很多人在抱怨，似乎在Apple Retina Display的SDK中，imageWithContentsOfFile存在一个bug，实际上并没有自动加载2x图像。
我偶然发现了一篇好文章，介绍了如何编写函数来检测UIScreen的比例因子，并正确加载低分辨率或高分辨率的图片（http://atastypixel.com/blog/uiimage-resolution-independence-and-the-iphone-4s-retina-display/），但是该解决方案加载了2x图像，而图像的比例因子仍设置为1.0，这导致2x图像缩放了2倍（因此，比它看起来要大4倍）
imageNamed似乎可以准确地加载低分辨率和高分辨率的图像，但对我来说不是选项。
是否有人有解决方案，可以加载低/高分辨率的图像而不使用imageNamed或imageWithContentsOfFile的自动加载？（或者有解决方案如何使imageWithContentsOfFile正常工作）
好的，Michael在这里找到了实际解决方案： http://atastypixel.com/blog/uiimage-resolution-independence-and-the-iphone-4s-retina-display/
他发现UIImage有一个名为"initWithCGImage"的方法，它也可以接受比例因子作为输入（我想这是唯一一个可以自行设置比例因子的方法）。
[UIImage initWithCGImage:scale:orientation:]
这种方法看起来很好，您可以自定义加载高分辨率图像，并将缩放因子设置为2.0
使用imageWithContentsOfFile的问题在于，由于它目前无法正常工作，即使修复了它，我们也不能信任它（因为一些用户仍会在设备上运行旧的iOS）
我们在工作中遇到了这个问题。下面是我的解决方法，看起来很有效：
NSString *imgFile = ...path to your file;
NSData *imgData = [[NSData alloc] initWithContentsOfFile:imgFile];
UIImage *img = [[UIImage alloc] initWithData:imgData];
imageWithContentsOfFile 函数在 iOS 4.1 及其以后版本中可以正常工作（考虑到正确比例的 @2x 图像）。
imageWithContentsOfFile:在绝对路径上无法正常工作。已在iOS 5.1上进行了测试。 - bentford增强Lisa Rossellis的答案，以保持视网膜图像在所需尺寸（而不是缩放它们）：
NSString *imagePath = ...Path to your image
UIImage *image = [UIImage imageWithData:[NSData dataWithContentsOfFile:imagePath] scale:[UIScreen mainScreen].scale];
#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
[UIImage imageWithContentsOfFile:] 方法如果你指定了绝对路径，就不能加载@2x分辨率的图像。
以下是解决方案：
- (UIImage *)loadRetinaImageIfAvailable:(NSString *)path {
NSString *retinaPath = [[path stringByDeletingLastPathComponent] stringByAppendingPathComponent:[NSString stringWithFormat:@"%@@2x.%@", [[path lastPathComponent] stringByDeletingPathExtension], [path pathExtension]]];
if( [UIScreen mainScreen].scale == 2.0 && [[NSFileManager defaultManager] fileExistsAtPath:retinaPath] == YES)
return [[[UIImage alloc] initWithCGImage:[[UIImage imageWithData:[NSData dataWithContentsOfFile:retinaPath]] CGImage] scale:2.0 orientation:UIImageOrientationUp] autorelease];
else
return [UIImage imageWithContentsOfFile:path];
}
感谢Christof Dorner提供的简单解决方案（我进行了修改并在此处粘贴）。