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;