【Some details of UIKit】
1、UIViewController的toolbarItems属性与UINavigationController配合使用。
2、The view for a navigation controller is just a container for several other views, including a navigation bar, an optional toolbar, and the view containing your custom content.
3、A UIBarButtonItem is a button specialized for placement on a UIToolbar or UINavigationBar object.
4、navigationItem第一次被引用的时候会被自动创建。只应该在与navigation controller配合的时候使用此属性。
5、navigation controller --> navigation bar --> navigation item -> bar button item.
