iOS 深入理解UINavigationController 和 UIViewController 之间的关系

创建三个类 

BasicViewController : UIViewController
SecondViewController : UIViewController
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： 

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    // Override point for customization after application launch.

    BasicViewController  *basicViewController = [BasicViewController new];//实例化内存后，_parentViewController-->nil
    UINavigationController *navigationController = [[UINavigationController alloc] initWithRootViewController:basicViewController];
    // self.window.rootViewController = basicViewController.parentViewController;
    self.window.rootViewController = navigationController;

    [self.window makeKeyAndVisible];
    return YES;
}

等价于 

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    // Override point for customization after application launch.

    BasicViewController  *basicViewController = [BasicViewController new];//实例化内存后，_parentViewController-->nil
    UINavigationController *navigationController = [[UINavigationController alloc] initWithRootViewController:basicViewController];
    self.window.rootViewController = basicViewController.parentViewController; //不可以注释上一句，因为上一句是为_parentViewController实例化
    // self.window.rootViewController = navigationController;

    [self.window makeKeyAndVisible];
    return YES;
}

分配内存如图