Creating Navigation Controllers
方法：initWithRootViewController:
Initializes and returns a newly created navigation controller.
- (instancetype)initWithRootViewController:(UIViewController *)rootViewController
参数：UIViewController
方法：initWithNavigationBarClass:toolbarClass
Initializes and returns a newly created navigation controller that uses your custom bar subclasses.
- (instancetype)initWithNavigationBarClass:(Class)navigationBarClass
toolbarClass:(Class)toolbarClass
参数：navigationBarClass、toolbarClass
附言：
Use this initialization method if you want to use custom navigation bar or toolbar subclasses with the navigation controller. If you use this method, you are responsible for adding a root view controller before presenting the navigation controller onscreen.