = timeOut
        let queue:DispatchQueue = DispatchQueue.global(qos: .default)
        
        // 在global线程里创建一个时间源
        let codeTimer = DispatchSource.makeTimerSource(queue:queue)
        
        
           codeTimer.scheduleRepeating(deadline: .now(), interval:.seconds(1))

        //每秒执行
        codeTimer.setEventHandler(handler: { () -> Void in
            if(timeout<=0){ //倒计时结束，关闭
                
                
                codeTimer.cancel()
                
                DispatchQueue.main.sync(execute: { () -> Void in
                    //设置界面的按钮显示 根据自己需求设置
                    self.sendCode.setTitle("重新发送", for: UIControlState.normal)
                    
                    self.sendCode.isUserInteractionEnabled = true
                    
                    
                })
            }else{//正在倒计时
                
                let seconds = timeout
                let strTime = NSString.localizedStringWithFormat("%.d", seconds)
                
                DispatchQueue.main.sync(execute: { () -> Void in
                    //                    NSLog("----%@", NSString.localizedStringWithFormat("%@S", strTime) as String)
                    
                    UIView.beginAnimations(nil, context: nil)
                    UIView.setAnimationDuration(1)
                    //设置界面的按钮显示 根据自己需求设置
                    self.sendCode.setTitle(NSString.localizedStringWithFormat("%@S", strTime) as String, for: UIControlState.normal)
                    UIView.commitAnimations()
                    self.sendCode.isUserInteractionEnabled = false
                })
                timeout -= 1;
            }
            
        })
        codeTimer.resume()
    }
        
        
}

貌似不用讲解，太简单了