SDWebImage是一个非常著名的iOS处理异步加载图片库，现在90%的iOS项目里都有它的身影。为此，本着提高技术的目的，解读SDWebImage

SDWebImageManager

• SDWebImageManager是UIImageView+WebCache后台工作的类
• 它是异步下载器(SDWebImageDownloader)和图片缓存存储(SDImageCache)之间的纽带
• 可以之间使用此类实现网络图片的下载

SDWebImageManager.h

#import "SDWebImageCompat.h"
#import "SDWebImageOperation.h"
#import "SDWebImageDownloader.h"
#import "SDImageCache.h"

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.
     */
    SDWebImageRetryFailed = 1 << 0, //失败之后尝试重修下载

    /**
     * By default, image downloads are started during UI interactions, this flags disable this feature,
     * leading to delayed download on UIScrollView deceleration for instance.
     */
    SDWebImageLowPriority = 1 << 1, //低优先级 图片下载推迟到滚动视图停止滚动之后

    /**
     * This flag disables on-disk caching
     */
    SDWebImageCacheMemoryOnly = 1 << 2, //只使用内存缓存，禁止磁盘缓存

    /**
     * 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.
     */
    SDWebImageProgressiveDownload = 1 << 3, //渐进式下载（图片逐步显示出来）

    /**
     * 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.
     */
    SDWebImageRefreshCached = 1 << 4,//刷新缓存 用于处理url指向图片发生变化，如果缓存的图片被刷新，会调用一次completion block，然后传递最后的图片

    /**
     * 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.
     */
    SDWebImageContinueInBackground = 1 << 5, // 后台下载 app进入后台时仍继续下载（如果后台任务过期，请求将被取消）

    /**
     * Handles cookies stored in NSHTTPCookieStore by setting
     * NSMutableURLRequest.HTTPShouldHandleCookies = YES;
     */
    SDWebImageHandleCookies = 1 << 6, //处理保存在NSHTTPCookieStore中的cookies

    /**
     * Enable to allow untrusted SSL certificates.
     * Useful for testing purposes. Use with caution in production.
     */
    SDWebImageAllowInvalidSSLCertificates = 1 << 7, //允许不信任的SSL证书

    /**
     * By default, images are loaded in the order in which they were queued. This flag moves them to
     * the front of the queue.
     */
    SDWebImageHighPriority = 1 << 8, //高优先级（正常情况下图片会按队列中的顺序下载，此选项直接把它们移动到队列最前端立即开始下载    ）
    
    /**
     * 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.
     */
    SDWebImageDelayPlaceholder = 1 << 9, // 延迟占位图片

    /**
     * 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.
     */
    SDWebImageTransformAnimatedImage = 1 << 10, //转换动画图片
    
    /**
     * 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
     */
    SDWebImageAvoidAutoSetImage = 1 << 11, //下载完成后手动设置图片（默认是自动放到ImageView上）
    
    /**
     * 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.
     */
    SDWebImageScaleDownLargeImages = 1 << 12 //缩小图片('SDWebImageProgressiveDownload'标志会禁止自动缩小图片)
};

typedef void(^SDExternalCompletionBlock)(UIImage * _Nullable image, NSError * _Nullable error, SDImageCacheType cacheType, NSURL * _Nullable imageURL);

typedef void(^SDInternalCompletionBlock)(UIImage * _Nullable image, NSData * _Nullable data, NSError * _Nullable error, SDImageCacheType cacheType, BOOL finished, NSURL * _Nullable imageURL);

typedef NSString * _Nullable (^SDWebImageCacheKeyFilterBlock)(NSURL * _Nullable url);


@class SDWebImageManager;

@protocol SDWebImageManagerDelegate <NSObject>

@optional

/**
 * Controls which image should be downloaded when the image is not found in the cache.
 *
 * @param imageManager The current `SDWebImageManager`
 * @param imageURL     The url of the image to be downloaded
 *
 * @return Return NO to prevent the downloading of the image on cache misses. If not implemented, YES is implied.
 */

//如果图片没有缓存，则下载图片 返回是否有缓存
- (BOOL)imageManager:(nonnull SDWebImageManager *)imageManager shouldDownloadImageForURL:(nullable NSURL *)imageURL;

/**
 * Allows to transform the image immediately after it has been downloaded and just before to cache it on disk and memory.
 * NOTE: This method is called from a global queue in order to not to block the main thread.
 *
 * @param imageManager The current `SDWebImageManager`
 * @param image        The image to transform
 * @param imageURL     The url of the image to transform
 *
 * @return The transformed image object.
 */
//允许在下载之后立即转换图片，然后再进行内存和磁盘缓存 返回转换后的图片对象
- (nullable UIImage *)imageManager:(nonnull SDWebImageManager *)imageManager transformDownloadedImage:(nullable UIImage *)image withURL:(nullable NSURL *)imageURL;

@end

/**
 * The SDWebImageManager is the class behind the UIImageView+WebCache category and likes.
 * It ties the asynchronous downloader (SDWebImageDownloader) with the image cache store (SDImageCache).
 * You can use this class directly to benefit from web image downloading with caching in another context than
 * a UIView.
 *
 * Here is a simple example of how to use SDWebImageManager:
 *
 * @code

SDWebImageManager *manager = [SDWebImageManager sharedManager];
[manager loadImageWithURL:imageURL
                  options:0
                 progress:nil
                completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
                    if (image) {
                        // do something with image
                    }
                }];

 * @endcode
 */


//

@interface SDWebImageManager : NSObject

@property (weak, nonatomic, nullable) id <SDWebImageManagerDelegate> delegate;

@property (strong, nonatomic, readonly, nullable) SDImageCache *imageCache; //缓存类
@property (strong, nonatomic, readonly, nullable) SDWebImageDownloader *imageDownloader; //图片下载工具类

/**
 * The cache filter is a block used each time SDWebImageManager need to convert an URL into a cache key. This can
 * be used to remove dynamic part of an image URL.
 *
 * The following example sets a filter in the application delegate that will remove any query-string from the
 * URL before to use it as a cache key:
 *
 * @code

[[SDWebImageManager sharedManager] setCacheKeyFilter:^(NSURL *url) {
    url = [[NSURL alloc] initWithScheme:url.scheme host:url.host path:url.path];
    return [url absoluteString];
}];

 * @endcode
 */
@property (nonatomic, copy, nullable) SDWebImageCacheKeyFilterBlock cacheKeyFilter; //缓存过滤器

/**
 * Returns global SDWebImageManager instance.
 *
 * @return SDWebImageManager shared instance
 */
//单例
+ (nonnull instancetype)sharedManager;

/**
 * Allows to specify instance of cache and image downloader used with image manager.
 * @return new instance of `SDWebImageManager` with specified cache and downloader.
 */
//根据SDImageCache和SDWebImageDownloader来创建一个SDWebImageManager实例
- (nonnull instancetype)initWithCache:(nonnull SDImageCache *)cache downloader:(nonnull SDWebImageDownloader *)downloader NS_DESIGNATED_INITIALIZER;

/**
 * Downloads the image at the given URL if not present in cache or return the cached version otherwise.
 *
 * @param url            The URL to the image
 * @param options        A mask to specify options to use for this request
 * @param progressBlock  A block called while image is downloading
 *                       @note the progress block is executed on a background queue
 * @param completedBlock A block called when operation has been completed.
 *
 *   This parameter is required.
 * 
 *   This block has no return value and takes the requested UIImage as first parameter and the NSData representation as second parameter.
 *   In case of error the image parameter is nil and the third parameter may contain an NSError.
 *
 *   The forth parameter is an `SDImageCacheType` enum indicating if the image was retrieved from the local cache
 *   or from the memory cache or from the network.
 *
 *   The fith parameter is set to NO when the SDWebImageProgressiveDownload option is used and the image is
 *   downloading. This block is thus called repeatedly with a partial image. When image is fully downloaded, the
 *   block is called a last time with the full image and the last parameter set to YES.
 *
 *   The last parameter is the original image URL
 *
 * @return Returns an NSObject conforming to SDWebImageOperation. Should be an instance of SDWebImageDownloaderOperation
 */

//如果URL对应的图像在缓存中不存在，那么就下载指定的图片 ，否则返回缓存的图像
//***主方法***
- (nullable id <SDWebImageOperation>)loadImageWithURL:(nullable NSURL *)url
                                              options:(SDWebImageOptions)options
                                             progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
                                            completed:(nullable SDInternalCompletionBlock)completedBlock;

/**
 * Saves image to cache for given URL
 *
 * @param image The image to cache
 * @param url   The URL to the image
 *
 */
//根据图片url保存图片到缓存
- (void)saveImageToCache:(nullable UIImage *)image forURL:(nullable NSURL *)url;

/**
 * Cancel all current operations
 */
//取消所有操作
- (void)cancelAll;

/**
 * Check one or more operations running
 */
//检查一个或多个操作是否在进行
- (BOOL)isRunning;

/**
 *  Async check if image has already been cached
 *
 *  @param url              image url
 *  @param completionBlock  the block to be executed when the check is finished
 *  
 *  @note the completion block is always executed on the main queue
 */
//检查图片是否在内存缓存中
- (void)cachedImageExistsForURL:(nullable NSURL *)url
                     completion:(nullable SDWebImageCheckCacheCompletionBlock)completionBlock;

/**
 *  Async check if image has already been cached on disk only
 *
 *  @param url              image url
 *  @param completionBlock  the block to be executed when the check is finished
 *
 *  @note the completion block is always executed on the main queue
 */
//检查图片是否在磁盘缓存中
- (void)diskImageExistsForURL:(nullable NSURL *)url
                   completion:(nullable SDWebImageCheckCacheCompletionBlock)completionBlock;

/**
 *Return the cache key for a given URL
 */
//返回图片的cacheKey
- (nullable NSString *)cacheKeyForURL:(nullable NSURL *)url;

@end

SDWebImageManager.m

#import "SDWebImageManager.h"
#import <objc/message.h>
#import "NSImage+WebCache.h"
//内部类
@interface SDWebImageCombinedOperation : NSObject <SDWebImageOperation>

@property (assign, nonatomic, getter = isCancelled) BOOL cancelled; //取消
@property (copy, nonatomic, nullable) SDWebImageNoParamsBlock cancelBlock; //取消的回调
@property (strong, nonatomic, nullable) NSOperation *cacheOperation; //处理缓存的操作

@end

@interface SDWebImageManager ()

@property (strong, nonatomic, readwrite, nonnull) SDImageCache *imageCache; //缓存类
@property (strong, nonatomic, readwrite, nonnull) SDWebImageDownloader *imageDownloader; //下载类
@property (strong, nonatomic, nonnull) NSMutableSet<NSURL *> *failedURLs; //url黑名单
@property (strong, nonatomic, nonnull) NSMutableArray<SDWebImageCombinedOperation *> *runningOperations; //当前正在执行的操作数组

@end

@implementation SDWebImageManager
//单例
+ (nonnull instancetype)sharedManager {
    static dispatch_once_t once;
    static id instance;
    dispatch_once(&once, ^{
        instance = [self new];
    });
    return instance;
}
//初始化
- (nonnull instancetype)init {
    SDImageCache *cache = [SDImageCache sharedImageCache];
    SDWebImageDownloader *downloader = [SDWebImageDownloader sharedDownloader];
    return [self initWithCache:cache downloader:downloader];
}

- (nonnull instancetype)initWithCache:(nonnull SDImageCache *)cache downloader:(nonnull SDWebImageDownloader *)downloader {
    if ((self = [super init])) {
        _imageCache = cache;
        _imageDownloader = downloader;
        _failedURLs = [NSMutableSet new];
        _runningOperations = [NSMutableArray new];
    }
    return self;
}

- (nullable NSString *)cacheKeyForURL:(nullable NSURL *)url {
    if (!url) {
        return @"";
    }
    //如果设置了缓存过滤器，走缓存过滤器处理url生成key，或者直接返回url的string当做key
    if (self.cacheKeyFilter) {
        return self.cacheKeyFilter(url);
    } else {
        return url.absoluteString;
    }
}

- (void)cachedImageExistsForURL:(nullable NSURL *)url
                     completion:(nullable SDWebImageCheckCacheCompletionBlock)completionBlock {
    NSString *key = [self cacheKeyForURL:url];
    //图片是否在内存缓存里
    BOOL isInMemoryCache = ([self.imageCache imageFromMemoryCacheForKey:key] != nil);
    
    if (isInMemoryCache) {
        // making sure we call the completion block on the main queue
        dispatch_async(dispatch_get_main_queue(), ^{
            if (completionBlock) {
                completionBlock(YES);
            }
        });
        return;
    }
    //图片是否在磁盘缓存里
    [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);
        }
    }];
}
//检测一张图片是否被磁盘缓存
- (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);
        }
    }];
}
//如果url对应的key找不到缓存，下载图片，否则返回缓存的图像  （主方法）
- (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
    //当没有completedBlock时，调用这个方法是无意义的
    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.
    //判断url是否正确，如果是string转换成url
    if ([url isKindOfClass:NSString.class]) {
        url = [NSURL URLWithString:(NSString *)url];
    }

    // Prevents app crashing on argument type error like sending NSNull instead of NSURL
    //错误处理
    if (![url isKindOfClass:NSURL.class]) {
        url = nil;
    }
    //初始化一个SDWebImageCombinedOperation块
    __block SDWebImageCombinedOperation *operation = [SDWebImageCombinedOperation new];
    __weak SDWebImageCombinedOperation *weakOperation = operation;

    BOOL isFailedUrl = NO;  //初始化url失败标志=0
    //查找url是否在黑名单内
    if (url) {
        @synchronized (self.failedURLs) {
            isFailedUrl = [self.failedURLs containsObject:url];
        }
    }
    //回调传递错误信息
    if (url.absoluteString.length == 0 || (!(options & SDWebImageRetryFailed) && isFailedUrl)) {
        [self callCompletionBlockForOperation:operation completion:completedBlock error:[NSError errorWithDomain:NSURLErrorDomain code:NSURLErrorFileDoesNotExist userInfo:nil] url:url];
        return operation;
    }

    //把当前下载任务添加进任务数组中
    @synchronized (self.runningOperations) {
        [self.runningOperations addObject:operation];
    }
    NSString *key = [self cacheKeyForURL:url];

    //查找key对应的图片缓存是否存在,查找完把图片和图片缓存方法通过block传回来
    operation.cacheOperation = [self.imageCache queryCacheOperationForKey:key done:^(UIImage *cachedImage, NSData *cachedData, SDImageCacheType cacheType) {
        //判断任务是否取消了
        if (operation.isCancelled) {
            [self safelyRemoveOperationFromRunning:operation];
            return;
        }
        //(缓存图片不存在||下载策略是刷新缓存) && (delegate不能响应 || 图片存在缓存)
        if ((!cachedImage || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) {
            
            //处理下载策略
            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
            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;
            
            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;
            }
            
            //使用SDWebImageDownloader下载图片
            SDWebImageDownloadToken *subOperationToken = [self.imageDownloader downloadImageWithURL:url options:downloaderOptions progress:progressBlock completed:^(UIImage *downloadedImage, NSData *downloadedData, NSError *error, BOOL finished) {
                __strong __typeof(weakOperation) strongOperation = weakOperation;
                //如果被取消了
                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
                }
                //如果error
                else if (error) {
                    [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];    //加入到黑名单
                        }
                    }
                }
                else {//下载成功
                    if ((options & SDWebImageRetryFailed)) {
                        @synchronized (self.failedURLs) {
                            [self.failedURLs removeObject:url];
                        }
                    }
                    
                    BOOL cacheOnDisk = !(options & SDWebImageCacheMemoryOnly); //是否要进行磁盘缓存

                    if (options & SDWebImageRefreshCached && cachedImage && !downloadedImage) {
                        // Image refresh hit the NSURLCache cache, do not call the completion block
                        //下载成功且是动画图片且实现了imageManager:transformDownloadedImage:withURL:
                    } 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), ^{
                            //转换图片
                            UIImage *transformedImage = [self.delegate imageManager:self transformDownloadedImage:downloadedImage withURL:url];

                            if (transformedImage && finished) {
                                BOOL imageWasTransformed = ![transformedImage isEqual:downloadedImage];
                                // pass nil if the image was transformed, so we can recalculate the data from the image
                                //图片转换成功后缓存
                                [self.imageCache storeImage:transformedImage imageData:(imageWasTransformed ? nil : downloadedData) forKey:key toDisk:cacheOnDisk completion:nil];
                            }
                            
                            [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:transformedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];
                        });
                    } else {
                        //下载成功,缓存图片
                        if (downloadedImage && finished) {
                            [self.imageCache storeImage:downloadedImage imageData:downloadedData forKey:key toDisk:cacheOnDisk completion:nil];
                        }
                        [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:downloadedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];
                    }
                }

                if (finished) { //从任务数组中移除任务
                    [self safelyRemoveOperationFromRunning:strongOperation];
                }
            }];
            //取消
            operation.cancelBlock = ^{
                [self.imageDownloader cancel:subOperationToken];
                __strong __typeof(weakOperation) strongOperation = weakOperation;
                [self safelyRemoveOperationFromRunning:strongOperation];
            };
            //缓存存在
        } else if (cachedImage) {
            __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 {
            //图片没下载和在缓存找到
            // 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;
}

- (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];
    }
}

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

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

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

- (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];
}
//保证回调在主线程
- (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);
        }
    });
}

@end


@implementation SDWebImageCombinedOperation

- (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];
    }
}

- (void)cancel {
    self.cancelled = YES;
    if (self.cacheOperation) {
        [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