// iPhone4S
#define IS_iPhone4S ([UIScreen instancesRespondToSelector:@selector(currentMode)] ? CGSizeEqualToSize(CGSizeMake(640, 960), [[UIScreen mainScreen] currentMode].size) : NO)
// iPhone5
#define IS_iPhone5 ([UIScreen instancesRespondToSelector:@selector(currentMode)] ? CGSizeEqualToSize(CGSizeMake(640, 1136), [[UIScreen mainScreen] currentMode].size) : NO)
// iPhone6
#define IS_iPhone6 ([UIScreen instancesRespondToSelector:@selector(currentMode)] ? (CGSizeEqualToSize(CGSizeMake(750, 1334), [[UIScreen mainScreen] currentMode].size) || CGSizeEqualToSize(CGSizeMake(640, 1136), [[UIScreen mainScreen] currentMode].size)) : NO)
// iPhone6plus
#define IS_iPhone6plus ([UIScreen instancesRespondToSelector:@selector(currentMode)] ? (CGSizeEqualToSize(CGSizeMake(1125, 2001), [[UIScreen mainScreen] currentMode].size) || CGSizeEqualToSize(CGSizeMake(1242, 2208), [[UIScreen mainScreen] currentMode].size)) : NO)
以 IS_iPhone4S为例，该宏定义的意思是：UIScreen类里面有没有currentMode这个方法，如果有就去判断CGSizeEqualToSize(CGSizeMake(640, 960), [[UIScreen mainScreen] currentMode].size)是YES还是NO，即判断CGSizeMake(640, 960)和[[UIScreen mainScreen] currentMode].size)是否相等，[[UIScreen mainScreen] currentMode].size是指屏幕像素(pixels)而不是points
