SDWebImage之工具类

SDWebImage使用了很多工具类来对图片的处理，比如获取图片类型、图片放大缩小、GIF图片处理、图片解压缩处理等。下面我们来看一下这几个工具类。

1.NSData+ImageContentType

这个类提供了一个类方法sd_imageFormatForImageData。通过这个方法传入图片的NSData数据，然后返回图片类型。

/**
 不同图片类型的枚举

 - SDImageFormatUndefined: 未知
 - SDImageFormatJPEG: JPG(FFD8FFE1)
 - SDImageFormatPNG: PNG(89504E47)
 - SDImageFormatGIF: GIF(47494638)
 - SDImageFormatTIFF: TIFF(49492A00或4D4D002A)
 - SDImageFormatWebP: WebP(524946462A73010057454250, 52494646对应ASCII字符为RIFF，57454250对应ASCII字符为WEBP。当第一个字节为52时，如果长度<12 我们就认定为不是图片。因此返回SDImageFormatUndefined。)
 */
typedef NS_ENUM(NSInteger, SDImageFormat) {
    SDImageFormatUndefined = -1,
    SDImageFormatJPEG = 0,
    SDImageFormatPNG,
    SDImageFormatGIF,
    SDImageFormatTIFF,
    SDImageFormatWebP
};

/**
 根据图片NSData获取图片的类型

 @param data NSData数据
 @return 图片数据类型
 */
+ (SDImageFormat)sd_imageFormatForImageData:(nullable NSData *)data {
    if (!data) {
        return SDImageFormatUndefined;
    }
    
    uint8_t c;
    //获取图片数据的第一个字节数据
    [data getBytes:&c length:1];
    //根据字母的ASCII码比较
    switch (c) {
        case 0xFF:
            return SDImageFormatJPEG;
        case 0x89:
            return SDImageFormatPNG;
        case 0x47:
            return SDImageFormatGIF;
        case 0x49:
        case 0x4D:
            return SDImageFormatTIFF;
        case 0x52:
            // R as RIFF for WEBP
            if (data.length < 12) {
                return SDImageFormatUndefined;
            }
            
            NSString *testString = [[NSString alloc] initWithData:[data subdataWithRange:NSMakeRange(0, 12)] encoding:NSASCIIStringEncoding];
            if ([testString hasPrefix:@"RIFF"] && [testString hasSuffix:@"WEBP"]) {
                return SDImageFormatWebP;
            }
    }
    return SDImageFormatUndefined;
}

2.UIImage+MultiFormat

该分类实现了NSData与UIImage对象之间的相互转换，并且是根据图片类型做转换。比如GIF的UIImage转换为GIF格式的NSData。 

/**
 根据图片的data数据，生成对应的图片对象

 @param data 图片的data
 @return 图片对象
 */
+ (nullable UIImage *)sd_imageWithData:(nullable NSData *)data {
    if (!data) {
        return nil;
    }
    
    UIImage *image;
    //获取data的图片类型，png，gif，jpg
    SDImageFormat imageFormat = [NSData sd_imageFormatForImageData:data];
    if (imageFormat == SDImageFormatGIF) {
        //gif处理：返回一张只包含数据第一张image 的gif图片
        image = [UIImage sd_animatedGIFWithData:data];
    }
#ifdef SD_WEBP
    else if (imageFormat == SDImageFormatWebP)
    {
        image = [UIImage sd_imageWithWebPData:data];
    }
#endif
    else {
        image = [[UIImage alloc] initWithData:data];
#if SD_UIKIT || SD_WATCH
        //获取方向
        UIImageOrientation orientation = [self sd_imageOrientationFromImageData:data];
        //如果不是向上的，还需要再次生成图片
        if (orientation != UIImageOrientationUp) {
            image = [UIImage imageWithCGImage:image.CGImage
                                        scale:image.scale
                                  orientation:orientation];
        }
#endif
    }


    return image;
}

#if SD_UIKIT || SD_WATCH

/**
 根据图片数据获取图片的方向

 @param imageData 图片数据
 @return 方向
 */
+(UIImageOrientation)sd_imageOrientationFromImageData:(nonnull NSData *)imageData {
    //默认是向上的
    UIImageOrientation result = UIImageOrientationUp;
    CGImageSourceRef imageSource = CGImageSourceCreateWithData((__bridge CFDataRef)imageData, NULL);
    if (imageSource) {
        //获取图片的属性列表
        CFDictionaryRef properties = CGImageSourceCopyPropertiesAtIndex(imageSource, 0, NULL);
        if (properties) {
            CFTypeRef val;
            int exifOrientation;
            //获取图片方向
            val = CFDictionaryGetValue(properties, kCGImagePropertyOrientation);
            if (val) {
                CFNumberGetValue(val, kCFNumberIntType, &exifOrientation);
                result = [self sd_exifOrientationToiOSOrientation:exifOrientation];
            } // else - if it's not set it remains at up
            CFRelease((CFTypeRef) properties);
        } else {
            //NSLog(@"NO PROPERTIES, FAIL");
        }
        CFRelease(imageSource);
    }
    return result;
}

#pragma mark EXIF orientation tag converter
// Convert an EXIF image orientation to an iOS one.
// reference see here: http://sylvana.net/jpegcrop/exif_orientation.html

/**
 根据不同的值返回不同的图片方向

 @param exifOrientation 输入值
 @return 图片的方向
 */
+ (UIImageOrientation) sd_exifOrientationToiOSOrientation:(int)exifOrientation {
    UIImageOrientation orientation = UIImageOrientationUp;
    switch (exifOrientation) {
        case 1:
            orientation = UIImageOrientationUp;
            break;

        case 3:
            orientation = UIImageOrientationDown;
            break;

        case 8:
            orientation = UIImageOrientationLeft;
            break;

        case 6:
            orientation = UIImageOrientationRight;
            break;

        case 2:
            orientation = UIImageOrientationUpMirrored;
            break;

        case 4:
            orientation = UIImageOrientationDownMirrored;
            break;

        case 5:
            orientation = UIImageOrientationLeftMirrored;
            break;

        case 7:
            orientation = UIImageOrientationRightMirrored;
            break;
        default:
            break;
    }
    return orientation;
}
#endif

- (nullable NSData *)sd_imageData {
    return [self sd_imageDataAsFormat:SDImageFormatUndefined];
}

/**
 根据指定的图片类型，把image对象转换为对应格式的data

 @param imageFormat 指定的image格式
 @return 返回data对象
 */
- (nullable NSData *)sd_imageDataAsFormat:(SDImageFormat)imageFormat {
    NSData *imageData = nil;
    if (self) {
#if SD_UIKIT || SD_WATCH
        int alphaInfo = CGImageGetAlphaInfo(self.CGImage);
        //是否有透明度
        BOOL hasAlpha = !(alphaInfo == kCGImageAlphaNone ||
                          alphaInfo == kCGImageAlphaNoneSkipFirst ||
                          alphaInfo == kCGImageAlphaNoneSkipLast);
        //只有png图片有alpha属性
        BOOL usePNG = hasAlpha;
        
        // the imageFormat param has priority here. But if the format is undefined, we relly on the alpha channel
        //是否是PNG类型的图片
        if (imageFormat != SDImageFormatUndefined) {
            usePNG = (imageFormat == SDImageFormatPNG);
        }
        //根据不同的图片类型获取到对应的图片data
        if (usePNG) {
            imageData = UIImagePNGRepresentation(self);
        } else {
            imageData = UIImageJPEGRepresentation(self, (CGFloat)1.0);
        }
#else
        NSBitmapImageFileType imageFileType = NSJPEGFileType;
        if (imageFormat == SDImageFormatGIF) {
            imageFileType = NSGIFFileType;
        } else if (imageFormat == SDImageFormatPNG) {
            imageFileType = NSPNGFileType;
        }
        
        imageData = [NSBitmapImageRep representationOfImageRepsInArray:self.representations
                                                             usingType:imageFileType
                                                            properties:@{}];
#endif
    }
    return imageData;
}

3.UIImage+GIF

该分类实现了对GIF图片的NSData的处理，处理方式是取出GIF图片的第一张UIImage来显示。如果要显示动态图片的话，需要使用FLAnimatedImageView来显示。

/**
 根据Gif图片的data生成对应的UIImage对象(只会取GIF图片的第一张UIImage)

 @param data data
 @return 生成的image对象。这里只获取Gif图片的第一张图像，如果要实现Gif完整图像，使用FLAnimatedImageView
 */
+ (UIImage *)sd_animatedGIFWithData:(NSData *)data {
    if (!data) {
        return nil;
    }

    CGImageSourceRef source = CGImageSourceCreateWithData((__bridge CFDataRef)data, NULL);
    //获取NSData中的图片数量
    size_t count = CGImageSourceGetCount(source);

    UIImage *staticImage;
    //如果图片数量小于或者等于1，直接转换
    if (count <= 1) {
        staticImage = [[UIImage alloc] initWithData:data];
    } else {
        // we will only retrieve the 1st frame. the full GIF support is available via the FLAnimatedImageView category.
        // this here is only code to allow drawing animated images as static ones
#if SD_WATCH
        CGFloat scale = 1;
        scale = [WKInterfaceDevice currentDevice].screenScale;
#elif SD_UIKIT
        CGFloat scale = 1;
        scale = [UIScreen mainScreen].scale;
#endif
        //获取第一张UIImage对象
        CGImageRef CGImage = CGImageSourceCreateImageAtIndex(source, 0, NULL);
#if SD_UIKIT || SD_WATCH
        //获取Gif图片的第一张图片
        UIImage *frameImage = [UIImage imageWithCGImage:CGImage scale:scale orientation:UIImageOrientationUp];
        //用第一张图片生成一个新的Gif图片
        staticImage = [UIImage animatedImageWithImages:@[frameImage] duration:0.0f];
#elif SD_MAC
        staticImage = [[UIImage alloc] initWithCGImage:CGImage size:NSZeroSize];
#endif
        CGImageRelease(CGImage);
    }

    CFRelease(source);

    return staticImage;
}

/**
 判断一张图片是否GIF图片

 @return YES/NO
 */
- (BOOL)isGIF {
    return (self.images != nil);
}

4.SDWebImageDecoder

 本类实现图片的解码操作，对于太大的图片，先按照一定比例缩小然后再解码。在这里，大家可能有一个问题：为什么要解码呢？

4.1为什么要解码？

在我们实际的项目开发中，我们经常使用imageNamed:方法来加载图片，系统默认会在主线程立即进行图片的解码工作，这一过程就是把图片解码成可供控件直接使用的位图。当在主线程调用了大量的imageNamed:方法后，就会产生卡顿。为了解决这个问题我们有两种处理方法：

不使用imageNamed:加载图片，使用imageWithContentsOfFile:来加载图片；

自己解码图片，把这个解码过程放到子线程。

关于图片的存储方式和处理，大家可以看一下这篇文章：图片格式。

4.2SDWebImageDecoder源码分析

#if SD_UIKIT || SD_WATCH
static const size_t kBytesPerPixel = 4;  //!<每个像素占用的字节数(图像在iOS设备上是以像素为单位显示的)
static const size_t kBitsPerComponent = 8;  //!<每一个组件占多少位(比方说RGBA，其中R（红色）G（绿色）B（蓝色）A（透明度）是4个组件，每个像素由这4个组件组成，那么我们就用8位来表示着每一个组件，所以这个RGBA就是8*4 = 32位)

/**
 解码图片

 @param image UIImage对象
 @return 返回解码以后的图片
 */
+ (nullable UIImage *)decodedImageWithImage:(nullable UIImage *)image {
    //图片是否能够解码
    if (![UIImage shouldDecodeImage:image]) {
        return image;
    }
    
    // autorelease the bitmap context and all vars to help system to free memory when there are memory warning.
    // on iOS7, do not forget to call [[SDImageCache sharedImageCache] clearMemory];
    //解码操作放入一个自动释放池里面，以便自动释放所有的变量
    @autoreleasepool{
        //获取和图像相关的各种参数
        CGImageRef imageRef = image.CGImage;
        //获取图片的色彩空间
        CGColorSpaceRef colorspaceRef = [UIImage colorSpaceForImageRef:imageRef];
        //宽度和高度
        size_t width = CGImageGetWidth(imageRef);
        size_t height = CGImageGetHeight(imageRef);
        //计算出每行的像素数
        size_t bytesPerRow = kBytesPerPixel * width;

        // kCGImageAlphaNone is not supported in CGBitmapContextCreate.
        // Since the original image here has no alpha info, use kCGImageAlphaNoneSkipLast
        // to create bitmap graphics contexts without alpha info.
        //创建一个绘制图片的上下文
        //这里创建的contexts是没有透明因素的。在UI渲染的时候，实际上是把多个图层按像素叠加计算的过程，需要对每一个像素进行 RGBA 的叠加计算。当某个 layer 的是不透明的，也就是 opaque 为 YES 时，GPU 可以直接忽略掉其下方的图层，这就减少了很多工作量。
        CGContextRef context = CGBitmapContextCreate(NULL,
                                                     width,
                                                     height,
                                                     kBitsPerComponent,
                                                     bytesPerRow,
                                                     colorspaceRef,
                                                     kCGBitmapByteOrderDefault|kCGImageAlphaNoneSkipLast);
        if (context == NULL) {
            return image;
        }
        
        // Draw the image into the context and retrieve the new bitmap image without alpha
        //绘制一个和图片大小一样的图片
        CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
        //创建一个没有alpha通道的图片
        CGImageRef imageRefWithoutAlpha = CGBitmapContextCreateImage(context);
        //得到解码以后的图片
        UIImage *imageWithoutAlpha = [UIImage imageWithCGImage:imageRefWithoutAlpha
                                                         scale:image.scale
                                                   orientation:image.imageOrientation];
        
        CGContextRelease(context);
        CGImageRelease(imageRefWithoutAlpha);
        
        return imageWithoutAlpha;
    }
}

/*
 * Defines the maximum size in MB of the decoded image when the flag `SDWebImageScaleDownLargeImages` is set
 * Suggested value for iPad1 and iPhone 3GS: 60.
 * Suggested value for iPad2 and iPhone 4: 120.
 * Suggested value for iPhone 3G and iPod 2 and earlier devices: 30.
 */
static const CGFloat kDestImageSizeMB = 60.0f;  //!<最大支持压缩图像源的大小，默认为60MB。当我们要压缩一张图像的时候，首先就是要定义最大支持的源文件的大小，不能没有任何限制。

/*
 * Defines the maximum size in MB of a tile used to decode image when the flag `SDWebImageScaleDownLargeImages` is set
 * Suggested value for iPad1 and iPhone 3GS: 20.
 * Suggested value for iPad2 and iPhone 4: 40.
 * Suggested value for iPhone 3G and iPod 2 and earlier devices: 10.
 */
static const CGFloat kSourceImageTileSizeMB = 20.0f;  //!<原图方块的大小，默认为20MB，这个方块将会被用来分割原图

static const CGFloat kBytesPerMB = 1024.0f * 1024.0f;  //!<1MB有多少字节
static const CGFloat kPixelsPerMB = kBytesPerMB / kBytesPerPixel;  //!<1MB可以存储多少像素
static const CGFloat kDestTotalPixels = kDestImageSizeMB * kPixelsPerMB;  //!<目标总像素
static const CGFloat kTileTotalPixels = kSourceImageTileSizeMB * kPixelsPerMB;  //!<原图方块总像素

static const CGFloat kDestSeemOverlap = 2.0f;   //重叠像素大小

/**
 如果原始图片占用的空间太大，则按照一定的比例解码，从而不让解码以后的图片占用的空间太大

 @param image UIImage对象
 @return 返回处理结束的UIImage对象
 */
+ (nullable UIImage *)decodedAndScaledDownImageWithImage:(nullable UIImage *)image {
    //图片是否支持解码
    if (![UIImage shouldDecodeImage:image]) {
        return image;
    }
    //图片不需要处理，直接解码
    if (![UIImage shouldScaleDownImage:image]) {
        return [UIImage decodedImageWithImage:image];
    }
    
    CGContextRef destContext;
    
    // autorelease the bitmap context and all vars to help system to free memory when there are memory warning.
    // on iOS7, do not forget to call [[SDImageCache sharedImageCache] clearMemory];
    @autoreleasepool {
        //获取和图像相关的各种参数
        CGImageRef sourceImageRef = image.CGImage;
        //获取原始图片的像素
        CGSize sourceResolution = CGSizeZero;
        sourceResolution.width = CGImageGetWidth(sourceImageRef);
        sourceResolution.height = CGImageGetHeight(sourceImageRef);
        //计算原始图片的总像素
        float sourceTotalPixels = sourceResolution.width * sourceResolution.height;
        // Determine the scale ratio to apply to the input image
        // that results in an output image of the defined size.
        // see kDestImageSizeMB, and how it relates to destTotalPixels.
        //根据一定的比例设置目标图片的宽度和高度
        float imageScale = kDestTotalPixels / sourceTotalPixels;
        //计算目标像素
        CGSize destResolution = CGSizeZero;
        destResolution.width = (int)(sourceResolution.width*imageScale);
        destResolution.height = (int)(sourceResolution.height*imageScale);
        
        // current color space
        //获取原始图片的像素空间，默认是RGB
        CGColorSpaceRef colorspaceRef = [UIImage colorSpaceForImageRef:sourceImageRef];
        //每一行像素占用的内存空间大小
        size_t bytesPerRow = kBytesPerPixel * destResolution.width;
        
        // Allocate enough pixel data to hold the output image.
        //目标图片占用的总内存空间大小，一行占用内存空间大小*高度
        void* destBitmapData = malloc( bytesPerRow * destResolution.height );
        if (destBitmapData == NULL) {
            return image;
        }
        
        // kCGImageAlphaNone is not supported in CGBitmapContextCreate.
        // Since the original image here has no alpha info, use kCGImageAlphaNoneSkipLast
        // to create bitmap graphics contexts without alpha info.
        //根据各种设置创建一个上下文环境
        destContext = CGBitmapContextCreate(destBitmapData,
                                            destResolution.width,
                                            destResolution.height,
                                            kBitsPerComponent,
                                            bytesPerRow,
                                            colorspaceRef,
                                            kCGBitmapByteOrderDefault|kCGImageAlphaNoneSkipLast);
        
        if (destContext == NULL) {
            free(destBitmapData);
            return image;
        }
        //设置目标图片的压缩质量
        CGContextSetInterpolationQuality(destContext, kCGInterpolationHigh);
        
        // Now define the size of the rectangle to be used for the
        // incremental blits from the input image to the output image.
        // we use a source tile width equal to the width of the source
        // image due to the way that iOS retrieves image data from disk.
        // iOS must decode an image from disk in full width 'bands', even
        // if current graphics context is clipped to a subrect within that
        // band. Therefore we fully utilize all of the pixel data that results
        // from a decoding opertion by achnoring our tile size to the full
        // width of the input image.
        //计算第一个原图方块，这个方块的宽度同原图一样，高度根据方块容量计算
        CGRect sourceTile = CGRectZero;
        sourceTile.size.width = sourceResolution.width;
        // The source tile height is dynamic. Since we specified the size
        // of the source tile in MB, see how many rows of pixels high it
        // can be given the input image width.
        sourceTile.size.height = (int)(kTileTotalPixels / sourceTile.size.width );
        sourceTile.origin.x = 0.0f;
        // The output tile is the same proportions as the input tile, but
        // scaled to image scale.
        //计算目标图像方块
        CGRect destTile;
        destTile.size.width = destResolution.width;
        destTile.size.height = sourceTile.size.height * imageScale;
        destTile.origin.x = 0.0f;
        // The source seem overlap is proportionate to the destination seem overlap.
        // this is the amount of pixels to overlap each tile as we assemble the ouput image.
        //计算原图像方块与方块重叠的像素大小
        float sourceSeemOverlap = (int)((kDestSeemOverlap/destResolution.height)*sourceResolution.height);
        CGImageRef sourceTileImageRef;
        // calculate the number of read/write operations required to assemble the
        // output image.
        //计算原图像需要被分割成多少个方块 iterations
        int iterations = (int)( sourceResolution.height / sourceTile.size.height );
        // If tile height doesn't divide the image height evenly, add another iteration
        // to account for the remaining pixels.
        int remainder = (int)sourceResolution.height % (int)sourceTile.size.height;
        if(remainder) {
            iterations++;
        }
        // Add seem overlaps to the tiles, but save the original tile height for y coordinate calculations.
        //根据重叠像素计算原图方块的大小后，获取原图中该方块内的数据，把该数据写入到相对应的目标方块中
        float sourceTileHeightMinusOverlap = sourceTile.size.height;
        sourceTile.size.height += sourceSeemOverlap;
        destTile.size.height += kDestSeemOverlap;
        for( int y = 0; y < iterations; ++y ) {
            @autoreleasepool {
                sourceTile.origin.y = y * sourceTileHeightMinusOverlap + sourceSeemOverlap;
                destTile.origin.y = destResolution.height - (( y + 1 ) * sourceTileHeightMinusOverlap * imageScale + kDestSeemOverlap);
                sourceTileImageRef = CGImageCreateWithImageInRect( sourceImageRef, sourceTile );
                if( y == iterations - 1 && remainder ) {
                    float dify = destTile.size.height;
                    destTile.size.height = CGImageGetHeight( sourceTileImageRef ) * imageScale;
                    dify -= destTile.size.height;
                    destTile.origin.y += dify;
                }
                CGContextDrawImage( destContext, destTile, sourceTileImageRef );
                CGImageRelease( sourceTileImageRef );
            }
        }
        
        CGImageRef destImageRef = CGBitmapContextCreateImage(destContext);
        CGContextRelease(destContext);
        if (destImageRef == NULL) {
            return image;
        }
        //生成处理结束以后的图片
        UIImage *destImage = [UIImage imageWithCGImage:destImageRef scale:image.scale orientation:image.imageOrientation];
        CGImageRelease(destImageRef);
        if (destImage == nil) {
            return image;
        }
        return destImage;
    }
}

/**
 图片是否能够解码

 @param image 图片
 @return 能否解码
 */
+ (BOOL)shouldDecodeImage:(nullable UIImage *)image {
    // Prevent "CGBitmapContextCreateImage: invalid context 0x0" error
    if (image == nil) {
        return NO;
    }

    // do not decode animated images
    //如果是动态图片不处理
    if (image.images != nil) {
        return NO;
    }
    
    CGImageRef imageRef = image.CGImage;
    //获取image的alpha通道。通过通道获取图片数据
    CGImageAlphaInfo alpha = CGImageGetAlphaInfo(imageRef);
    BOOL anyAlpha = (alpha == kCGImageAlphaFirst ||
                     alpha == kCGImageAlphaLast ||
                     alpha == kCGImageAlphaPremultipliedFirst ||
                     alpha == kCGImageAlphaPremultipliedLast);
    // do not decode images with alpha
    //如果有alpha通道值，则不处理
    if (anyAlpha) {
        return NO;
    }
    
    return YES;
}

/**
 是否需要压缩原始图片的大小(图像大于目标尺寸才需要压缩)

 @param image UIImage对象
 @return 是否支持压缩
 */
+ (BOOL)shouldScaleDownImage:(nonnull UIImage *)image {
    BOOL shouldScaleDown = YES;
        
    CGImageRef sourceImageRef = image.CGImage;
    CGSize sourceResolution = CGSizeZero;
    sourceResolution.width = CGImageGetWidth(sourceImageRef);
    sourceResolution.height = CGImageGetHeight(sourceImageRef);
    //图片总共像素
    float sourceTotalPixels = sourceResolution.width * sourceResolution.height;
    //如果图片的总像素大于一定比例，则需要做简化处理
    float imageScale = kDestTotalPixels / sourceTotalPixels;
    if (imageScale < 1) {
        shouldScaleDown = YES;
    } else {
        shouldScaleDown = NO;
    }
    
    return shouldScaleDown;
}

/**
 获取图片的色彩空间

 @param imageRef 图片
 @return 色彩空间
 */
+ (CGColorSpaceRef)colorSpaceForImageRef:(CGImageRef)imageRef {
    // current
    CGColorSpaceModel imageColorSpaceModel = CGColorSpaceGetModel(CGImageGetColorSpace(imageRef));
    CGColorSpaceRef colorspaceRef = CGImageGetColorSpace(imageRef);
    
    BOOL unsupportedColorSpace = (imageColorSpaceModel == kCGColorSpaceModelUnknown ||
                                  imageColorSpaceModel == kCGColorSpaceModelMonochrome ||
                                  imageColorSpaceModel == kCGColorSpaceModelCMYK ||
                                  imageColorSpaceModel == kCGColorSpaceModelIndexed);
    if (unsupportedColorSpace) {
        colorspaceRef = CGColorSpaceCreateDeviceRGB();
        CFAutorelease(colorspaceRef);
    }
    return colorspaceRef;
}

5.UIView+WebCacheOperation

UIView+WebCacheOperation主要用来记录 UIView 加载 Operation 操作，大多数情况下一个 View 仅拥有一个 Operation ，默认的 key 是当前类的类名，如果设置了不同的 key，将保存不同的 Operation 。比如一个 UIButton，可以设置不同状态下的图片，那么需要记录多个 Operation ，它主要采用一个字典来保存所有的 Operation 。

/**
 关联属性

 @return 属性值
 */
- (SDOperationsDictionary *)operationDictionary {
    SDOperationsDictionary *operations = objc_getAssociatedObject(self, &loadOperationKey);
    if (operations) {
        return operations;
    }
    operations = [NSMutableDictionary dictionary];
    objc_setAssociatedObject(self, &loadOperationKey, operations, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    return operations;
}

/**
 关联Operation对象与key对象

 @param operation Operation对象
 @param key key
 */
- (void)sd_setImageLoadOperation:(nullable id)operation forKey:(nullable NSString *)key {
    if (key) {
        [self sd_cancelImageLoadOperationWithKey:key];
        if (operation) {
            SDOperationsDictionary *operationDictionary = [self operationDictionary];
            operationDictionary[key] = operation;
        }
    }
}

/**
 取消当前key对应的所有实现了SDWebImageOperation协议的Operation对象

 @param key Operation对应的key
 */
- (void)sd_cancelImageLoadOperationWithKey:(nullable NSString *)key {
    // Cancel in progress downloader from queue
    //获取当前View对应的所有key
    SDOperationsDictionary *operationDictionary = [self operationDictionary];
    //获取对应的图片加载Operation
    id operations = operationDictionary[key];
    //取消所有当前View对应的所有Operation
    if (operations) {
        if ([operations isKindOfClass:[NSArray class]]) {
            for (id <SDWebImageOperation> operation in operations) {
                if (operation) {
                    //SDWebImageCombinedOperation对象的cancel方法
                    [operation cancel];
                }
            }
        } else if ([operations conformsToProtocol:@protocol(SDWebImageOperation)]){
            [(id<SDWebImageOperation>) operations cancel];
        }
        [operationDictionary removeObjectForKey:key];
    }
}

/**
 根据key移除相应的Operation对象

 @param key key
 */
- (void)sd_removeImageLoadOperationWithKey:(nullable NSString *)key {
    if (key) {
        SDOperationsDictionary *operationDictionary = [self operationDictionary];
        [operationDictionary removeObjectForKey:key];
    }
}