UINavigationBar,UIBarButtonItem 相关
1. Adding a Custom UIBarButtonItem
We will use an image in the resources folder called “back.png”. To insert the image as the back button, add this piece of code to the AppDelegate.m file. This should be in the application:didFinishLaunchingWihOptions method.

if ([self.navigationController.navigationBar respondsToSelector:@selector( setBackgroundImage:forBarMetrics:)]){[self.navigationController.navigationBar setBackgroundImage:[UIImage imageNamed:@"navbar.png"] forBarMetrics:UIBarMetricsDefault];}

If you see, here we are asking if navbar will respondToSelector to avoid crash on iOS4!