typedef NS_OPTIONS(NSUInteger, SDWebImageOptions) {
    /**
     * By default, when a URL fail to be downloaded, the URL is blacklisted so the library won‘t keep trying.
     * This flag disable this blacklisting.
     */
    //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,

    /**
     * By default, image downloads are started during UI interactions, this flags disable this feature,
     * leading to delayed download on UIScrollView deceleration for instance.
     */
    //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,

    /**
     * This flag disables on-disk caching
     */
    //这个属性禁止磁盘缓存
    SDWebImageCacheMemoryOnly = 1 << 2,

    /**
     * This flag enables progressive download, the image is displayed progressively during download as a browser would do.
     * By default, the image is only displayed once completely downloaded.
     */
    //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,

    /**
     * Even if the image is cached, respect the HTTP response cache control, and refresh the image from remote location if needed.
     * The disk caching will be handled by NSURLCache instead of SDWebImage leading to slight performance degradation.
     * This option helps deal with images changing behind the same request URL, e.g. Facebook graph api profile pics.
     * If a cached image is refreshed, the completion block is called once with the cached image and again with the final image.
     *
     * Use this flag only if you can‘t make your URLs static with embedded cache busting parameter.
     */
    /*
     *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根据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对应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和它对应的内容不能完全对应的时候才使用这个标记。
     */
    SDWebImageRefreshCached = 1 << 4,

    /**
     * In iOS 4+, continue the download of the image if the app goes to background. This is achieved by asking the system for
     * extra time in background to let the request finish. If the background task expires the operation will be cancelled.
     */
    //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,

    /**
     * Handles cookies stored in NSHTTPCookieStore by setting
     * NSMutableURLRequest.HTTPShouldHandleCookies = YES;
     */
    //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,

    /**
     * Enable to allow untrusted SSL certificates.
     * Useful for testing purposes. Use with caution in production.
     */
    //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,

    /**
     * By default, images are loaded in the order in which they were queued. This flag moves them to
     * the front of the queue.
     */
    //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,
    
    /**
     * By default, placeholder images are loaded while the image is loading. This flag will delay the loading
     * of the placeholder image until after the image has finished loading.
     */
    //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,

    /**
     * We usually don‘t call transformDownloadedImage delegate method on animated images,
     * as most transformation code would mangle it.
     * Use this flag to transform them anyway.
     */
    //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,
    
    /**
     * By default, image is added to the imageView after download. But in some cases, we want to
     * have the hand before setting the image (apply a filter or add it with cross-fade animation for instance)
     * Use this flag if you want to manually set the image in the completion when success
     */
    //默认情况下，图片在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,
    
    /**
     * By default, images are decoded respecting their original size. On iOS, this flag will scale down the
     * images to a size compatible with the constrained memory of devices.
     * If `SDWebImageProgressiveDownload` flag is set the scale down is deactivated.
     */
    //默认情况下，图片会按照它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码。
    //如果`SDWebImageProgressiveDownload`标记被设置了，则这个flag不起作用。
    SDWebImageScaleDownLargeImages = 1 << 12
};

/**
 图片下载完成回调Block(外部使用，一般在我们常使用的视图分类方法中用)

 @param image 图片
 @param error 错误信息
 @param cacheType 图片获取方式(本地缓存;内存缓存;网络)
 @param imageURL 图片URL
 */
typedef void(^SDExternalCompletionBlock)(UIImage * _Nullable image, NSError * _Nullable error, SDImageCacheType cacheType, NSURL * _Nullable imageURL);

/**
 图片下载完成回调Block(内部使用，仅在本类中使用)
 
 @param image 图片，如果请求出错，那么image参数为nil
 @param data 图片数据
 @param error 错误信息
 @param cacheType 图片获取方式(本地缓存;内存缓存;网络)
 @param finished 是否下载完成
 @param imageURL 图片URL
 */
typedef void(^SDInternalCompletionBlock)(UIImage * _Nullable image, NSData * _Nullable data, NSError * _Nullable error, SDImageCacheType cacheType, BOOL finished, NSURL * _Nullable imageURL);

/**
 对URL格式做处理，在方法cacheKeyForURL:中使用

 @param url URL
 @return 处理之后的URL
 */
typedef NSString * _Nullable (^SDWebImageCacheKeyFilterBlock)(NSURL * _Nullable url);

@protocol SDWebImageManagerDelegate <NSObject>

@optional

/**
 在缓存中没有找到图片，控制是否去下载图片

 @param imageManager SDWebImageManager
 @param imageURL 图片URL
 @return YES/NO
 */
- (BOOL)imageManager:(nonnull SDWebImageManager *)imageManager shouldDownloadImageForURL:(nullable NSURL *)imageURL;

/**
 图片下载完成，在保存到磁盘和内存之前，对图片进行转换(主要是针对动态图)

 @param imageManager SDWebImageManager
 @param image 图片
 @param imageURL 图片URL
 @return 转换之后的图片
 */
- (nullable UIImage *)imageManager:(nonnull SDWebImageManager *)imageManager transformDownloadedImage:(nullable UIImage *)image withURL:(nullable NSURL *)imageURL;

@end

@interface SDWebImageCombinedOperation : NSObject <SDWebImageOperation>

@property (assign, nonatomic, getter = isCancelled) BOOL cancelled;  //!<用于判断Operation是否已经取消
@property (copy, nonatomic, nullable) SDWebImageNoParamsBlock cancelBlock;  //!<取消回调的Block
@property (strong, nonatomic, nullable) NSOperation *cacheOperation;  //!<NSOperation对象。可以通过这个属性取消一个NSOperation

@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];
    }
}

/**
 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

//.h文件
@property (weak, nonatomic, nullable) id <SDWebImageManagerDelegate> delegate;  //!<代理

@property (strong, nonatomic, readonly, nullable) SDImageCache *imageCache;  //!<缓存对象
@property (strong, nonatomic, readonly, nullable) SDWebImageDownloader *imageDownloader;  //!<下载对象

//.m文件
@property (strong, nonatomic, readwrite, nonnull) SDImageCache *imageCache;  //!<重写为可读写的缓存对象
@property (strong, nonatomic, readwrite, nonnull) SDWebImageDownloader *imageDownloader;  //!<重写为可读写的下载对象
@property (strong, nonatomic, nonnull) NSMutableSet<NSURL *> *failedURLs;  //!<加载失败的URL
@property (strong, nonatomic, nonnull) NSMutableArray<SDWebImageCombinedOperation *> *runningOperations;  //!<当前正在加载的任务

+ (nonnull instancetype)sharedManager {
    static dispatch_once_t once;
    static id instance;
    dispatch_once(&once, ^{
        instance = [self new];
    });
    return instance;
}

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

/**
 初始化

 @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;
}

/**
 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的绝对路径作为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;
    }
}

/**
 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 缓存结果回调
 */
- (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);
        }
    }];
}

/**
 核心方法。UIView、UIImageView等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 获取图片加载处理的属性
 @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 {
    // Invoking this method without a completedBlock is pointless
    //如果想预先下载图片，使用[SDWebImagePrefetcher prefetchURLs]取代本方法。预下载图片是有很多种使用场景的，当我们使用SDWebImagePrefetcher下载图片后，之后使用该图片时就不用从网络上下载了。
    NSAssert(completedBlock != nil, @"If you mean to prefetch the image, use -[SDWebImagePrefetcher prefetchURLs] instead");

    // Very common mistake is to send the URL using NSString object instead of NSURL. For some strange reason, Xcode won‘t
    // throw any warning for this type mismatch. Here we failsafe this error by allowing URLs to be passed as NSString.
    //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不是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。在图片的下载中，会有一些下载失败的情况，这时候会把这些下载失败的url放到一个集合中去，也就是加入了黑名单，默认是不会再继续下载黑名单中的url了，但是也有例外，当options被设置为SDWebImageRetryFailed的时候，会尝试进行重新下载。
    if (url) {
        @synchronized (self.failedURLs) {
            isFailedUrl = [self.failedURLs containsObject:url];
        }
    }
    //如果url长度为0 或者是 失败的url且图片加载处理属性不包含SDWebImageRetryFailed，则直接返回
    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operation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];
    //key为空或者图片从内存中加载，则返回的cacheOperation是nil；其他情况cacheOperation为NSOperation对象
    operation.cacheOperation = [self.imageCache queryCacheOperationForKey:key done:^(UIImage *cachedImage, NSData *cachedData, SDImageCacheType cacheType) {
        //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;
        }
        //(如果未能在缓存中找到图片||强制刷新缓存) && (代理中未实现图片是否下载的方法||代理中图片是否应该下载方法返回值为YES
        if ((!cachedImage || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) {
            //如果从缓存中获取了图片并且设置了SDWebImageRefreshCached来忽略缓存，则先把缓存的图片返回
            if (cachedImage && options & SDWebImageRefreshCached) {
                // If image was found in the cache but SDWebImageRefreshCached is provided, notify about the cached image
                // AND try to re-download it in order to let a chance to NSURLCache to refresh it from server.
                [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) {
                    // Do nothing if the operation was cancelled
                    // See #699 for more details
                    // if we would call the completedBlock, there could be a race condition between this block and another completedBlock for the same object, so if this one is called second, we will overwrite the new data
                } 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 { //网络图片下载成功
                    //如果有重试失败下载的选项，则把url从failedURLS中移除
                    if ((options & SDWebImageRetryFailed)) {
                        @synchronized (self.failedURLs) {
                            [self.failedURLs removeObject:url];
                        }
                    }
                    //是否需要保存到磁盘
                    BOOL cacheOnDisk = !(options & SDWebImageCacheMemoryOnly);
                    //如果设置了强制刷新缓存 && 缓存图片存在 && 下载图片不存在，那么直接返回，不执行回调Block
                    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 && 图片是动态图片 && 实现了imageManager:transformDownloadedImage:withURL:代理方法，则进行图片的处理
                    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，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，则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 {  //图片没有缓存并且图片也没有下载
            // Image not in cache and download disallowed by delegate
            __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;
}

/**
 保存图片到缓存

 @param image 图片
 @param url URL
 */
- (void)saveImageToCache:(nullable UIImage *)image forURL:(nullable NSURL *)url {
    if (image && url) {
        NSString *key = [self cacheKeyForURL:url];
        [self.imageCache storeImage:image forKey:key toDisk:YES completion:nil];
    }
}

/**
 取消所有的下载
 */
- (void)cancelAll {
    @synchronized (self.runningOperations) {
        NSArray<SDWebImageCombinedOperation *> *copiedOperations = [self.runningOperations copy];
        [copiedOperations makeObjectsPerformSelector:@selector(cancel)];
        [self.runningOperations removeObjectsInArray:copiedOperations];
    }
}

/**
 查看是否正在执行任务

 @return YES/NO
 */
- (BOOL)isRunning {
    BOOL isRunning = NO;
    @synchronized (self.runningOperations) {
        isRunning = (self.runningOperations.count > 0);
    }
    return isRunning;
}

/**
 安全移除任务

 @param operation 任务
 */
- (void)safelyRemoveOperationFromRunning:(nullable SDWebImageCombinedOperation*)operation {
    @synchronized (self.runningOperations) {
        if (operation) {
            [self.runningOperations removeObject:operation];
        }
    }
}

/**
 拼接回调Block并进行回调

 @param operation 任务
 @param completionBlock 回调Block
 @param error error
 @param url URL
 */
- (void)callCompletionBlockForOperation:(nullable SDWebImageCombinedOperation*)operation
                             completion:(nullable SDInternalCompletionBlock)completionBlock
                                  error:(nullable NSError *)error
                                    url:(nullable NSURL *)url {
    [self callCompletionBlockForOperation:operation completion:completionBlock image:nil data:nil error:error cacheType:SDImageCacheTypeNone finished:YES url:url];
}

/**
 拼接回调Block并进行回调

 @param operation 任务
 @param completionBlock 回调Block
 @param image 图片
 @param data 图片数据
 @param error error
 @param cacheType 缓存类型
 @param finished 是否完成
 @param url URL
 */
- (void)callCompletionBlockForOperation:(nullable SDWebImageCombinedOperation*)operation
                             completion:(nullable SDInternalCompletionBlock)completionBlock
                                  image:(nullable UIImage *)image
                                   data:(nullable NSData *)data
                                  error:(nullable NSError *)error
                              cacheType:(SDImageCacheType)cacheType
                               finished:(BOOL)finished
                                    url:(nullable NSURL *)url {
    dispatch_main_async_safe(^{
        if (operation && !operation.isCancelled && completionBlock) {
            completionBlock(image, data, error, cacheType, finished, url);
        }
    });
}