@protocol SDWebImageOperation <NSObject>
- (void)cancel;

 Apple's defines from TargetConditionals.h are a bit weird.
 Apple的从TargetConditionals.h引用的宏有点诡异。
 Seems like TARGET_OS_MAC is always defined (on all platforms).
 好像TARGET_OS_MAC 是在所有的平台上被默认（都是1）
 To determine if we are running on OSX, we can only rely on TARGET_OS_IPHONE=0 and all the other platforms
为了确定我们是否正在运行OSX，我们只能靠target_os_iphone = 0 和 其他所有的平台

 +------------------------------------------------+
 |                TARGET_OS_MAC                   |
 | +---+  +-------------------------------------+ |
 | |   |  |          TARGET_OS_IPHONE           | |
 | |OSX|  | +-----+ +----+ +-------+ +--------+ | |
 | |   |  | | IOS | | TV | | WATCH | | BRIDGE | | |
 | |   |  | +-----+ +----+ +-------+ +--------+ | |
 | +---+  +-------------------------------------+ |
 +------------------------------------------------+
 

 #ifndef NS_ENUM
#define NS_ENUM(_type, _name) enum _name : _type _name; enum _name : _type
#endif

#ifndef NS_OPTIONS
#define NS_OPTIONS(_type, _name) enum _name : _type _name; enum _name : _type
#endif

#define dispatch_main_sync_safe(block)\
    if ([NSThread isMainThread]) {\
        block();\
    } else {\
        dispatch_sync(dispatch_get_main_queue(), block);\
    }

#ifndef dispatch_queue_async_safe
#define dispatch_queue_async_safe(queue, block)\
    if (strcmp(dispatch_queue_get_label(DISPATCH_CURRENT_QUEUE_LABEL), dispatch_queue_get_label(queue)) == 0) {\
        block();\
    } else {\
        dispatch_async(queue, block);\
    }
#endif

#ifndef dispatch_main_async_safe
#define dispatch_main_async_safe(block) dispatch_queue_async_safe(dispatch_get_main_queue(), block)
#endif

inline UIImage *SDScaledImageForKey(NSString * _Nullable key, UIImage * _Nullable image) {
    if (!image)
    {
        return nil;
    }
    
#if SD_MAC
    return image;
#elif SD_UIKIT || SD_WATCH
    //如果是动态图片，比如GIF图片，则迭代处理
    if ((image.images).count > 0)
    {
        NSMutableArray<UIImage *> *scaledImages = [NSMutableArray array];

        for (UIImage *tempImage in image.images) {
            [scaledImages addObject:SDScaledImageForKey(key, tempImage)];
        }
        
        UIImage *animatedImage = [UIImage animatedImageWithImages:scaledImages duration:image.duration];
        if (animatedImage) {
            animatedImage.sd_imageLoopCount = image.sd_imageLoopCount;
        }
        //把处理结束的图片再合成一张动态图片
        return animatedImage;
    }
    else
    {
#if SD_WATCH
        //判断screenScale方法是否存在
        if ([[WKInterfaceDevice currentDevice] respondsToSelector:@selector(screenScale)]) {
#elif SD_UIKIT
        //判断scale方法是否存在
        if ([[UIScreen mainScreen] respondsToSelector:@selector(scale)])
        {
#endif
            CGFloat scale = 1;
            // “@2x.png”的长度为7，所以此处添加了这个判断，很巧妙
            if (key.length >= 8)
            {
                NSRange range = [key rangeOfString:@"@2x."];
                if (range.location != NSNotFound) {
                    scale = 2.0;
                }
                //这里需要判断图片的大小
                range = [key rangeOfString:@"@3x."];
                if (range.location != NSNotFound) {
                    scale = 3.0;
                }
            }
            // 使用initWithCGImage来根据Core Graphics的图片构建UIImage。
            UIImage *scaledImage = [[UIImage alloc] initWithCGImage:image.CGImage scale:scale orientation:image.imageOrientation];
            image = scaledImage;
        }
        return image;
    }
#endif
}