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// This method requires ImageIO.framework
#import <ImageIO/ImageIO.h>

- (CGSize)sizeOfImageAtURL:(NSURL *)imageURL
{
    // With CGImageSource we avoid loading the whole image into memory
    CGSize imageSize = CGSizeZero;
    CGImageSourceRef source = CGImageSourceCreateWithURL((CFURLRef)imageURL, NULL);
    if (source) {
        NSDictionary *options = [NSDictionary dictionaryWithObject:[NSNumber numberWithBool:NO] forKey:(NSString *)kCGImageSourceShouldCache];