1 概述

这篇博文中，我将分析SDWebImageManager和SDImageCache。SDWebImageManager拥有一个SDWebImageCache和SDWebImageDownloader属性分别用于图片的缓存和加载处理。为UIView及其子类提供了加载图片的统一接口。管理正在加载操作的集合,这个类是一个单列。同时管理各种加载选项的处理。SDImageCache负责SDWebImage的整个缓存工作，是一个单列对象。缓存路径处理、缓存名字处理、管理内存缓存和磁盘缓存的创建和删除、根据指定key获取图片、存入图片的类型处理、根据缓存的创建和修改日期删除缓存。

2 SDWebImageManager分析

UIImageView等各种视图通过UIView+WebCache分类的sd_internalSetImageWithURL方法来调用SDWebImageManager类的如下方法实现图片加载：

/**
 这个方法是核心方法。UIImageView等这种分类都默认通过调用这个方法来获取数据。

 @param url 图片的url地址
 @param options 获取图片的属性
 @param progressBlock 加载进度回调
 @param completedBlock 加载完成回调
 @return 返回一个加载的载体对象。以便提供给后面取消删除等。
 */
- (nullable id <SDWebImageOperation>)loadImageWithURL:(nullable NSURL *)url
                                              options:(SDWebImageOptions)options
                                             progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                            completed:(nullable SDInternalCompletionBlock)completedBlock;

首先看他地第二个参数options。这个参数指定了图片加载过程中的不同选项。指定不同选项，SDWebImage可以根据选项做不同的处理，这是一个枚举类型,多个选项之间可以组合使用。

/**
 枚举，定义了图片加载处理过程中的选项
 */
typedef NS_OPTIONS(NSUInteger, SDWebImageOptions) {
    /*
     默认情况下，当一个URL下载失败的时候，这个URL会被加入黑名单列表，下次再有这个url的请求则停止请求。
     如果为true，这个值表示需要再尝试请求。
     */
    SDWebImageRetryFailed = 1 << 0,

    /*
     默认情况下，当UI可以交互的时候就开始加载图片。这个标记可以阻止这个时候加载。
     而是当UIScrollView开始减速滑动的时候开始加载。
     */
    SDWebImageLowPriority = 1 << 1,

    /*
     这个属性禁止磁盘缓存
     */
    SDWebImageCacheMemoryOnly = 1 << 2,

    /*
     这个标记允许图片在加载过程中显示，就像浏览器那样。
     默认情况下，图片只会在加载完成以后再显示。
     */
    SDWebImageProgressiveDownload = 1 << 3,

    /*
     *即使本地已经缓存了图片，但是根据HTTP的缓存策略去网络上加载图片。也就是说本地缓存了也不管了，尝试从网络上加载数据。但是具体是从代理加载、HTTP缓存加载、还是原始服务器加载这个就更具HTTP的请求头配置。
     *使用NSURLCache而不是SDWebImage来处理磁盘缓存。从而可能会导致轻微的性能损害。
     *这个选项专门用于处理，url地址没有变，但是url对于的图片数据在服务器改变的情况。
     *如果一个缓存图片更新了，则completion这个回调会被调用两次，一次返回缓存图片，一次返回最终图片。
     *我们只有在不能确保URL和他对应的内容不能完全对应的时候才使用这个标记。
     */
    SDWebImageRefreshCached = 1 << 4,

    /*
     当应用进入后台以后，图片继续下载。应用进入后台以后，通过向系统申请额外的时间来完成。如果时间超时，那么下载操作会被取消。
     */
    SDWebImageContinueInBackground = 1 << 5,

    /*
     处理缓存在`NSHTTPCookieStore`对象里面的cookie。通过设置`NSMutableURLRequest.HTTPShouldHandleCookies = YES`来实现的。
     */
    SDWebImageHandleCookies = 1 << 6,

    /*
     *允许非信任的SSL证书请求。
     *在测试的时候很有用。但是正式环境要小心使用。
     */
    SDWebImageAllowInvalidSSLCertificates = 1 << 7,

    /*
     * 默认情况下，图片加载的顺序是根据加入队列的顺序加载的。但是这个标记会把任务加入队列的最前面。
     */
    SDWebImageHighPriority = 1 << 8,
    
    /*
     默认情况下，在图片加载的过程中，会显示占位图。
     但是这个标记会阻止显示占位图直到图片加载完成。
     */
    SDWebImageDelayPlaceholder = 1 << 9,

    /*
     *默认情况下，我们不会去调用`animated images`(估计就是多张图片循环显示或者GIF图片)的`transformDownloadedImage`代理方法来处理图片。因为大部分transformation操作会对图片做无用处理。
     *用这个标记表示无论如何都要对图片做transform处理。
     */
    SDWebImageTransformAnimatedImage = 1 << 10,
    
    /*
     *默认情况下，图片再下载完成以后都会被自动加载到UIImageView对象上面。但是有时我们希望UIImageView加载我们手动处理以后的图片。
     *这个标记允许我们在completion这个Block中手动设置处理好以后的图片。
     */
    SDWebImageAvoidAutoSetImage = 1 << 11,
    
    /*
     *默认情况下，图片会按照他的原始大小来解码显示。根据设备的内存限制，这个属性会调整图片的尺寸到合适的大小再解压缩。
     *如果`SDWebImageProgressiveDownload`标记被设置了，则这个flag不起作用。
     */
    SDWebImageScaleDownLargeImages = 1 << 12
};

接下来我先看SDWebImageManager的初始化过程。

- (nonnull instancetype)init {
    SDImageCache *cache = [SDImageCache sharedImageCache];
    SDWebImageDownloader *downloader = [SDWebImageDownloader sharedDownloader];
    return [self initWithCache:cache downloader:downloader];
}

/**
 初始化SDImageCache和SDWebImageDownloader对象

 @param cache SDImageCache对象
 @param downloader SDWebImageDownloader对象
 @return 返回初始化结果
 */
- (nonnull instancetype)initWithCache:(nonnull SDImageCache *)cache downloader:(nonnull SDWebImageDownloader *)downloader {
    if ((self = [super init])) {
        _imageCache = cache;
        _imageDownloader = downloader;
        //用于保存加载失败的url集合
        _failedURLs = [NSMutableSet new];
        //用于保存当前正在加载的Operation
        _runningOperations = [NSMutableArray new];
    }
    return self;
}

loadImageWithURL方法是SDWebImageManager最核心的方法，实现过程：

/**
 这个方法是核心方法。UIImageView等这种分类都默认通过调用这个方法来获取数据。

 @param url 图片的url地址
 @param options 获取图片的属性
 @param progressBlock 加载进度回调
 @param completedBlock 加载完成回调
 @return 返回一个加载的载体对象。以便提供给后面取消删除等。
 */
- (id <SDWebImageOperation>)loadImageWithURL:(nullable NSURL *)url
                                     options:(SDWebImageOptions)options
                                    progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                   completed:(nullable SDInternalCompletionBlock)completedBlock {
    /*
     如果传入的url是NSString格式的。则转换为NSURL类型再处理
     */
    if ([url isKindOfClass:NSString.class]) {
        url = [NSURL URLWithString:(NSString *)url];
    }

    // Prevents app crashing on argument type error like sending NSNull instead of NSURL
    //如果url不会NSURL类型的对象。则置为nil
    if (![url isKindOfClass:NSURL.class]) {
        url = nil;
    }
    /*
     图片加载获取获取过程中绑定一个`SDWebImageCombinedOperation`对象。以方便后续再通过这个对象对url的加载控制。
     */
    __block SDWebImageCombinedOperation *operation = [SDWebImageCombinedOperation new];
    __weak SDWebImageCombinedOperation *weakOperation = operation;

    BOOL isFailedUrl = NO;
    //当前url是否在失败url的集合里面
    if (url) {
        @synchronized (self.failedURLs) {
            isFailedUrl = [self.failedURLs containsObject:url];
        }
    }
    /*
     如果url是失败的url或者url有问题等各种问题。则直接根据opeation来做异常情况的处理
     */
    if (url.absoluteString.length == 0 || (!(options & SDWebImageRetryFailed) && isFailedUrl)) {
        //构建回调Block
        [self callCompletionBlockForOperation:operation completion:completedBlock error:[NSError errorWithDomain:NSURLErrorDomain code:NSURLErrorFileDoesNotExist userInfo:nil] url:url];
        return operation;
    }
    //把加载图片的一个载体存入runningOperations。里面是所有正在做图片加载过程的operation的集合。
    @synchronized (self.runningOperations) {
        [self.runningOperations addObject:operation];
    }
    //根据url获取url对应的key
    NSString *key = [self cacheKeyForURL:url];
    /*
    *如果图片是从内存加载，则返回的cacheOperation是nil，
    *如果是从磁盘加载，则返回的cacheOperation是`NSOperation`对象。
    *如果是从网络加载，则返回的cacheOperation对象是`SDWebImageDownloaderOperation`对象。
    */
    operation.cacheOperation = [self.imageCache queryCacheOperationForKey:key done:^(UIImage *cachedImage, NSData *cachedData, SDImageCacheType cacheType) {
        //从缓存中获取图片数据返回
        //如果已经取消了操作。则直接返回并且移除对应的opetation对象
        if (operation.isCancelled) {
            [self safelyRemoveOperationFromRunning:operation];
            return;
        }
        
        if ((!cachedImage || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) {
            /**
             如果从缓存获取图片失败。或者设置了SDWebImageRefreshCached来忽略缓存。则先把缓存的图片返回。
             */
            if (cachedImage && options & SDWebImageRefreshCached) {
                //构建回调Block
                [self callCompletionBlockForOperation:weakOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
            }

            // download if no image or requested to refresh anyway, and download allowed by delegate
            /*
             把图片加载的`SDWebImageOptions`类型枚举转换为图片下载的`SDWebImageDownloaderOptions`类型的枚举
             */
            SDWebImageDownloaderOptions downloaderOptions = 0;
            if (options & SDWebImageLowPriority) downloaderOptions |= SDWebImageDownloaderLowPriority;
            if (options & SDWebImageProgressiveDownload) downloaderOptions |= SDWebImageDownloaderProgressiveDownload;
            if (options & SDWebImageRefreshCached) downloaderOptions |= SDWebImageDownloaderUseNSURLCache;
            if (options & SDWebImageContinueInBackground) downloaderOptions |= SDWebImageDownloaderContinueInBackground;
            if (options & SDWebImageHandleCookies) downloaderOptions |= SDWebImageDownloaderHandleCookies;
            if (options & SDWebImageAllowInvalidSSLCertificates) downloaderOptions |= SDWebImageDownloaderAllowInvalidSSLCertificates;
            if (options & SDWebImageHighPriority) downloaderOptions |= SDWebImageDownloaderHighPriority;
            if (options & SDWebImageScaleDownLargeImages) downloaderOptions |= SDWebImageDownloaderScaleDownLargeImages;
            /*
             如果设置了强制刷新缓存的选项。则`SDWebImageDownloaderProgressiveDownload`选项失效并且添加`SDWebImageDownloaderIgnoreCachedResponse`选项。
             */
            if (cachedImage && options & SDWebImageRefreshCached) {
                // force progressive off if image already cached but forced refreshing
                downloaderOptions &= ~SDWebImageDownloaderProgressiveDownload;
                // ignore image read from NSURLCache if image if cached but force refreshing
                downloaderOptions |= SDWebImageDownloaderIgnoreCachedResponse;
            }
            /*
             新建一个网络下载的操作。
             */
            SDWebImageDownloadToken *subOperationToken = [self.imageDownloader downloadImageWithURL:url options:downloaderOptions progress:progressBlock completed:^(UIImage *downloadedImage, NSData *downloadedData, NSError *error, BOOL finished) {
                __strong __typeof(weakOperation) strongOperation = weakOperation;
                //如果图片下载结束以后，对应的图片加载操作已经取消。则什么处理都不做
                if (!strongOperation || strongOperation.isCancelled) {
                   //如果operation已经被取消了，则什么也不做
                } else if (error) {
                    //如果加载出错。则直接返回回调。并且添加到failedURLs中
                    [self callCompletionBlockForOperation:strongOperation completion:completedBlock error:error url:url];

                    if (   error.code != NSURLErrorNotConnectedToInternet
                        && error.code != NSURLErrorCancelled
                        && error.code != NSURLErrorTimedOut
                        && error.code != NSURLErrorInternationalRoamingOff
                        && error.code != NSURLErrorDataNotAllowed
                        && error.code != NSURLErrorCannotFindHost
                        && error.code != NSURLErrorCannotConnectToHost
                        && error.code != NSURLErrorNetworkConnectionLost) {
                        @synchronized (self.failedURLs) {
                            [self.failedURLs addObject:url];
                        }
                    }
                }
                else {
                    //网络图片加载成功
                    if ((options & SDWebImageRetryFailed)) {
                        //如果有重试失败下载的选项。则把url从failedURLS中移除
                        @synchronized (self.failedURLs) {
                            [self.failedURLs removeObject:url];
                        }
                    }
                    
                    BOOL cacheOnDisk = !(options & SDWebImageCacheMemoryOnly);

                    if (options & SDWebImageRefreshCached && cachedImage && !downloadedImage) {
                        // Image refresh hit the NSURLCache cache, do not call the completion block
                        //如果成功下载图片。并且图片是动态图片。并且设置了SDWebImageTransformAnimatedImage属性。则处理图片
                    } else if (downloadedImage && (!downloadedImage.images || (options & SDWebImageTransformAnimatedImage)) && [self.delegate respondsToSelector:@selector(imageManager:transformDownloadedImage:withURL:)]) {
                        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
                            //获取transform以后的图片
                            UIImage *transformedImage = [self.delegate imageManager:self transformDownloadedImage:downloadedImage withURL:url];
                            //存储transform以后的的图片
                            if (transformedImage && finished) {
                                BOOL imageWasTransformed = ![transformedImage isEqual:downloadedImage];
                                // pass nil if the image was transformed, so we can recalculate the data from the image
                                [self.imageCache storeImage:transformedImage imageData:(imageWasTransformed ? nil : downloadedData) forKey:key toDisk:cacheOnDisk completion:nil];
                            }
                            //回调拼接
                            [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:transformedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];
                        });
                    } else {
                        //如果成功下载图片。并且图片不是图片。则直接缓存和回调
                        if (downloadedImage && finished) {
                            [self.imageCache storeImage:downloadedImage imageData:downloadedData forKey:key toDisk:cacheOnDisk completion:nil];
                        }
                        //回调拼接
                        [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:downloadedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];
                    }
                }
                //从正在加载的图片操作集合中移除当前操作
                if (finished) {
                    [self safelyRemoveOperationFromRunning:strongOperation];
                }
            }];
            //重置cancelBlock，取消下载operation
            operation.cancelBlock = ^{
                [self.imageDownloader cancel:subOperationToken];
                __strong __typeof(weakOperation) strongOperation = weakOperation;
                [self safelyRemoveOperationFromRunning:strongOperation];
            };
        } else if (cachedImage) {
            //如果获取到了缓存图片。在直接通过缓存图片处理
            __strong __typeof(weakOperation) strongOperation = weakOperation;
            [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
            [self safelyRemoveOperationFromRunning:operation];
        } else {
            // Image not in cache and download disallowed by delegate
            //图片么有缓存、并且图片也没有下载
            __strong __typeof(weakOperation) strongOperation = weakOperation;
            [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:nil data:nil error:nil cacheType:SDImageCacheTypeNone finished:YES url:url];
            [self safelyRemoveOperationFromRunning:operation];
        }
    }];

    return operation;
}

通过对这个方法的分析，他主要实现的功能有：

• 创建一个SDWebImageCombinedOperation对象，调用者可以通过这个对象来对加载做取消等操作。这个对象的cancelOperation属性有如下几种情况。
  • 如果图片是从内存加载，则返回的cacheOperation是nil。
  • 如果是从磁盘加载，则返回的cacheOperation是NSOperation对象。
  • 如果是从网络加载，则返回的cacheOperation对象是SDWebImageDownloaderOperation对象。
• 通过failedURLs属性来保存加载失败的url。通过它可以阻止失败的url再次加载，提高用户体验。
• 通过runningOperations属性记录当前正在加载的Operation列表。
• 加载结束以后，通过callCompletionBlockForOperation方法来拼接回调Block。
• 把SDWebImageOptions类型的枚举值转换为SDWebImageDownloaderOptions类型的枚举值。
• 图片成功从网络加载以后，通过imageCache属性的storeImage方法来缓存图片。

另外还有一些辅助性的方法,用于处理缓存判断、url与key转换等功能:

/**
 根据url获取url对应的缓存key。
 如果有实现指定的url转换key的Block，则用这个方式转换为key。
 否则直接用url的绝对值多为key

 @param url url
 @return 缓存的key
 */
- (nullable NSString *)cacheKeyForURL:(nullable NSURL *)url {
    if (!url) {
        return @"";
    }
    if (self.cacheKeyFilter) {
        return self.cacheKeyFilter(url);
    } else {
        return url.absoluteString;
    }
}

/**
 一个url的缓存是否存在

 @param url 缓存数据对应的url
 @param completionBlock 缓存结果回调
 */
- (void)cachedImageExistsForURL:(nullable NSURL *)url
                     completion:(nullable SDWebImageCheckCacheCompletionBlock)completionBlock {
    NSString *key = [self cacheKeyForURL:url];
    //内存里面是否有key的缓存
    BOOL isInMemoryCache = ([self.imageCache imageFromMemoryCacheForKey:key] != nil);
    //内存缓存
    if (isInMemoryCache) {
        // making sure we call the completion block on the main queue
        dispatch_async(dispatch_get_main_queue(), ^{
            if (completionBlock) {
                completionBlock(YES);
            }
        });
        return;
    }
    //磁盘缓存
    [self.imageCache diskImageExistsWithKey:key completion:^(BOOL isInDiskCache) {
        // the completion block of checkDiskCacheForImageWithKey:completion: is always called on the main queue, no need to further dispatch
        if (completionBlock) {
            completionBlock(isInDiskCache);
        }
    }];
}

/**
 url是否有磁盘缓存数据

 @param url url
 @param completionBlock 回调
 */
- (void)diskImageExistsForURL:(nullable NSURL *)url
                   completion:(nullable SDWebImageCheckCacheCompletionBlock)completionBlock {
    NSString *key = [self cacheKeyForURL:url];
    
    [self.imageCache diskImageExistsWithKey:key completion:^(BOOL isInDiskCache) {
        // the completion block of checkDiskCacheForImageWithKey:completion: is always called on the main queue, no need to further dispatch
        if (completionBlock) {
            completionBlock(isInDiskCache);
        }
    }];
}

2.1 SDWebImageCombinedOperation分析

从上面的方法，发现loadImageWithURL方法会返回一个SDWebImageCombinedOperation给调用者,这个对象有个cancel方法,这个方法继承自SDWebImageOperation协议。方法里面会调用SDWebImageDownlaoderOperation或者NSOperation的cancel方法

/**
 通过这个对象关联一个`SDWebImageDownloaderOperation`对象
 */
@interface SDWebImageCombinedOperation : NSObject <SDWebImageOperation>

/**
 用于判断Operation是否已经取消
 */
@property (assign, nonatomic, getter = isCancelled) BOOL cancelled;

/**
 取消回调
 */
@property (copy, nonatomic, nullable) SDWebImageNoParamsBlock cancelBlock;

/**
 SDWebImageDownloaderOperation对象。可以通过这个属性取消一个NSOperation
 */
@property (strong, nonatomic, nullable) NSOperation *cacheOperation;

@end

@implementation SDWebImageCombinedOperation

/**
 取消Operation的回调Block

 @param cancelBlock 回调Block
 */
- (void)setCancelBlock:(nullable SDWebImageNoParamsBlock)cancelBlock {
    // check if the operation is already cancelled, then we just call the cancelBlock
    if (self.isCancelled) {
        if (cancelBlock) {
            cancelBlock();
        }
        _cancelBlock = nil; // don't forget to nil the cancelBlock, otherwise we will get crashes
    } else {
        _cancelBlock = [cancelBlock copy];
    }
}

/**
 调用cancel方法。这个方法继承自`SDWebImageOperation`协议。方法里面会调用`SDWebImageDownlaoderOperation`或者`NSOperation`的cancel方法
 */
- (void)cancel {
    self.cancelled = YES;
    if (self.cacheOperation) {
        //调用`SDWebImageDownlaoderOperation`或者`NSOperation`的cancel方法
        [self.cacheOperation cancel];
        self.cacheOperation = nil;
    }
    if (self.cancelBlock) {
        self.cancelBlock();
        
        // TODO: this is a temporary fix to #809.
        // Until we can figure the exact cause of the crash, going with the ivar instead of the setter
        //        self.cancelBlock = nil;
        _cancelBlock = nil;
    }
}

@end

那么取消一个Operation是在哪里呢？其实就在UIView+WebCache中,所有使用SDWebImage的View都可以使用这个方法来取消下载Operation：

/**
 取消当前Class对应的所有加载请求
 */
- (void)sd_cancelCurrentImageLoad {
    [self sd_cancelImageLoadOperationWithKey:NSStringFromClass([self class])];
}

上面这个方法又会调用UIView+WebCacheOperation分类的sd_cancelImageLoadOperationWithKey方法来实现，这个方法会调用SDWebImageCombinedOperation对象的cancel方法,然后在cancel方法中再调用SDWebImageDownloadOperation或者NSOperation的cancel方法：

/**
 取消当前key对应的所有`SDWebImageCombinedOperation`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];
    }
}

3 SDImageCache分析

从上面的代码中，发现SDWebImageManager通过SDImageCache来获取/存储图片，而且SDImageCache是一个单列对象。他的具体实现可以总结为如下几点：

• 通过AutoPurgeCache这个NSCache子类来管理内存缓存。当接收到内存警告的时候，移除内存缓存的所有对象。
• 接收到UIApplicationDidReceiveMemoryWarningNotification通知以后，会删除内存中缓存的图片。
• 接收到UIApplicationWillTerminateNotification通知以后，会通过deleteOldFiles方法删除老的图片。具体删除规则如下：
  • 缓存大小、过期日期、是否解压缩缓存、是否允许内存缓存都是通过SDImageCacheConfig这个对象来配置的。
  • 首先会迭代缓存目录下的所有文件，对于大于一周的图片数据全部删除。
  • 然后会记录缓存目录的所有大小，如果当前缓存大于默认缓存，则按照创建日期开始删除图片缓存，直到缓存大小小于默认缓存大小。
• 当接收到UIApplicationDidEnterBackgroundNotification通知以后，会调用deleteOldFilesWithCompletionBlock来清理缓存数据。
• 定义了一些列方法来处理图片的获取、缓存、移除操作。主要有下面几个方法：
  • queryCacheOperationForKey查询指定key对应的缓存图片，先从内存查找，然后从磁盘查找。
  • removeImageForKey移除指定的缓存图片。
  • diskImageDataBySearchingAllPathsForKey在磁盘上查找指定key对应的图片。
  • storeImageDataToDisk把指定的图片数据存入磁盘。
• 通过cachedFileNameForKey方法获取一张图片对应的MD5加密的缓存名字。

SDImageCache的完整实现如下：

@interface AutoPurgeCache : NSCache
@end

/**
 如果接收到内存警告、移除所有的缓存对象
 */
@implementation AutoPurgeCache

- (nonnull instancetype)init {
    self = [super init];
    if (self) {
#if SD_UIKIT
        [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(removeAllObjects) name:UIApplicationDidReceiveMemoryWarningNotification object:nil];
#endif
    }
    return self;
}

- (void)dealloc {
#if SD_UIKIT
    [[NSNotificationCenter defaultCenter] removeObserver:self name:UIApplicationDidReceiveMemoryWarningNotification object:nil];
#endif
}

@end
/**
 计算一张图片占用内存的大小

 @param image 图片
 @return 占用内存大小
 */
FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
#if SD_MAC
    return image.size.height * image.size.width;
#elif SD_UIKIT || SD_WATCH
    return image.size.height * image.size.width * image.scale * image.scale;
#endif
}
/**
 单列对象、用处管理图片的缓存以及缓存图片的处理
 */
@interface SDImageCache ()

#pragma mark - Properties
@property (strong, nonatomic, nonnull) NSCache *memCache;
@property (strong, nonatomic, nonnull) NSString *diskCachePath;
@property (strong, nonatomic, nullable) NSMutableArray<NSString *> *customPaths;
@property (SDDispatchQueueSetterSementics, nonatomic, nullable) dispatch_queue_t ioQueue;

@end

@implementation SDImageCache {
    NSFileManager *_fileManager;
}
#pragma mark - Singleton, init, dealloc
+ (nonnull instancetype)sharedImageCache {
    static dispatch_once_t once;
    static id instance;
    dispatch_once(&once, ^{
        instance = [self new];
    });
    return instance;
}

- (instancetype)init {
    return [self initWithNamespace:@"default"];
}

- (nonnull instancetype)initWithNamespace:(nonnull NSString *)ns {
    NSString *path = [self makeDiskCachePath:ns];
    return [self initWithNamespace:ns diskCacheDirectory:path];
}

- (nonnull instancetype)initWithNamespace:(nonnull NSString *)ns
                       diskCacheDirectory:(nonnull NSString *)directory {
    if ((self = [super init])) {
        NSString *fullNamespace = [@"com.hackemist.SDWebImageCache." stringByAppendingString:ns];
        // 初始化一个串行的dispatch_queue_t
        _ioQueue = dispatch_queue_create("com.hackemist.SDWebImageCache", DISPATCH_QUEUE_SERIAL);
        //初始化缓存策略配置对象
        _config = [[SDImageCacheConfig alloc] init];
        
        // 初始化内存缓存对象
        _memCache = [[AutoPurgeCache alloc] init];
        _memCache.name = fullNamespace;

        // 初始化磁盘缓存路径
        if (directory != nil) {
            _diskCachePath = [directory stringByAppendingPathComponent:fullNamespace];
        } else {
            NSString *path = [self makeDiskCachePath:ns];
            _diskCachePath = path;
        }

        dispatch_sync(_ioQueue, ^{
            _fileManager = [NSFileManager new];
        });

#if SD_UIKIT
        /*
         当应用收到内存警告的时候，清除内存缓存。
         */
        [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(clearMemory)
                                                     name:UIApplicationDidReceiveMemoryWarningNotification
                                                   object:nil];
        /*
         当应用终止的时候，清除老数据
         */
        [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(deleteOldFiles)
                                                     name:UIApplicationWillTerminateNotification
                                                   object:nil];
        /*
         当应用进入后台的时候，在后台删除老数据
         */
        [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(backgroundDeleteOldFiles)
                                                     name:UIApplicationDidEnterBackgroundNotification
                                                   object:nil];
#endif
    }

    return self;
}

- (void)dealloc {
    [[NSNotificationCenter defaultCenter] removeObserver:self];
    SDDispatchQueueRelease(_ioQueue);
}

/**
 当前queue是否在ioQueue
 */
- (void)checkIfQueueIsIOQueue {
    const char *currentQueueLabel = dispatch_queue_get_label(DISPATCH_CURRENT_QUEUE_LABEL);
    const char *ioQueueLabel = dispatch_queue_get_label(self.ioQueue);
    if (strcmp(currentQueueLabel, ioQueueLabel) != 0) {
        NSLog(@"This method should be called from the ioQueue");
    }
}

#pragma mark - Cache paths
/*
 添加只能读的缓存目录
 */
- (void)addReadOnlyCachePath:(nonnull NSString *)path {
    if (!self.customPaths) {
        self.customPaths = [NSMutableArray new];
    }

    if (![self.customPaths containsObject:path]) {
        [self.customPaths addObject:path];
    }
}

/**
 获取指定key对应的完整缓存路径

 @param key key，对应一张图片。比如图片的名字
 @param path 指定根目录
 @return 完整目录
 */
- (nullable NSString *)cachePathForKey:(nullable NSString *)key inPath:(nonnull NSString *)path {
    NSString *filename = [self cachedFileNameForKey:key];
    return [path stringByAppendingPathComponent:filename];
}

- (nullable NSString *)defaultCachePathForKey:(nullable NSString *)key {
    return [self cachePathForKey:key inPath:self.diskCachePath];
}


/**
 MD5加密
 X 表示以十六进制形式输出
 02 表示不足两位，前面补0输出；出过两位，不影响

 @param key key
 @return 加密后的数据
 */
- (nullable NSString *)cachedFileNameForKey:(nullable NSString *)key {
    const char *str = key.UTF8String;
    if (str == NULL) {
        str = "";
    }
    unsigned char r[CC_MD5_DIGEST_LENGTH];
    CC_MD5(str, (CC_LONG)strlen(str), r);
    NSString *filename = [NSString stringWithFormat:@"%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%@",
                          r[0], r[1], r[2], r[3], r[4], r[5], r[6], r[7], r[8], r[9], r[10],
                          r[11], r[12], r[13], r[14], r[15], [key.pathExtension isEqualToString:@""] ? @"" : [NSString stringWithFormat:@".%@", key.pathExtension]];

    return filename;
}

/**
 图片缓存目录

 @param fullNamespace 自定义目录
 @return 完整目录
 */
- (nullable NSString *)makeDiskCachePath:(nonnull NSString*)fullNamespace {
    NSArray<NSString *> *paths = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES);
    return [paths[0] stringByAppendingPathComponent:fullNamespace];
}

#pragma mark - 图片缓存具体实现的一些列方法

- (void)storeImage:(nullable UIImage *)image
            forKey:(nullable NSString *)key
        completion:(nullable SDWebImageNoParamsBlock)completionBlock {
    [self storeImage:image imageData:nil forKey:key toDisk:YES completion:completionBlock];
}

- (void)storeImage:(nullable UIImage *)image
            forKey:(nullable NSString *)key
            toDisk:(BOOL)toDisk
        completion:(nullable SDWebImageNoParamsBlock)completionBlock {
    [self storeImage:image imageData:nil forKey:key toDisk:toDisk completion:completionBlock];
}

/**
 把一张图片存入缓存的具体实现

 @param image 缓存的图片对象
 @param imageData 缓存的图片数据
 @param key 缓存对应的key
 @param toDisk 是否缓存到瓷片
 @param completionBlock 缓存完成回调
 */
- (void)storeImage:(nullable UIImage *)image
         imageData:(nullable NSData *)imageData
            forKey:(nullable NSString *)key
            toDisk:(BOOL)toDisk
        completion:(nullable SDWebImageNoParamsBlock)completionBlock {
    if (!image || !key) {
        if (completionBlock) {
            completionBlock();
        }
        return;
    }
    //缓存到内存
    if (self.config.shouldCacheImagesInMemory) {
        //计算缓存数据的大小
        NSUInteger cost = SDCacheCostForImage(image);
        //加入缓存对此昂
        [self.memCache setObject:image forKey:key cost:cost];
    }
    
    if (toDisk) {
        /*
         在一个线性队列中做磁盘缓存操作。
         */
        dispatch_async(self.ioQueue, ^{
            NSData *data = imageData;
            if (!data && image) {
                //获取图片的类型GIF/PNG等
                SDImageFormat imageFormatFromData = [NSData sd_imageFormatForImageData:data];
                //根据指定的SDImageFormat。把图片转换为对应的data数据
                data = [image sd_imageDataAsFormat:imageFormatFromData];
            }
            //把处理好了的数据存入磁盘
            [self storeImageDataToDisk:data forKey:key];
            if (completionBlock) {
                dispatch_async(dispatch_get_main_queue(), ^{
                    completionBlock();
                });
            }
        });
    } else {
        if (completionBlock) {
            completionBlock();
        }
    }
}

/**
 把图片资源存入磁盘

 @param imageData 图片数据
 @param key key
 */
- (void)storeImageDataToDisk:(nullable NSData *)imageData forKey:(nullable NSString *)key {
    if (!imageData || !key) {
        return;
    }
    
    [self checkIfQueueIsIOQueue];
    //缓存目录是否已经初始化
    if (![_fileManager fileExistsAtPath:_diskCachePath]) {
        [_fileManager createDirectoryAtPath:_diskCachePath withIntermediateDirectories:YES attributes:nil error:NULL];
    }
    
    // get cache Path for image key
    //获取key对应的完整缓存路径
    NSString *cachePathForKey = [self defaultCachePathForKey:key];
    // transform to NSUrl
    NSURL *fileURL = [NSURL fileURLWithPath:cachePathForKey];
    //把数据存入路径
    [_fileManager createFileAtPath:cachePathForKey contents:imageData attributes:nil];
    
    // disable iCloud backup
    if (self.config.shouldDisableiCloud) {
        //给文件添加到运行存储到iCloud属性
        [fileURL setResourceValue:@YES forKey:NSURLIsExcludedFromBackupKey error:nil];
    }
}

#pragma mark - Query and Retrieve Ops

- (void)diskImageExistsWithKey:(nullable NSString *)key completion:(nullable SDWebImageCheckCacheCompletionBlock)completionBlock {
    dispatch_async(_ioQueue, ^{
        BOOL exists = [_fileManager fileExistsAtPath:[self defaultCachePathForKey:key]];

        // fallback because of https://github.com/rs/SDWebImage/pull/976 that added the extension to the disk file name
        // checking the key with and without the extension
        if (!exists) {
            exists = [_fileManager fileExistsAtPath:[self defaultCachePathForKey:key].stringByDeletingPathExtension];
        }

        if (completionBlock) {
            dispatch_async(dispatch_get_main_queue(), ^{
                completionBlock(exists);
            });
        }
    });
}

/**
 根据key获取缓存在内存中的图片

 @param key key
 @return 缓存的图片
 */
- (nullable UIImage *)imageFromMemoryCacheForKey:(nullable NSString *)key {
    return [self.memCache objectForKey:key];
}

- (nullable UIImage *)imageFromDiskCacheForKey:(nullable NSString *)key {
    UIImage *diskImage = [self diskImageForKey:key];
    if (diskImage && self.config.shouldCacheImagesInMemory) {
        NSUInteger cost = SDCacheCostForImage(diskImage);
        [self.memCache setObject:diskImage forKey:key cost:cost];
    }

    return diskImage;
}

- (nullable UIImage *)imageFromCacheForKey:(nullable NSString *)key {
    // First check the in-memory cache...
    UIImage *image = [self imageFromMemoryCacheForKey:key];
    if (image) {
        return image;
    }
    
    // Second check the disk cache...
    image = [self imageFromDiskCacheForKey:key];
    return image;
}

/**
 根据指定的key，获取存储在磁盘上的数据

 @param key 图片对应的key
 @return 返回图片数据
 */
- (nullable NSData *)diskImageDataBySearchingAllPathsForKey:(nullable NSString *)key {
    //获取key对应的path
    NSString *defaultPath = [self defaultCachePathForKey:key];
    NSData *data = [NSData dataWithContentsOfFile:defaultPath];
    if (data) {
        return data;
    }

    // fallback because of https://github.com/rs/SDWebImage/pull/976 that added the extension to the disk file name
    // checking the key with and without the extension
    /*
     如果key么有后缀名，则会走到这里通过这里读取
     */
    data = [NSData dataWithContentsOfFile:defaultPath.stringByDeletingPathExtension];
    if (data) {
        return data;
    }
    /*
     如果在默认路径没有找到图片，则在自定义路径迭代查找
     */
    NSArray<NSString *> *customPaths = [self.customPaths copy];
    for (NSString *path in customPaths) {
        NSString *filePath = [self cachePathForKey:key inPath:path];
        NSData *imageData = [NSData dataWithContentsOfFile:filePath];
        if (imageData) {
            return imageData;
        }

        // fallback because of https://github.com/rs/SDWebImage/pull/976 that added the extension to the disk file name
        // checking the key with and without the extension
        imageData = [NSData dataWithContentsOfFile:filePath.stringByDeletingPathExtension];
        if (imageData) {
            return imageData;
        }
    }

    return nil;
}

/**
 根据指定的key获取image对象

 @param key key
 @return image对象
 */
- (nullable UIImage *)diskImageForKey:(nullable NSString *)key {
    //获取磁盘数据
    NSData *data = [self diskImageDataBySearchingAllPathsForKey:key];
    if (data) {
        UIImage *image = [UIImage sd_imageWithData:data];
        image = [self scaledImageForKey:key image:image];
        if (self.config.shouldDecompressImages) {
            image = [UIImage decodedImageWithImage:image];
        }
        return image;
    }
    else {
        return nil;
    }
}

- (nullable UIImage *)scaledImageForKey:(nullable NSString *)key image:(nullable UIImage *)image {
    return SDScaledImageForKey(key, image);
}

/**
 在缓存中查询对应key的数据。通过一个NSOperation来完成

 @param key 要查询的key
 @param doneBlock 查询结束以后的Block
 @return 返回做查询操作的Block
 */
- (nullable NSOperation *)queryCacheOperationForKey:(nullable NSString *)key done:(nullable SDCacheQueryCompletedBlock)doneBlock {
    if (!key) {
        if (doneBlock) {
            doneBlock(nil, nil, SDImageCacheTypeNone);
        }
        return nil;
    }
    // First check the in-memory cache...
    //首先从内测中查找图片
    UIImage *image = [self imageFromMemoryCacheForKey:key];
    if (image) {
        NSData *diskData = nil;
        //是否是gif图片
        if ([image isGIF]) {
            diskData = [self diskImageDataBySearchingAllPathsForKey:key];
        }
        if (doneBlock) {
            doneBlock(image, diskData, SDImageCacheTypeMemory);
        }
        return nil;
    }
    //新建一个NSOperation来获取磁盘图片
    NSOperation *operation = [NSOperation new];
    dispatch_async(self.ioQueue, ^{
        if (operation.isCancelled) {
            // do not call the completion if cancelled
            return;
        }
        //在一个自动释放池中处理图片从磁盘加载
        @autoreleasepool {
            NSData *diskData = [self diskImageDataBySearchingAllPathsForKey:key];
            UIImage *diskImage = [self diskImageForKey:key];
            if (diskImage && self.config.shouldCacheImagesInMemory) {
                NSUInteger cost = SDCacheCostForImage(diskImage);
                //把从磁盘取出的缓存图片加入内存缓存中
                [self.memCache setObject:diskImage forKey:key cost:cost];
            }
            //图片处理完成以后回调Block
            if (doneBlock) {
                dispatch_async(dispatch_get_main_queue(), ^{
                    doneBlock(diskImage, diskData, SDImageCacheTypeDisk);
                });
            }
        }
    });
    return operation;
}

#pragma mark - Remove Ops

- (void)removeImageForKey:(nullable NSString *)key withCompletion:(nullable SDWebImageNoParamsBlock)completion {
    [self removeImageForKey:key fromDisk:YES withCompletion:completion];
}

/**
 移除指定key对应的缓存数据

 @param key key
 @param fromDisk 是否也清除磁盘缓存
 @param completion 回调
 */
- (void)removeImageForKey:(nullable NSString *)key fromDisk:(BOOL)fromDisk withCompletion:(nullable SDWebImageNoParamsBlock)completion {
    if (key == nil) {
        return;
    }
    //移除内存缓存
    if (self.config.shouldCacheImagesInMemory) {
        [self.memCache removeObjectForKey:key];
    }
    //移除磁盘缓存
    if (fromDisk) {
        dispatch_async(self.ioQueue, ^{
            [_fileManager removeItemAtPath:[self defaultCachePathForKey:key] error:nil];
            
            if (completion) {
                dispatch_async(dispatch_get_main_queue(), ^{
                    completion();
                });
            }
        });
    } else if (completion){
        completion();
    }
    
}

# pragma mark - Mem Cache settings

- (void)setMaxMemoryCost:(NSUInteger)maxMemoryCost {
    self.memCache.totalCostLimit = maxMemoryCost;
}

- (NSUInteger)maxMemoryCost {
    return self.memCache.totalCostLimit;
}

- (NSUInteger)maxMemoryCountLimit {
    return self.memCache.countLimit;
}

- (void)setMaxMemoryCountLimit:(NSUInteger)maxCountLimit {
    self.memCache.countLimit = maxCountLimit;
}

#pragma mark - 内存缓存清理相关操作

/**
 清理当前SDImageCache对象的内存缓存
 */
- (void)clearMemory {
    [self.memCache removeAllObjects];
}

/**
 移除所有的缓存图片数据

 @param completion 移除完成以后回调
 */
- (void)clearDiskOnCompletion:(nullable SDWebImageNoParamsBlock)completion {
    dispatch_async(self.ioQueue, ^{
        [_fileManager removeItemAtPath:self.diskCachePath error:nil];
        [_fileManager createDirectoryAtPath:self.diskCachePath
                withIntermediateDirectories:YES
                                 attributes:nil
                                      error:NULL];
        if (completion) {
            dispatch_async(dispatch_get_main_queue(), ^{
                completion();
            });
        }
    });
}

- (void)deleteOldFiles {
    [self deleteOldFilesWithCompletionBlock:nil];
}

/**
 当应用终止或者进入后台都回调用这个方法来清除缓存图片。
 这里会根据图片存储时间来清理图片、默认是一周，从最老的图片开始清理。如果图片缓存空间小于一个规定值，则不考虑。

 @param completionBlock 清除完成以后的回调
 */
- (void)deleteOldFilesWithCompletionBlock:(nullable SDWebImageNoParamsBlock)completionBlock {
    dispatch_async(self.ioQueue, ^{
        //获取磁盘缓存的默认根目录
        NSURL *diskCacheURL = [NSURL fileURLWithPath:self.diskCachePath isDirectory:YES];
        
        NSArray<NSString *> *resourceKeys = @[NSURLIsDirectoryKey, NSURLContentModificationDateKey, NSURLTotalFileAllocatedSizeKey];

        // This enumerator prefetches useful properties for our cache files.
        /*
         第二个参数制定了需要获取的属性集合
         第三个参数表示不迭代隐藏文件
        */
        NSDirectoryEnumerator *fileEnumerator = [_fileManager enumeratorAtURL:diskCacheURL
                                                   includingPropertiesForKeys:resourceKeys
                                                                      options:NSDirectoryEnumerationSkipsHiddenFiles
                                                                 errorHandler:NULL];

        NSDate *expirationDate = [NSDate dateWithTimeIntervalSinceNow:-self.config.maxCacheAge];
        NSMutableDictionary<NSURL *, NSDictionary<NSString *, id> *> *cacheFiles = [NSMutableDictionary dictionary];
        NSUInteger currentCacheSize = 0;

        // Enumerate all of the files in the cache directory.  This loop has two purposes:
        //
        //  1. Removing files that are older than the expiration date.
        //  2. Storing file attributes for the size-based cleanup pass.
        /*
         迭代缓存目录。有两个目的：
         1 删除比指定日期更老的图片
         2 记录文件的大小，以提供给后面删除使用
         */
        NSMutableArray<NSURL *> *urlsToDelete = [[NSMutableArray alloc] init];
        for (NSURL *fileURL in fileEnumerator) {
            NSError *error;
            //获取指定url对应文件的指定三种属性的key和value
            NSDictionary<NSString *, id> *resourceValues = [fileURL resourceValuesForKeys:resourceKeys error:&error];
            // Skip directories and errors.
            //如果是文件夹则返回
            if (error || !resourceValues || [resourceValues[NSURLIsDirectoryKey] boolValue]) {
                continue;
            }

            // Remove files that are older than the expiration date;
            //获取指定url文件对应的修改日期
            NSDate *modificationDate = resourceValues[NSURLContentModificationDateKey];
            //如果修改日期大于指定日期，则加入要移除的数组里
            if ([[modificationDate laterDate:expirationDate] isEqualToDate:expirationDate]) {
                [urlsToDelete addObject:fileURL];
                continue;
            }

            // Store a reference to this file and account for its total size.
            //获取指定的url对应的文件的大小，并且把url与对应大小存入一个字典中
            NSNumber *totalAllocatedSize = resourceValues[NSURLTotalFileAllocatedSizeKey];
            currentCacheSize += totalAllocatedSize.unsignedIntegerValue;
            cacheFiles[fileURL] = resourceValues;
        }
        //删除所有最后修改日期大于指定日期的所有文件
        for (NSURL *fileURL in urlsToDelete) {
            [_fileManager removeItemAtURL:fileURL error:nil];
        }

        // If our remaining disk cache exceeds a configured maximum size, perform a second
        // size-based cleanup pass.  We delete the oldest files first.
        /*
         如果我们当前缓存的大小超过了默认大小，则按照日期删除，直到缓存大小<默认大小的一半
         */
        if (self.config.maxCacheSize > 0 && currentCacheSize > self.config.maxCacheSize) {
            // Target half of our maximum cache size for this cleanup pass.
            const NSUInteger desiredCacheSize = self.config.maxCacheSize / 2;

            // Sort the remaining cache files by their last modification time (oldest first).
            //根据文件创建的时间排序
            NSArray<NSURL *> *sortedFiles = [cacheFiles keysSortedByValueWithOptions:NSSortConcurrent
                                                                     usingComparator:^NSComparisonResult(id obj1, id obj2) {
                                                                         return [obj1[NSURLContentModificationDateKey] compare:obj2[NSURLContentModificationDateKey]];
                                                                     }];

            // Delete files until we fall below our desired cache size.
            /*
             迭代删除缓存，直到缓存大小是默认缓存大小的一半
             */
            for (NSURL *fileURL in sortedFiles) {
                if ([_fileManager removeItemAtURL:fileURL error:nil]) {
                    NSDictionary<NSString *, id> *resourceValues = cacheFiles[fileURL];
                    NSNumber *totalAllocatedSize = resourceValues[NSURLTotalFileAllocatedSizeKey];
                    //总得缓存大小减去当前要删除文件的大小
                    currentCacheSize -= totalAllocatedSize.unsignedIntegerValue;

                    if (currentCacheSize < desiredCacheSize) {
                        break;
                    }
                }
            }
        }
        //执行完毕，主线程回调
        if (completionBlock) {
            dispatch_async(dispatch_get_main_queue(), ^{
                completionBlock();
            });
        }
    });
}

#if SD_UIKIT

/**
 应用进入后台的时候，调用这个方法
 */
- (void)backgroundDeleteOldFiles {
    Class UIApplicationClass = NSClassFromString(@"UIApplication");
    if(!UIApplicationClass || ![UIApplicationClass respondsToSelector:@selector(sharedApplication)]) {
        return;
    }
    UIApplication *application = [UIApplication performSelector:@selector(sharedApplication)];
    //如果backgroundTask对应的时间结束了。任务还么有处理完成。则直接终止任务
    __block UIBackgroundTaskIdentifier bgTask = [application beginBackgroundTaskWithExpirationHandler:^{
        // Clean up any unfinished task business by marking where you
        // stopped or ending the task outright.
        //当任务非正常终止的时候，做清理工作
        [application endBackgroundTask:bgTask];
        bgTask = UIBackgroundTaskInvalid;
    }];

    // Start the long-running task and return immediately.
    //图片清理结束以后。处理完成
    [self deleteOldFilesWithCompletionBlock:^{
        //清理完成以后，终止任务
        [application endBackgroundTask:bgTask];
        bgTask = UIBackgroundTaskInvalid;
    }];
}
#endif

#pragma mark - Cache Info

- (NSUInteger)getSize {
    __block NSUInteger size = 0;
    dispatch_sync(self.ioQueue, ^{
        NSDirectoryEnumerator *fileEnumerator = [_fileManager enumeratorAtPath:self.diskCachePath];
        for (NSString *fileName in fileEnumerator) {
            NSString *filePath = [self.diskCachePath stringByAppendingPathComponent:fileName];
            NSDictionary<NSString *, id> *attrs = [[NSFileManager defaultManager] attributesOfItemAtPath:filePath error:nil];
            size += [attrs fileSize];
        }
    });
    return size;
}

- (NSUInteger)getDiskCount {
    __block NSUInteger count = 0;
    dispatch_sync(self.ioQueue, ^{
        NSDirectoryEnumerator *fileEnumerator = [_fileManager enumeratorAtPath:self.diskCachePath];
        count = fileEnumerator.allObjects.count;
    });
    return count;
}

- (void)calculateSizeWithCompletionBlock:(nullable SDWebImageCalculateSizeBlock)completionBlock {
    NSURL *diskCacheURL = [NSURL fileURLWithPath:self.diskCachePath isDirectory:YES];

    dispatch_async(self.ioQueue, ^{
        NSUInteger fileCount = 0;
        NSUInteger totalSize = 0;

        NSDirectoryEnumerator *fileEnumerator = [_fileManager enumeratorAtURL:diskCacheURL
                                                   includingPropertiesForKeys:@[NSFileSize]
                                                                      options:NSDirectoryEnumerationSkipsHiddenFiles
                                                                 errorHandler:NULL];

        for (NSURL *fileURL in fileEnumerator) {
            NSNumber *fileSize;
            [fileURL getResourceValue:&fileSize forKey:NSURLFileSizeKey error:NULL];
            totalSize += fileSize.unsignedIntegerValue;
            fileCount += 1;
        }

        if (completionBlock) {
            dispatch_async(dispatch_get_main_queue(), ^{
                completionBlock(fileCount, totalSize);
            });
        }
    });
}
@end

最后原文地址,demo地址。