iOS 深入理解UINavigationController 和 UIViewController 之间的关系

创建三个类

1. BasicViewController : UIViewController
2. SecondViewController : UIViewController
3. ThirdViewController : UIViewController

然后我们在BasicViewController .m文件中push一个viewController： 
SecondViewController *svc = [SecondViewController new]; 
[self.navigationController pushViewController:svc animated:true]; 
在SecondViewController.m文件中pop出viewController： 
[self.navigationController popViewControllerAnimated:true]

问题就来了， 
push和pop是同一个viewController，那为什么用self.navigationController 就可以知道了

其中在AppDelegate.m：

1. - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
2. {
3. self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
4. // Override point for customization after application launch.
5. BasicViewController  *basicViewController = [BasicViewController new];//实例化内存后，_parentViewController-->nil
6. UINavigationController *navigationController = [[UINavigationController alloc] initWithRootViewController:basicViewController];
7. // self.window.rootViewController = basicViewController.parentViewController;
8. self.window.rootViewController = navigationController;
9. [self.window makeKeyAndVisible];
10. return YES;
11. }

等价于

1. - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
2. {
3. self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
4. // Override point for customization after application launch.
5. BasicViewController  *basicViewController = [BasicViewController new];//实例化内存后，_parentViewController-->nil
6. UINavigationController *navigationController = [[UINavigationController alloc] initWithRootViewController:basicViewController];
7. self.window.rootViewController = basicViewController.parentViewController; //不可以注释上一句，因为上一句是为_parentViewController实例化
8. // self.window.rootViewController = navigationController;
9. [self.window makeKeyAndVisible];
10. return YES;
11. }

分配内存如图