[[UIScreen mainScreen] scale]是计算屏幕分辨率的
- [[UIScreen main] scale] == 1; //代表320 x 480 的分辨率(就是iphone4之前的设备,非Retain屏幕)
- [[UIScreen main] scale] == 2; //代表640 x 960 的分辨率(Retain屏幕)
- [[UIScreen main] scale] == 3; //代表1242 x 2208 的分辨率
[[UIScreen mainScreen] scale]是计算屏幕分辨率的