判断运行在那种设备上：UI_USER_INTERFACE_IDIOM()
UI_USER_INTERFACE_IDIOM() ([[UIDevice currentDevice] respondsToSelector:@selector(userInterfaceIdiom)] ? [[UIDevice currentDevice] userInterfaceIdiom] : UIUserInterfaceIdiomPhone)
定义在UIKit库的UIDevice.h中，（ios3.2以下的系统中没有userInterfaceIdiom属性，所以用这宏判断）
(UIUserInterfaceIdiom)userInterfaceIdiom是一个枚举，包括：UIUserInterfaceIdiomPhone，UIUserInterfaceIdiomPad
用法：if (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPhone){//do something}