ios - 这个方法的原理是什么？

读sdwebimage源码看到这个方法，完全不懂
这个方法涉及到的技术在哪可以深入研究？

+ (UIImage *)+ (UIImage *)decodedImageWithImage:(UIImage *)image{
    if (img.images) {
        return img;
    }
    
    CGImageRef imageRef = img.CGImage;
    CGSize imgSize = CGSizeMake(CGImageGetWidth(imageRef), CGImageGetHeight(imageRef));
    CGRect imgRect = (CGRect){.origin = CGPointZero, .size = imgSize};
    
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    CGBitmapInfo bitmapInfo = CGImageGetBitmapInfo(imageRef);
    
    int infoMask = (bitmapInfo & kCGBitmapAlphaInfoMask);
    BOOL anyNonAlpha = (infoMask == kCGImageAlphaNone || infoMask == kCGImageAlphaNoneSkipFirst
                        || infoMask == kCGImageAlphaNoneSkipLast);
    // CGBitmapContextCreate 不支持 kCGImageAlphaNone with rgb
    // https://developer.apple.com/library/mac/#qa/qa1037/_index.html
    if (infoMask == kCGImageAlphaNone  && CGColorSpaceGetNumberOfComponents(colorSpace) > 1) {
        bitmapInfo &= ~kCGBitmapAlphaInfoMask;
        
        bitmapInfo |= kCGImageAlphaNoneSkipFirst;
    }
    // Some PNGs tell us they have alpha but only 3 components. Odd.
    else if (!anyNonAlpha && CGColorSpaceGetNumberOfComponents(colorSpace) == 3){
        bitmapInfo &= ~kCGBitmapAlphaInfoMask;
        bitmapInfo |= kCGImageAlphaPremultipliedFirst;
    }
    
    CGContextRef context = CGBitmapContextCreate(NULL
                                                 ,imgSize.width
                                                 ,imgSize.height
                                                 ,CGImageGetBitsPerComponent(imageRef)
                                                 ,0
                                                 ,colorSpace
    ,bitmapInfo);
    CGColorSpaceRelease(colorSpace);
    
    if (!context) {
        return img;
    }
    
    CGContextDrawImage(context, imgRect, imageRef);
    CGImageRef decompressedImgRef = CGBitmapContextCreateImage(context);
    
    CGContextRelease(context);
    
    UIImage *decompressedImg = [UIImage imageWithCGImage:decompressedImgRef scale:img.scale orientation:img.imageOrientation];
    CGImageRelease(decompressedImgRef);
    return decompressedImg;
}