UINavigationController的代理方法

如果要改变导航条的背景颜色的透明度 viewDidLoad和 viewWillAppear里面修改是实现不了的，因为在这个里面导航条早就已经创建好了这个时候需要用到导航的代理方法 当然前提是先设置代理和遵循代理协议 

@interface ViewController : UIViewController<UINavigationControllerDelegate>其次实现导航的两个代理方法- (void) navigationController:(UINavigationController *)navigationController willShowViewController:(UIViewController *)viewController animated:(BOOL)animated{    self.navigationController.navigationBar.alpha = 0.7;}- (void)navigationController:(UINavigationController *)navigationController didShowViewController:(UIViewController *)viewController animated:(BOOL)animated{    self.navigationController.navigationBar.alpha = 0.7;}