UINavigationBar UINavigationItem UIBarButtonItem

摘要: UINavigationBar继承于UIView UINavigationItem UIBarButtonItem 继承于UIBarItem

UINavigationBar 主要用来设置导航栏的背景图片

UINavigationItem 主要是作为UIBarButtonItem的容器用来存放leftBarButtonItem

主要接口：

- (void)setLeftBarButtonItems:(nullable NSArray<UIBarButtonItem *> *)items animated:(BOOL)animated NS_AVAILABLE_IOS(5_0);
- (void)setRightBarButtonItems:(nullable NSArray<UIBarButtonItem *> *)items animated:(BOOL)animated NS_AVAILABLE_IOS(5_0);
- (void)setLeftBarButtonItem:(nullable UIBarButtonItem *)item animated:(BOOL)animated;
- (void)setRightBarButtonItem:(nullable UIBarButtonItem *)item animated:(BOOL)animated;
属性：
@property(nullable, nonatomic,strong) UIBarButtonItem *leftBarButtonItem;
@property(nullable, nonatomic,strong) UIBarButtonItem *rightBarButtonItem;