控制器NavigationController常用于用来做层次化UI导航。类名为UINavigationController。

NavigationController可以通过属性包含多个ViewController、一个UINavigationBar、一个可选的UIToolbar。

以一个共三层的ViewController的案例，来展示NavigationController的使用：

1. 共三层层次化UI

2. 每一级页面内有一个按钮，可以继续导航到下一级页面

3. 每一个页面的导航条的左侧按钮可以返还到上一级

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代码如下：

import UIKit
    @UIApplicationMain
    class AppDelegate: UIResponder,UIApplicationDelegate {
        var window: UIWindow?
        var nav : Nav?
        func application(_ application: UIApplication,didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
            self.window = UIWindow(frame: UIScreen.main.bounds)
            nav = Nav()
            self.window!.rootViewController = nav
            self.window?.makeKeyAndVisible()
            return true
        }
    }
    class Nav: UINavigationController {
        var count = 0
        var label : UILabel!
        override func viewDidLoad() {
            super.viewDidLoad()
            self.view.backgroundColor = .white
            self.pushViewController(Level1(),animated: true)
        }
    }
    
    class Level1: UIViewController {
        var count = 0
        var label : UILabel!
        override func viewDidLoad() {
            super.viewDidLoad()
            self.title = "Level 1"
                    self.view.backgroundColor = .white
                    let button   = UIButton(type: .system)
                    button.frame = CGRect(x: 120,y: 100,width: 100,height: 50)
                    button.setTitle("Dive Into 2",for: .normal)
                    button.addTarget(self,action: #selector(Level1.buttonAction(_:)),for: .touchUpInside)
                    view.addSubview(button)
        }
        func buttonAction(_ sender:UIButton!){
            let appDelegate = UIApplication.shared.delegate as! AppDelegate
            appDelegate.nav?.pushViewController(Level2(),animated: true)
        }
    }
    
    class Level2: UIViewController {
        var count = 0
        var label : UILabel!
        override func viewDidLoad() {
            super.viewDidLoad()
            self.title = "Level 2"
            self.view.backgroundColor = .white
            let button   = UIButton(type: .system)
            button.frame = CGRect(x: 120,height: 50)
            button.setTitle("Dive Into 3",for: .normal)
            button.addTarget(self,action: #selector(Level2.buttonAction(_:)),for: .touchUpInside)
            view.addSubview(button)
        }
        func buttonAction(_ sender:UIButton!){
            let appDelegate = UIApplication.shared.delegate as! AppDelegate
            appDelegate.nav?.pushViewController(Level3(),animated: true)
        }
    }
    class Level3: UIViewController {
        var count = 0
        var label : UILabel!
        override func viewDidLoad() {
            super.viewDidLoad()
            self.title = "Level 3"
            self.view.backgroundColor = .white
            let button   = UIButton(type: .system)
            button.frame = CGRect(x: 120,height: 50)
            button.setTitle("End",action: #selector(Level3.buttonAction(_:)),for: .touchUpInside)
            view.addSubview(button)
        }
        func buttonAction(_ sender:UIButton!){
            
        }
    }

解释下：

1. 把Nav作为UINavigationController的子类，实例化此类并设置到window.rootViewController上

2. 每一层都是继承于UIViewController，类名分别为Level1、Level2、Level3

3. Nav类视图加载完成后，把第一层Level1压入导航视图

4. 无需任何用户代码，UINavigationController本身默认提供页面的顶部条状区域，它被称为导航条(UINavigationBar)

5. 点击此导航条的左侧按钮可以返还到上一级页面；导航条中间显示当前ViewController的title属性值

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