iOS: image with 2x

應該準備2套image

一個是for standard,format：<ImageName><device_modifier>.<filename_extension>

一個是for 高清：<ImageName>@2x<device_modifier>.<filename_extension>

其中<device_modifier>是optional的，它可以是"~ipad" or "~iphone"

當你使用下列代碼時：

[button setImage:[UIImage imageNamed:@"play.png"] forState:UIControlStateNormal];

如果是retina device會自動detect是否有play@2x.png的圖片，如果有則載入@2x圖片

ref links:

http://developer.apple.com/library/ios/#documentation/2DDrawing/Conceptual/DrawingPrintingiOS/SupportingHiResScreens/SupportingHiResScreens.html#//apple_ref/doc/uid/TP40010156-CH15-SW8

http://stackoverflow.com/questions/3442239/retina-icons-2x-arent-being-used-when-images-are-specified-in-code

http://stackoverflow.com/questions/9770990/uiimage-imagenamed-not-work-on-ios-5-0