Decode UIImage in background

gistfile1.m

__weak Photo *p = self;
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^
{
    CGImageRef imageRef = image.CGImage;
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    CGContextRef context = CGBitmapContextCreate(NULL, 
                                                 CGImageGetWidth(imageRef), 
                                                 CGImageGetHeight(imageRef), 
                                                 8, 
                                                 CGImageGetWidth(imageRef) * 4, 
                                                 colorSpace, 
                                                 kCGImageAlphaPremultipliedLast);
    CGColorSpaceRelease(colorSpace);
    if (!context)
        return;
    
    CGRect rect = (CGRect){CGPointZero, CGImageGetWidth(imageRef), CGImageGetHeight(imageRef)};
    CGContextDrawImage(context, rect, imageRef);
    CGImageRef decompressedImageRef = CGBitmapContextCreateImage(context);
    CGContextRelease(context);
    
    UIImage *decompressedImage = [[UIImage alloc] initWithCGImage:decompressedImageRef];
    CGImageRelease(decompressedImageRef);
    
    dispatch_async(dispatch_get_main_queue(), ^{p.thumbImage = decompressedImage;});
});