import UIKit

class ViewController: UIViewController {
    
    var count = 0
    //声明导航条
    var navigationBar:UINavigationBar?
    
    override func viewDidLoad() {
        super.viewDidLoad()
        //实例化导航条
        navigationBar = UINavigationBar(frame: CGRect(x:0, y:20, width:320, height:44))
        self.view.addSubview(navigationBar!)
        onAdd()
    }
    
    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
    
    //增加导航项函数
    @objc func onAdd(){
        count = count + 1
        //给导航条增加导航项
        navigationBar?.pushItem(onMakeNavitem(), animated: true)
    }
    
    //删除导航项函数
    @objc func onRemove(){
        if count > 1{
            //减少导航项数量
            count = count - 1
            //从导航条中移除最后一个导航项
            navigationBar?.popItem(animated: true)
        }
    }
    
    //创建一个导航项
    func onMakeNavitem()->UINavigationItem{
        let navigationItem = UINavigationItem()
        //创建左边按钮
        let leftBtn = UIBarButtonItem(barButtonSystemItem: UIBarButtonSystemItem.add,
                                      target: self, action: #selector(onAdd))
        //创建右边按钮
        let rightBtn = UIBarButtonItem(barButtonSystemItem: UIBarButtonSystemItem.cancel,
                                       target: self, action: #selector(onRemove))
        //设置导航栏标题
        navigationItem.title = "第\(count)个导航项"
        //设置导航项左边的按钮
        navigationItem.setLeftBarButton(leftBtn, animated: true)
        //设置导航项右边的按钮
        navigationItem.setRightBarButton(rightBtn, animated: true)
        return navigationItem
    }
}