该文章阅读的SDWebImage的版本为4.3.3。

1.全局静态常量

/**
 保存自定义调度组的key
 */
FOUNDATION_EXPORT NSString * _Nonnull const SDWebImageInternalSetImageGroupKey;
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/**
 保存自定义图像管理者的key
 */
FOUNDATION_EXPORT NSString * _Nonnull const SDWebImageExternalCustomManagerKey;
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/**
 获取不到进度时的默认数值，为 long long 类型的1
 */
FOUNDATION_EXPORT const int64_t SDWebImageProgressUnitCountUnknown;
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2.公共类型定义

/**
 用于自定义图像设置的block
 */
typedef void(^SDSetImageBlock)(UIImage * _Nullable image, NSData * _Nullable imageData);
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3.公共属性

/**
 图像加载进度
 */
@property (nonatomic, strong, null_resettable) NSProgress *sd_imageProgress;
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/**
 图像加载完成时的转换动画对象
 */
@property (nonatomic, strong, nullable) SDWebImageTransition *sd_imageTransition;
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3.公共方法

/**
 获取当前加载的url
 */
- (nullable NSURL *)sd_imageURL;
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/**
 为控件设置图像的方法
 */
- (void)sd_internalSetImageWithURL:(nullable NSURL *)url
                  placeholderImage:(nullable UIImage *)placeholder
                           options:(SDWebImageOptions)options
                      operationKey:(nullable NSString *)operationKey
                     setImageBlock:(nullable SDSetImageBlock)setImageBlock
                          progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                         completed:(nullable SDExternalCompletionBlock)completedBlock;
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/**
 为控件设置图像的方法，比上一个方法多了一个context参数
 */
- (void)sd_internalSetImageWithURL:(nullable NSURL *)url
                  placeholderImage:(nullable UIImage *)placeholder
                           options:(SDWebImageOptions)options
                      operationKey:(nullable NSString *)operationKey
                     setImageBlock:(nullable SDSetImageBlock)setImageBlock
                          progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                         completed:(nullable SDExternalCompletionBlock)completedBlock
                           context:(nullable NSDictionary<NSString *, id> *)context;
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/**
 取消当前图像加载
 */
- (void)sd_cancelCurrentImageLoad;
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/**
 是否展示UIActivityIndicatorView
 */
- (void)sd_setShowActivityIndicatorView:(BOOL)show;
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/**
 设置UIActivityIndicatorView的类型
 */
- (void)sd_setIndicatorStyle:(UIActivityIndicatorViewStyle)style;
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/**
 获取UIActivityIndicatorView的展示状态
 */
- (BOOL)sd_showActivityIndicatorView;
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/**
 添加UIActivityIndicatorView
 */
- (void)sd_addActivityIndicator;
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/**
 移除UIActivityIndicatorView
 */
- (void)sd_removeActivityIndicator;
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4.私有静态变量

/**
 用于保存图像路径
 */
static char imageURLKey;
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/**
 用于保存加载小菊花
 */
static char TAG_ACTIVITY_INDICATOR;
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/**
 用于保存加载小菊花的类型
 */
static char TAG_ACTIVITY_STYLE;
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/**
 用于保存加载小菊花的展示状态
 */
static char TAG_ACTIVITY_SHOW;
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5.实现

5.1.私有方法

/**
 向控件上设置图像
 */
- (void)sd_setImage:(UIImage *)image imageData:(NSData *)imageData basedOnClassOrViaCustomSetImageBlock:(SDSetImageBlock)setImageBlock {
    // 调用下面的图像设置方法
    [self sd_setImage:image imageData:imageData basedOnClassOrViaCustomSetImageBlock:setImageBlock transition:nil cacheType:0 imageURL:nil];
}
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/**
 向控件上设置图像
 */
- (void)sd_setImage:(UIImage *)image imageData:(NSData *)imageData basedOnClassOrViaCustomSetImageBlock:(SDSetImageBlock)setImageBlock transition:(SDWebImageTransition *)transition cacheType:(SDImageCacheType)cacheType imageURL:(NSURL *)imageURL {
    // 获取当前视图对象
    UIView *view = self;
    // 创建变量保存设置图像操作
    SDSetImageBlock finalSetImageBlock;
    if (setImageBlock) {
        // 如果设置了就直接保存
        finalSetImageBlock = setImageBlock;
    }
#if SD_UIKIT || SD_MAC
    else if ([view isKindOfClass:[UIImageView class]]) {
        // 如果没设置，但是是UIImageView类型，就在block中设置图像
        UIImageView *imageView = (UIImageView *)view;
        finalSetImageBlock = ^(UIImage *setImage, NSData *setImageData) {
            imageView.image = setImage;
        };
    }
#endif
#if SD_UIKIT
    else if ([view isKindOfClass:[UIButton class]]) {
        // 如果没设置，但是是UIButton类型，就在block中为UIControlStateNormal状态设置图像
        UIButton *button = (UIButton *)view;
        finalSetImageBlock = ^(UIImage *setImage, NSData *setImageData){
            [button setImage:setImage forState:UIControlStateNormal];
        };
    }
#endif
    
    if (transition) {
#if SD_UIKIT
        // 如果设置了展示动画就添加动画
        [UIView transitionWithView:view duration:0 options:0 animations:^{
            // 0 duration to let UIKit render placeholder and prepares block
            if (transition.prepares) {
                transition.prepares(view, image, imageData, cacheType, imageURL);
            }
        } completion:^(BOOL finished) {
            [UIView transitionWithView:view duration:transition.duration options:transition.animationOptions animations:^{
                if (finalSetImageBlock && !transition.avoidAutoSetImage) {
                    finalSetImageBlock(image, imageData);
                }
                if (transition.animations) {
                    transition.animations(view, image);
                }
            } completion:transition.completion];
        }];
#elif SD_MAC
        [NSAnimationContext runAnimationGroup:^(NSAnimationContext * _Nonnull prepareContext) {
            // 0 duration to let AppKit render placeholder and prepares block
            prepareContext.duration = 0;
            if (transition.prepares) {
                transition.prepares(view, image, imageData, cacheType, imageURL);
            }
        } completionHandler:^{
            [NSAnimationContext runAnimationGroup:^(NSAnimationContext * _Nonnull context) {
                context.duration = transition.duration;
                context.timingFunction = transition.timingFunction;
                context.allowsImplicitAnimation = (transition.animationOptions & SDWebImageAnimationOptionAllowsImplicitAnimation);
                if (finalSetImageBlock && !transition.avoidAutoSetImage) {
                    finalSetImageBlock(image, imageData);
                }
                if (transition.animations) {
                    transition.animations(view, image);
                }
            } completionHandler:^{
                if (transition.completion) {
                    transition.completion(YES);
                }
            }];
        }];
#endif
    } else {
        // 如果没设置动画就直接设置图像
        if (finalSetImageBlock) {
            finalSetImageBlock(image, imageData);
        }
    }
}
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/**
 更新布局
 */
- (void)sd_setNeedsLayout {
    // 兼容不同系统
#if SD_UIKIT
    [self setNeedsLayout];
#elif SD_MAC
    [self setNeedsLayout:YES];
#endif
}
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- (int)sd_getIndicatorStyle{
    // 通过关联对象获取到加载小菊花对象类型
    return [objc_getAssociatedObject(self, &TAG_ACTIVITY_STYLE) intValue];
}
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5.2.自定义getter/setter方法

- (NSProgress *)sd_imageProgress {
    // 通过关联对象获取到进度对象
    NSProgress *progress = objc_getAssociatedObject(self, @selector(sd_imageProgress));
    // 如果没有就创建一个
    if (!progress) {
        progress = [[NSProgress alloc] initWithParent:nil userInfo:nil];
        self.sd_imageProgress = progress;
    }
    // 返回进度对象
    return progress;
}
- (void)setSd_imageProgress:(NSProgress *)sd_imageProgress {
    // 通过关联对象保存进度对象
    objc_setAssociatedObject(self, @selector(sd_imageProgress), sd_imageProgress, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
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- (SDWebImageTransition *)sd_imageTransition {
    // 通过关联对象获取到展示动画对象
    return objc_getAssociatedObject(self, @selector(sd_imageTransition));
}
- (void)setSd_imageTransition:(SDWebImageTransition *)sd_imageTransition {
    // 通过关联对象保存展示动画对象
    objc_setAssociatedObject(self, @selector(sd_imageTransition), sd_imageTransition, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
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- (UIActivityIndicatorView *)activityIndicator {
    // 通过关联对象获取到加载小菊花对象
    return (UIActivityIndicatorView *)objc_getAssociatedObject(self, &TAG_ACTIVITY_INDICATOR);
}
- (void)setActivityIndicator:(UIActivityIndicatorView *)activityIndicator {
    // 通过关联对象保存加载小菊花对象
    objc_setAssociatedObject(self, &TAG_ACTIVITY_INDICATOR, activityIndicator, OBJC_ASSOCIATION_RETAIN);
}
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5.3.公共方法

- (nullable NSURL *)sd_imageURL {
    // 通过关联对象获取到图像路径
    return objc_getAssociatedObject(self, &imageURLKey);
}
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- (void)sd_internalSetImageWithURL:(nullable NSURL *)url
                  placeholderImage:(nullable UIImage *)placeholder
                           options:(SDWebImageOptions)options
                      operationKey:(nullable NSString *)operationKey
                     setImageBlock:(nullable SDSetImageBlock)setImageBlock
                          progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                         completed:(nullable SDExternalCompletionBlock)completedBlock {
    // 调用下面的全能方法
    return [self sd_internalSetImageWithURL:url placeholderImage:placeholder options:options operationKey:operationKey setImageBlock:setImageBlock progress:progressBlock completed:completedBlock context:nil];
}
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- (void)sd_internalSetImageWithURL:(nullable NSURL *)url
                  placeholderImage:(nullable UIImage *)placeholder
                           options:(SDWebImageOptions)options
                      operationKey:(nullable NSString *)operationKey
                     setImageBlock:(nullable SDSetImageBlock)setImageBlock
                          progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                         completed:(nullable SDExternalCompletionBlock)completedBlock
                           context:(nullable NSDictionary<NSString *, id> *)context {
    // 创建变量保存key，如果没有就用当前类名
    NSString *validOperationKey = operationKey ?: NSStringFromClass([self class]);
    // 取消掉key对应的图像加载操作
    [self sd_cancelImageLoadOperationWithKey:validOperationKey];
    // 通过关联对象保存图像路径
    objc_setAssociatedObject(self, &imageURLKey, url, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    
    // 如果没选择延迟加载占位图的选项
    if (!(options & SDWebImageDelayPlaceholder)) {
        // 如果自定义了调度组就添加任务
        if ([context valueForKey:SDWebImageInternalSetImageGroupKey]) {
            dispatch_group_t group = [context valueForKey:SDWebImageInternalSetImageGroupKey];
            dispatch_group_enter(group);
        }
        // 主队列异步调用
        dispatch_main_async_safe(^{
            // 设置占位图
            [self sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock];
        });
    }
    
    if (url) {
        // 如果传入了图像路径
        // 如果要展示加载小菊花就添加加载小菊花控件
        if ([self sd_showActivityIndicatorView]) {
            [self sd_addActivityIndicator];
        }
        
        // 重置进度数据
        self.sd_imageProgress.totalUnitCount = 0;
        self.sd_imageProgress.completedUnitCount = 0;
        
        // 获取图像管理者对象
        SDWebImageManager *manager;
        // 如果自定义了图像管理者对象就用自定义的，否则就用单例对象
        if ([context valueForKey:SDWebImageExternalCustomManagerKey]) {
            manager = (SDWebImageManager *)[context valueForKey:SDWebImageExternalCustomManagerKey];
        } else {
            manager = [SDWebImageManager sharedManager];
        }
        
        // 创建代码块用来处理进度数据
        __weak __typeof(self)wself = self;
        SDWebImageDownloaderProgressBlock combinedProgressBlock = ^(NSInteger receivedSize, NSInteger expectedSize, NSURL * _Nullable targetURL) {
            // 保存进度数据
            wself.sd_imageProgress.totalUnitCount = expectedSize;
            wself.sd_imageProgress.completedUnitCount = receivedSize;
            // 回调进度数据
            if (progressBlock) {
                progressBlock(receivedSize, expectedSize, targetURL);
            }
        };
        // 创建图像加载操作
        id <SDWebImageOperation> operation = [manager loadImageWithURL:url options:options progress:combinedProgressBlock completed:^(UIImage *image, NSData *data, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
            __strong __typeof (wself) sself = wself;
            // 如果当前对象不在了就中止执行
            if (!sself) { return; }
            // 移除加载小菊花
            [sself sd_removeActivityIndicator];
            // 如果完成但是没有进度数据，就为进度数据设置默认值
            if (finished && !error && sself.sd_imageProgress.totalUnitCount == 0 && sself.sd_imageProgress.completedUnitCount == 0) {
                sself.sd_imageProgress.totalUnitCount = SDWebImageProgressUnitCountUnknown;
                sself.sd_imageProgress.completedUnitCount = SDWebImageProgressUnitCountUnknown;
            }
            // 如果完成，或者没完成但是设置了手动设置图像的选项，就需要调用完成block
            BOOL shouldCallCompletedBlock = finished || (options & SDWebImageAvoidAutoSetImage);
            // 如果有图并且设置了手动设置图像的选项，
            // 或者没有图并且没设置延迟加载占位图的选项，
            // 就不需要设置图像
            BOOL shouldNotSetImage = ((image && (options & SDWebImageAvoidAutoSetImage)) ||
                                      (!image && !(options & SDWebImageDelayPlaceholder)));
            // 创建代码块用来处理完成操作
            SDWebImageNoParamsBlock callCompletedBlockClojure = ^{
                // 如果当前对象不在了就中止执行
                if (!sself) { return; }
                // 如果需要设置图像就设置
                if (!shouldNotSetImage) {
                    [sself sd_setNeedsLayout];
                }
                // 如果需要回调完成情况就回调
                if (completedBlock && shouldCallCompletedBlock) {
                    completedBlock(image, error, cacheType, url);
                }
            };
            
            如果不需要设置图像就直接回调完成情况
            if (shouldNotSetImage) {
                dispatch_main_async_safe(callCompletedBlockClojure);
                return;
            }
            
            // 创建变量保存图像对象和图像数据
            UIImage *targetImage = nil;
            NSData *targetData = nil;
            if (image) {
                // 如果有图就直接保存
                targetImage = image;
                targetData = data;
            } else if (options & SDWebImageDelayPlaceholder) {
                // 如果没图但是设置了延迟加载占位图的选项，就职保存占位图
                targetImage = placeholder;
                targetData = nil;
            }
            
            // 创建变量保存图片加载动画对象
            SDWebImageTransition *transition = nil;
            // 如果加载完成，并且设置了强制展示动画选项，
            // 或者加载完成，并且没设置强制展示动画选项但是是从网络下载的图像
            if (finished && (options & SDWebImageForceTransition || cacheType == SDImageCacheTypeNone)) {
                // 获取图片加载动画对象
                transition = sself.sd_imageTransition;
            }
            if ([context valueForKey:SDWebImageInternalSetImageGroupKey]) {
                // 如果自定义了调度组就添加任务
                dispatch_group_t group = [context valueForKey:SDWebImageInternalSetImageGroupKey];
                dispatch_group_enter(group);
                // 主线程异步调用设置图像
                dispatch_main_async_safe(^{
                    [sself sd_setImage:targetImage imageData:targetData basedOnClassOrViaCustomSetImageBlock:setImageBlock transition:transition cacheType:cacheType imageURL:imageURL];
                });
                // 调度完成后调用完成回调block
                dispatch_group_notify(group, dispatch_get_main_queue(), ^{
                    callCompletedBlockClojure();
                });
            } else {
                // 正常情况下就直接主线程异步调用设置图像并回调完成情况
                dispatch_main_async_safe(^{
                    [sself sd_setImage:targetImage imageData:targetData basedOnClassOrViaCustomSetImageBlock:setImageBlock transition:transition cacheType:cacheType imageURL:imageURL];
                    callCompletedBlockClojure();
                });
            }
        }];
        // 将key与图像加载操作相关联
        [self sd_setImageLoadOperation:operation forKey:validOperationKey];
    } else {
        // 如果没有传图像路径
        // 主队列异步调用
        dispatch_main_async_safe(^{
            // 移除加载小菊花
            [self sd_removeActivityIndicator];
            // 回调错误信息
            if (completedBlock) {
                NSError *error = [NSError errorWithDomain:SDWebImageErrorDomain code:-1 userInfo:@{NSLocalizedDescriptionKey : @"Trying to load a nil url"}];
                completedBlock(nil, error, SDImageCacheTypeNone, url);
            }
        });
    }
}
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- (void)sd_cancelCurrentImageLoad {
    // 调用UIView+WebCacheOperation分类方法取消当前控件的图像加载操作
    [self sd_cancelImageLoadOperationWithKey:NSStringFromClass([self class])];
}
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- (void)sd_setShowActivityIndicatorView:(BOOL)show {
    // 通过关联对象保存加载小菊花对象状态
    objc_setAssociatedObject(self, &TAG_ACTIVITY_SHOW, @(show), OBJC_ASSOCIATION_RETAIN);
}
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- (BOOL)sd_showActivityIndicatorView {
    // 通过关联对象获取到加载小菊花对象状态
    return [objc_getAssociatedObject(self, &TAG_ACTIVITY_SHOW) boolValue];
}
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- (void)sd_setIndicatorStyle:(UIActivityIndicatorViewStyle)style{
    // 通过关联对象保存加载小菊花对象类型
    objc_setAssociatedObject(self, &TAG_ACTIVITY_STYLE, [NSNumber numberWithInt:style], OBJC_ASSOCIATION_RETAIN);
}
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- (void)sd_addActivityIndicator {
#if SD_UIKIT
    // 主队列异步调用
    dispatch_main_async_safe(^{
        // 如果没有加载小菊花对象就创建加载小菊花对象
        if (!self.activityIndicator) {
            self.activityIndicator = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:[self sd_getIndicatorStyle]];
            self.activityIndicator.translatesAutoresizingMaskIntoConstraints = NO;
        
            [self addSubview:self.activityIndicator];
            
            [self addConstraint:[NSLayoutConstraint constraintWithItem:self.activityIndicator
                                                             attribute:NSLayoutAttributeCenterX
                                                             relatedBy:NSLayoutRelationEqual
                                                                toItem:self
                                                             attribute:NSLayoutAttributeCenterX
                                                            multiplier:1.0
                                                              constant:0.0]];
            [self addConstraint:[NSLayoutConstraint constraintWithItem:self.activityIndicator
                                                             attribute:NSLayoutAttributeCenterY
                                                             relatedBy:NSLayoutRelationEqual
                                                                toItem:self
                                                             attribute:NSLayoutAttributeCenterY
                                                            multiplier:1.0
                                                              constant:0.0]];
        }
        // 开始动画
        [self.activityIndicator startAnimating];
    });
#endif
}
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- (void)sd_removeActivityIndicator {
#if SD_UIKIT
    // 主队列异步调用
    dispatch_main_async_safe(^{
        // 如果有加载小菊花对象就移除并将保存的属性置空
        if (self.activityIndicator) {
            [self.activityIndicator removeFromSuperview];
            self.activityIndicator = nil;
        }
    });
#endif
}
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6.总结

这个分类实现了向控件上设置图像的基本操作，其他的分类，如UIImageView+WebCache、UIButton+WebCache等基本上没有做什么额外的操作，只是调用该分类的方法。

源码阅读系列：SDWebImage

源码阅读：SDWebImage（一）——从使用入手

源码阅读：SDWebImage（二）——SDWebImageCompat

源码阅读：SDWebImage（三）——NSData+ImageContentType

源码阅读：SDWebImage（四）——SDWebImageCoder

源码阅读：SDWebImage（五）——SDWebImageFrame

源码阅读：SDWebImage（六）——SDWebImageCoderHelper

源码阅读：SDWebImage（七）——SDWebImageImageIOCoder

源码阅读：SDWebImage（八）——SDWebImageGIFCoder

源码阅读：SDWebImage（九）——SDWebImageCodersManager

源码阅读：SDWebImage（十）——SDImageCacheConfig

源码阅读：SDWebImage（十一）——SDImageCache

源码阅读：SDWebImage（十二）——SDWebImageDownloaderOperation

源码阅读：SDWebImage（十三）——SDWebImageDownloader

源码阅读：SDWebImage（十四）——SDWebImageManager

源码阅读：SDWebImage（十五）——SDWebImagePrefetcher

源码阅读：SDWebImage（十六）——SDWebImageTransition

源码阅读：SDWebImage（十七）——UIView+WebCacheOperation

源码阅读：SDWebImage（十八）——UIView+WebCache

源码阅读：SDWebImage（十九）——UIImage+ForceDecode/UIImage+GIF/UIImage+MultiFormat

源码阅读：SDWebImage（二十）——UIButton+WebCache

源码阅读：SDWebImage（二十一）——UIImageView+WebCache/UIImageView+HighlightedWebCache