I've developed a drop-in workaround for this problem.
It uses method swizzling to replace the behavior of the "imageWithContentsOfFile:" method of UIImage.
It works fine on iPhones/iPods pre/post retina.
Not sure about the iPad.
Hope this is of help.
#import </usr/include/objc/objc-class.h>
@implementation NSString(LoadHighDef)
/** If self is the path to an image, returns the nominal path to the high-res variant of that image */
-(NSString*) stringByInsertingHighResPathModifier {
NSString *path = [self stringByDeletingPathExtension];
// We determine whether a device modifier is present, and in case it is, where is
// the "split position" at which the "@2x" token is to be added
NSArray *deviceModifiers = [NSArray arrayWithObjects:@"~iphone", @"~ipad", nil];
NSInteger splitIdx = [path length];
for (NSString *modifier in deviceModifiers) {
if ([path hasSuffix:modifier]) {
splitIdx -= [modifier length];
break;
}
}
// We insert the "@2x" token in the string at the proper position; if no
// device modifier is present the token is added at the end of the string
NSString *highDefPath = [NSString stringWithFormat:@"%@@2x%@",[path substringToIndex:splitIdx], [path substringFromIndex:splitIdx]];
// We possibly add the extension, if there is any extension at all
NSString *ext = [self pathExtension];
return [ext length]>0? [highDefPath stringByAppendingPathExtension:ext] : highDefPath;
}
@end
@implementation UIImage (LoadHighDef)
/* Upon loading this category, the implementation of "imageWithContentsOfFile:" is exchanged with the implementation
* of our custom "imageWithContentsOfFile_custom:" method, whereby we replace and fix the behavior of the system selector. */
+(void)load {
Method originalMethod = class_getClassMethod([UIImage class], @selector(imageWithContentsOfFile:));
Method replacementMethod = class_getClassMethod([UIImage class], @selector(imageWithContentsOfFile_custom:));
method_exchangeImplementations(replacementMethod, originalMethod);
}
/** This method works just like the system "imageWithContentsOfFile:", but it loads the high-res version of the image
* instead of the default one in case the device's screen is high-res and the high-res variant of the image is present.
*
* We assume that the original "imageWithContentsOfFile:" implementation properly sets the "scale" factor upon
* loading a "@2x" image . (this is its behavior as of OS 4.0.1).
*
* Note: The "imageWithContentsOfFile_custom:" invocations in this code are not recursive calls by virtue of
* method swizzling. In fact, the original UIImage implementation of "imageWithContentsOfFile:" gets called.
*/
+ (UIImage*) imageWithContentsOfFile_custom:(NSString*)imgName {
// If high-res is supported by the device...
UIScreen *screen = [UIScreen mainScreen];
if ([screen respondsToSelector:@selector(scale)] && [screen scale]>=2.0) {
// then we look for the high-res version of the image first
UIImage *hiDefImg = [UIImage imageWithContentsOfFile_custom:[imgName stringByInsertingHighResPathModifier]];
// If such high-res version exists, we return it
// The scale factor will be correctly set because once you give imageWithContentsOfFile:
// the full hi-res path it properly takes it into account
if (hiDefImg!=nil)
return hiDefImg;
}
// If the device does not support high-res of it does but there is
// no high-res variant of imgName, we return the base version
return [UIImage imageWithContentsOfFile_custom:imgName];
}
@end
