UIViewController navigationItem 属性

1. This is a unique instance of UINavigationItem created to represent the view controller when it is pushed onto a navigation controller. The first time the property is accessed, the UINavigationItem object is created. Therefore, you shouldn’t access this property if you are not using a navigation controller to display the view controller. To ensure the navigation item is configured, you can either override this property and add code to create the bar button items when first accessed or create the items in your view controller’s initialization code.

2.Avoid tying the creation of bar button items in your navigation item to the creation of your view controller’s view. The navigation item of a view controller may be retrieved independently of the view controller’s view. For example, when pushing two view controllers onto a navigation stack, the topmost view controller becomes visible, but the other view controller’s navigation item may be retrieved in order to present its back button.(没懂)