如何让 [UIImage imageWithContentsOfFile:] 和高分辨率图像工作

Question

正如许多人抱怨的那样，在用于 Retina 显示器的 Apple SDK 中似乎存在一个错误，并且 imageWithContentsOfFile 实际上不会自动加载 2x 图像。

我偶然发现了一篇不错的帖子，如何制作一个检测 UIScreen 比例因子并正确加载低分辨率或高分辨率图像 (http://atastypixel.com/blog/uiimage-resolution-independence-and-the-iphone-4s-retina-display/) 的函数，但该解决方案加载了 2x 图像并且仍然具有图像设置为 1.0，这将导致 2x 图像缩放 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图像，但我没有选择。

是否有人有不使用自动加载 imageNamed 或 imageWithContentsOfFile 来加载低/高分辨率图像的解决方案？ （或最终解决如何使 imageWithContentsOfFile 正常工作）

Answer 1

好的，Michael 在这里找到的实际解决方案： http://atastypixel.com/blog/uiimage-resolution-independence-and-the-iphone-4s-retina-display/

他发现 UIImage 有方法“initWithCGImage”，它也接受一个比例因子作为输入（我猜你可以设置自己的比例因子的唯一方法）

[UIImage initWithCGImage:scale:orientation:]

这似乎很好用，您可以自定义加载高分辨率图像并将比例因子设置为 2.0

imageWithContentsOfFile 的问题在于，由于它目前无法正常工作，因此即使它已修复，我们也无法信任它（因为某些用户的设备上仍会使用较旧的 iOS）

Answer 2

我们刚刚在工作中遇到了这个问题。 这是我的解决方法，似乎站得住脚：

NSString *imgFile = ...path to your file;
NSData *imgData = [[NSData alloc] initWithContentsOfFile:imgFile];
UIImage *img = [[UIImage alloc] initWithData:imgData];

Answer 3

imageWithContentsOfFile 从 iOS 4.1 及更高版本开始正常工作（考虑具有正确比例的@2x 图像）。

Answer 4

增强 Lisa Rossellis 的答案以将视网膜图像保持在所需大小（而不是按比例放大）：

NSString *imagePath = ...Path to your image
UIImage *image = [UIImage imageWithData:[NSData dataWithContentsOfFile:imagePath] scale:[UIScreen mainScreen].scale];

Answer 5

我已经为这个问题开发了一个临时解决方法。 它使用方法调配来替换 UIImage 的“imageWithContentsOfFile:”方法的行为。 它在视网膜前/后的 iPhone/iPod 上运行良好。 不确定 iPad。

希望对您有所帮助。

#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

Answer 6

如果您指定绝对路径，[UIImage imageWithContentsOfFile:] 不会加载 @2x 图形。

这里有一个解决方案：

- (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 他的简单解决方案（我在此处修改并粘贴）。