SDWebImageManager主要关联SDWebImageDownloader和SDImageCache操作，即异步下载图片后进行缓存处理。

SDWebImageOptions说明：

p.p1 {margin: 0.0px 0.0px 0.0px 0.0px; font: 14.0px Menlo; color: #1d9421}p.p2 {margin: 0.0px 0.0px 0.0px 0.0px; font: 14.0px Menlo; min-height: 16.0px}p.p3 {margin: 0.0px 0.0px 0.0px 0.0px; font: 14.0px Menlo; color: #c91b13}p.p4 {margin: 0.0px 0.0px 0.0px 0.0px; font: 14.0px Menlo}p.p5 {margin: 0.0px 0.0px 0.0px 0.0px; font: 14.0px 'PingFang SC'; color: #1d9421}p.p6 {margin: 0.0px 0.0px 0.0px 0.0px; font: 14.0px Menlo; color: #c32275}span.s1 {font-variant-ligatures: no-common-ligatures}span.s2 {font-variant-ligatures: no-common-ligatures; color: #822e0e}span.s3 {font-variant-ligatures: no-common-ligatures; color: #c32275}span.s4 {font: 14.0px Menlo; font-variant-ligatures: no-common-ligatures}span.s5 {font-variant-ligatures: no-common-ligatures; color: #000000}span.s6 {font: 14.0px 'PingFang SC'; font-variant-ligatures: no-common-ligatures}span.s7 {font-variant-ligatures: no-common-ligatures; color: #c91b13}span.s8 {font: 14.0px Menlo; font-variant-ligatures: no-common-ligatures; color: #000000}span.s9 {font-variant-ligatures: no-common-ligatures; color: #0435ff}span.s10 {font-variant-ligatures: no-common-ligatures; color: #1d9421}span.s11 {font: 14.0px 'PingFang SC'; font-variant-ligatures: no-common-ligatures; color: #1d9421}

/*
 * This file is part of the SDWebImage package.
 * (c) Olivier Poitrey <rs@dailymotion.com>
 *
 * For the full copyright and license information, please view the LICENSE
 * file that was distributed with this source code.
 */

#import "SDWebImageManager.h"
#import <objc/message.h>

@interface SDWebImageCombinedOperation : NSObject <SDWebImageOperation>

// 是否取消当前所有操作
@property (assign, nonatomic, getter = isCancelled) BOOL cancelled;
// 没有参数取消回调
@property (copy, nonatomic) SDWebImageNoParamsBlock cancelBlock;
// 执行缓存的操作
@property (strong, nonatomic) NSOperation *cacheOperation;

@end

@interface SDWebImageManager ()

// 图片缓存对象
@property (strong, nonatomic, readwrite) SDImageCache *imageCache;
// 图片下载对象
@property (strong, nonatomic, readwrite) SDWebImageDownloader *imageDownloader;
// 下载失败的图片URL
@property (strong, nonatomic) NSMutableSet *failedURLs;
// 装载正在执行操作的数组
@property (strong, nonatomic) NSMutableArray *runningOperations;

@end

@implementation SDWebImageManager

/**
 *  生成一个SDWebImagemanager的单例
 *
 *  @return <#return value description#>
 */
+ (id)sharedManager {
    static dispatch_once_t once;
    static id instance;
    dispatch_once(&once, ^{
        instance = [self new];
    });
    return instance;
}

/**
 *  初始化SDImageCache、SDWebImageDownloader
 *
 *  @return <#return value description#>
 */
- (instancetype)init {
    SDImageCache *cache = [SDImageCache sharedImageCache];
    SDWebImageDownloader *downloader = [SDWebImageDownloader sharedDownloader];
    return [self initWithCache:cache downloader:downloader];
}

/**
 *  初始化SDImageCache、SDWebImageDownloader
 *
 *  @return <#return value description#>
 */
- (instancetype)initWithCache:(SDImageCache *)cache downloader:(SDWebImageDownloader *)downloader {
    if ((self = [super init])) {
        _imageCache = cache;
        _imageDownloader = downloader;
        _failedURLs = [NSMutableSet new];
        _runningOperations = [NSMutableArray new];
    }
    return self;
}

/**
 *  根绝URL获取缓存中的key
 *
 *  @param url 图片URL
 *
 *  @return 图片在缓存中对应的Key
 */
- (NSString *)cacheKeyForURL:(NSURL *)url {
    if (!url) {
        return @"";
    }
    
    if (self.cacheKeyFilter) {
        return self.cacheKeyFilter(url);
    } else {
        return [url absoluteString];
    }
}

/**
 *  根据URL判断缓存中是否存在图片：先判断内存缓存、在判断磁盘缓存
 *
 *  @param url 图片URL
 *
 *  @return 是否存在图片
 */
- (BOOL)cachedImageExistsForURL:(NSURL *)url {
    NSString *key = [self cacheKeyForURL:url];
    if ([self.imageCache imageFromMemoryCacheForKey:key] != nil) return YES;
    return [self.imageCache diskImageExistsWithKey:key];
}

/**
 *  根据URL判断磁盘缓存中是否存在图片
 *
 *  @param url 图片URL
 *
 *  @return 是否存在图片
 */
- (BOOL)diskImageExistsForURL:(NSURL *)url {
    NSString *key = [self cacheKeyForURL:url];
    return [self.imageCache diskImageExistsWithKey:key];
}

/**
 *  根据URL判断缓存中是否存在图片并进行回调：先判断内存缓存、在判断磁盘缓存
 *
 *  @param url             图片URL
 *  @param completionBlock 图片在缓存中是否存在回调
 */
- (void)cachedImageExistsForURL:(NSURL *)url
                     completion:(SDWebImageCheckCacheCompletionBlock)completionBlock {
    NSString *key = [self cacheKeyForURL:url];
    
    BOOL isInMemoryCache = ([self.imageCache imageFromMemoryCacheForKey:key] != nil);
    
    if (isInMemoryCache) {
        // 确保回调在主线程中执行：making sure we call the completion block on the main queue
        dispatch_async(dispatch_get_main_queue(), ^{
            if (completionBlock) {
                completionBlock(YES);
            }
        });
        return;
    }
    
    // 该方法的回调已在主线程中执行
    [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);
        }
    }];
}

/**
 *  根据URL判断磁盘缓存中是否存在图片并进行回调
 *
 *  @param url             图片URL
 *  @param completionBlock 图片在缓存中是否存在回调
 */
- (void)diskImageExistsForURL:(NSURL *)url
                   completion:(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);
        }
    }];
}

/**
 *  根据URL下载图片
 *
 *  @param url            图片URL
 *  @param options        下载图片选项：SDWebImageOptions
 *  @param progressBlock  图片下载进度回调
 *  @param completedBlock 图片下载完成回调
 *
 *  @return SDWebImageOperation
 */
- (id <SDWebImageOperation>)downloadImageWithURL:(NSURL *)url
                                         options:(SDWebImageOptions)options
                                        progress:(SDWebImageDownloaderProgressBlock)progressBlock
                                       completed:(SDWebImageCompletionWithFinishedBlock)completedBlock {
    // completedBlock为空时，断言将被执行，程序Crash，并打印出断言中的描述信息。Invoking this method without a completedBlock is pointless
    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.
    // 确保传进来的URL类型为NSURL，如果为NSString则进行类型转化
    if ([url isKindOfClass:NSString.class]) {
        url = [NSURL URLWithString:(NSString *)url];
    }

    // Prevents app crashing on argument type error like sending NSNull instead of NSURL
    // 防止URL为空时程序崩溃
    if (![url isKindOfClass:NSURL.class]) {
        url = nil;
    }

    // 执行图片下载操作的集合
    __block SDWebImageCombinedOperation *operation = [SDWebImageCombinedOperation new];
    __weak SDWebImageCombinedOperation *weakOperation = operation;

    // 图片URL是否失败
    BOOL isFailedUrl = NO;
    // 创建一个互斥锁，保证此时没有其它线程对self对象进行修改
    @synchronized (self.failedURLs) {
        isFailedUrl = [self.failedURLs containsObject:url];
    }

    // URL长度为0，或 获取该URL已下载失败过且操作类型没有禁用掉黑名单列表，则该图片就下载失败，返回失败block
    if (url.absoluteString.length == 0 || (!(options & SDWebImageRetryFailed) && isFailedUrl)) {
        // 在主线程中执行回调
        dispatch_main_sync_safe(^{
            NSError *error = [NSError errorWithDomain:NSURLErrorDomain code:NSURLErrorFileDoesNotExist userInfo:nil];
            completedBlock(nil, error, SDImageCacheTypeNone, YES, url);
        });
        return operation;
    }

    // 创建一个互斥锁，保证此时没有其它线程对self对象进行修改
    @synchronized (self.runningOperations) {
        // 将操作加入到当前运行操作的数组中
        [self.runningOperations addObject:operation];
    }
    NSString *key = [self cacheKeyForURL:url];

    // 根据key从缓存里面查找
    operation.cacheOperation = [self.imageCache queryDiskCacheForKey:key done:^(UIImage *image, SDImageCacheType cacheType) {
        // 操作被取消，则从当前运行操作的数组中删除该操作
        if (operation.isCancelled) {
            @synchronized (self.runningOperations) {
                [self.runningOperations removeObject:operation];
            }

            return;
        }

        // 没有图片或操作类型会刷新缓存 且 delegate允许下载图片
        if ((!image || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) {
            // // 有图片 并且操作类型为 SDWebImageRefreshCached（先用缓存图片回调block，图片下载完成后再回调block）
            if (image && options & SDWebImageRefreshCached) {
                dispatch_main_sync_safe(^{
                    // 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.
                    completedBlock(image, nil, cacheType, YES, url);
                });
            }

            // download if no image or requested to refresh anyway, and download allowed by delegate
            // 如果缓存没有图片或请求刷新，并通过委托下载图片，则下载图片
            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 (image && 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;
            }
            // 下载图片
            id <SDWebImageOperation> subOperation = [self.imageDownloader downloadImageWithURL:url options:downloaderOptions progress:progressBlock completed:^(UIImage *downloadedImage, NSData *data, NSError *error, BOOL finished) {
                __strong __typeof(weakOperation) strongOperation = weakOperation;
                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
                    // 操作取消则不做任何处理
                    // 如果这里进行了completedBlock回调，由于对同一个对象是竞争条件，可能会覆盖新数据？？？？？
                }
                else if (error) {
                    // 出错
                    dispatch_main_sync_safe(^{
                        if (strongOperation && !strongOperation.isCancelled) {
                            completedBlock(nil, error, SDImageCacheTypeNone, finished, url);
                        }
                    });

                    if (   error.code != NSURLErrorNotConnectedToInternet
                        && error.code != NSURLErrorCancelled
                        && error.code != NSURLErrorTimedOut
                        && error.code != NSURLErrorInternationalRoamingOff
                        && error.code != NSURLErrorDataNotAllowed
                        && error.code != NSURLErrorCannotFindHost
                        && error.code != NSURLErrorCannotConnectToHost) {
                        // 下载失败图片的URL添加到失败URL数组
                        @synchronized (self.failedURLs) {
                            [self.failedURLs addObject:url];
                        }
                    }
                }
                // 下载成功
                else {
                    //
                    if ((options & SDWebImageRetryFailed)) {
                        // 操作类型为失败重新刷新下载，失败图片的URL添加到失败URL数组
                        @synchronized (self.failedURLs) {
                            [self.failedURLs removeObject:url];
                        }
                    }
                    
                    // 是否存储在磁盘
                    BOOL cacheOnDisk = !(options & SDWebImageCacheMemoryOnly);

                    if (options & SDWebImageRefreshCached && image && !downloadedImage) {
                        // Image refresh hit the NSURLCache cache, do not call the completion block
                    }
                    // 图片下载成功且需要转换图片
                    else if (downloadedImage && (!downloadedImage.images || (options & SDWebImageTransformAnimatedImage)) && [self.delegate respondsToSelector:@selector(imageManager:transformDownloadedImage:withURL:)]) {
                        // 在全局队列中异步进行操作
                        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
                            // 根据代理获取转换后的图片
                            UIImage *transformedImage = [self.delegate imageManager:self transformDownloadedImage:downloadedImage withURL:url];

                            // 转换图片存在 并且下载图片操作已完成 则存储图片
                            if (transformedImage && finished) {
                                BOOL imageWasTransformed = ![transformedImage isEqual:downloadedImage];
                                [self.imageCache storeImage:transformedImage recalculateFromImage:imageWasTransformed imageData:(imageWasTransformed ? nil : data) forKey:key toDisk:cacheOnDisk];
                            }

                            // completedBlock回调
                            dispatch_main_sync_safe(^{
                                if (strongOperation && !strongOperation.isCancelled) {
                                    completedBlock(transformedImage, nil, SDImageCacheTypeNone, finished, url);
                                }
                            });
                        });
                    }
                    // 存储图片并completedBlock回调
                    else {
                        if (downloadedImage && finished) {
                            [self.imageCache storeImage:downloadedImage recalculateFromImage:NO imageData:data forKey:key toDisk:cacheOnDisk];
                        }

                        dispatch_main_sync_safe(^{
                            if (strongOperation && !strongOperation.isCancelled) {
                                completedBlock(downloadedImage, nil, SDImageCacheTypeNone, finished, url);
                            }
                        });
                    }
                }

                // 图片下载完成，则在运行的操作数组中删除该操作
                if (finished) {
                    @synchronized (self.runningOperations) {
                        if (strongOperation) {
                            [self.runningOperations removeObject:strongOperation];
                        }
                    }
                }
            }];
            // 取消操作,从数组中删除该操作
            operation.cancelBlock = ^{
                [subOperation cancel];
                
                @synchronized (self.runningOperations) {
                    __strong __typeof(weakOperation) strongOperation = weakOperation;
                    if (strongOperation) {
                        [self.runningOperations removeObject:strongOperation];
                    }
                }
            };
        }
        // 有图片且线程没有被取消,则返回有图片的completedBlock
        else if (image) {
            dispatch_main_sync_safe(^{
                __strong __typeof(weakOperation) strongOperation = weakOperation;
                if (strongOperation && !strongOperation.isCancelled) {
                    completedBlock(image, nil, cacheType, YES, url);
                }
            });
            @synchronized (self.runningOperations) {
                [self.runningOperations removeObject:operation];
            }
        }
        // 图片没有在缓存且不允许委托方法下载图片
        else {
            // Image not in cache and download disallowed by delegate
            dispatch_main_sync_safe(^{
                __strong __typeof(weakOperation) strongOperation = weakOperation;
                if (strongOperation && !weakOperation.isCancelled) {
                    completedBlock(nil, nil, SDImageCacheTypeNone, YES, url);
                }
            });
            @synchronized (self.runningOperations) {
                [self.runningOperations removeObject:operation];
            }
        }
    }];

    return operation;
}

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

/**
 *  取消掉当前所有的下载图片的operation
 */
- (void)cancelAll {
    @synchronized (self.runningOperations) {
        NSArray *copiedOperations = [self.runningOperations copy];
        [copiedOperations makeObjectsPerformSelector:@selector(cancel)];
        [self.runningOperations removeObjectsInArray:copiedOperations];
    }
}

/**
 *  检查是否有一个或者多个operation正在执行(简单来说就是check是否有图片在下载)
 *
 *  @return <#return value description#>
 */
- (BOOL)isRunning {
    BOOL isRunning = NO;
    @synchronized(self.runningOperations) {
        isRunning = (self.runningOperations.count > 0);
    }
    return isRunning;
}

@end

@implementation SDWebImageCombinedOperation

- (void)setCancelBlock:(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

@implementation SDWebImageManager (Deprecated)

// 废弃的方法：deprecated method, uses the non deprecated method
// adapter for the completion block
- (id <SDWebImageOperation>)downloadWithURL:(NSURL *)url options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletedWithFinishedBlock)completedBlock {
    return [self downloadImageWithURL:url
                              options:options
                             progress:progressBlock
                            completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
                                if (completedBlock) {
                                    completedBlock(image, error, cacheType, finished);
                                }
                            }];
}

@end

SDWebImagePrefecher注释：

p.p1 {margin: 0.0px 0.0px 0.0px 0.0px; font: 14.0px Menlo; color: #1d9421}p.p2 {margin: 0.0px 0.0px 0.0px 0.0px; font: 14.0px Menlo; min-height: 16.0px}p.p3 {margin: 0.0px 0.0px 0.0px 0.0px; font: 14.0px Menlo; color: #c91b13}p.p4 {margin: 0.0px 0.0px 0.0px 0.0px; font: 14.0px Menlo}p.p5 {margin: 0.0px 0.0px 0.0px 0.0px; font: 14.0px 'PingFang SC'; color: #1d9421}p.p6 {margin: 0.0px 0.0px 0.0px 0.0px; font: 14.0px Menlo; color: #c32275}span.s1 {font-variant-ligatures: no-common-ligatures}span.s2 {font-variant-ligatures: no-common-ligatures; color: #822e0e}span.s3 {font-variant-ligatures: no-common-ligatures; color: #c32275}span.s4 {font: 14.0px Menlo; font-variant-ligatures: no-common-ligatures}span.s5 {font: 14.0px 'PingFang SC'; font-variant-ligatures: no-common-ligatures}span.s6 {font-variant-ligatures: no-common-ligatures; color: #0435ff}span.s7 {font-variant-ligatures: no-common-ligatures; color: #000000}span.s8 {font-variant-ligatures: no-common-ligatures; color: #1d9421}span.s9 {font: 14.0px 'PingFang SC'; font-variant-ligatures: no-common-ligatures; color: #1d9421}span.s10 {font: 14.0px Menlo; font-variant-ligatures: no-common-ligatures; color: #000000}

/*
 * This file is part of the SDWebImage package.
 * (c) Olivier Poitrey <rs@dailymotion.com>
 *
 * For the full copyright and license information, please view the LICENSE
 * file that was distributed with this source code.
 */

#import "SDWebImagePrefetcher.h"

@interface SDWebImagePrefetcher ()

@property (strong, nonatomic) SDWebImageManager *manager;
// 预取图片地址数组
@property (strong, nonatomic) NSArray *prefetchURLs;
// 请求数
@property (assign, nonatomic) NSUInteger requestedCount;
@property (assign, nonatomic) NSUInteger skippedCount;
// 完成数
@property (assign, nonatomic) NSUInteger finishedCount;
@property (assign, nonatomic) NSTimeInterval startedTime;
@property (copy, nonatomic) SDWebImagePrefetcherCompletionBlock completionBlock;
@property (copy, nonatomic) SDWebImagePrefetcherProgressBlock progressBlock;

@end

@implementation SDWebImagePrefetcher

+ (SDWebImagePrefetcher *)sharedImagePrefetcher {
    static dispatch_once_t once;
    static id instance;
    dispatch_once(&once, ^{
        instance = [self new];
    });
    return instance;
}

- (id)init {
    return [self initWithImageManager:[SDWebImageManager new]];
}

- (id)initWithImageManager:(SDWebImageManager *)manager {
    if ((self = [super init])) {
        _manager = manager;
        _options = SDWebImageLowPriority;
        _prefetcherQueue = dispatch_get_main_queue();
        self.maxConcurrentDownloads = 3;
    }
    return self;
}

- (void)setMaxConcurrentDownloads:(NSUInteger)maxConcurrentDownloads {
    self.manager.imageDownloader.maxConcurrentDownloads = maxConcurrentDownloads;
}

- (NSUInteger)maxConcurrentDownloads {
    return self.manager.imageDownloader.maxConcurrentDownloads;
}

/**
 *  开始预取第几张图片
 *
 *  @param index 图片位置
 */
- (void)startPrefetchingAtIndex:(NSUInteger)index {
    if (index >= self.prefetchURLs.count) return;
    
    // 请求数＋1
    self.requestedCount++;
    [self.manager downloadImageWithURL:self.prefetchURLs[index] options:self.options progress:nil completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
        if (!finished) return;
        // 完成数＋1
        self.finishedCount++;

        if (image) {
            if (self.progressBlock) {
                self.progressBlock(self.finishedCount,[self.prefetchURLs count]);
            }
        }
        else {
            if (self.progressBlock) {
                self.progressBlock(self.finishedCount,[self.prefetchURLs count]);
            }
            // 图片没有下载完，跳过数＋1:Add last failed
            self.skippedCount++;
        }
        if ([self.delegate respondsToSelector:@selector(imagePrefetcher:didPrefetchURL:finishedCount:totalCount:)]) {
            [self.delegate imagePrefetcher:self
                            didPrefetchURL:self.prefetchURLs[index]
                             finishedCount:self.finishedCount
                                totalCount:self.prefetchURLs.count
             ];
        }
        if (self.prefetchURLs.count > self.requestedCount) {
            // 预取图片数 > 请求数，继续预取图片
            dispatch_async(self.prefetcherQueue, ^{
                [self startPrefetchingAtIndex:self.requestedCount];
            });
        } else if (self.finishedCount == self.requestedCount) {
            // 完成数 ＝ 请求数，该图片下载完成，进行回调
            [self reportStatus]; // 报告图片完成状态
            if (self.completionBlock) {
                self.completionBlock(self.finishedCount, self.skippedCount);
                self.completionBlock = nil;
            }
            self.progressBlock = nil;
        }
    }];
}

/**
 *  报告预取图片完成状态
 */
- (void)reportStatus {
    NSUInteger total = [self.prefetchURLs count];
    if ([self.delegate respondsToSelector:@selector(imagePrefetcher:didFinishWithTotalCount:skippedCount:)]) {
        [self.delegate imagePrefetcher:self
               didFinishWithTotalCount:(total - self.skippedCount)
                          skippedCount:self.skippedCount
         ];
    }
}

/**
 *  根据图片地址数组预取图片
 *
 *  @param urls 图片地址数组
 */
- (void)prefetchURLs:(NSArray *)urls {
    [self prefetchURLs:urls progress:nil completed:nil];
}

- (void)prefetchURLs:(NSArray *)urls progress:(SDWebImagePrefetcherProgressBlock)progressBlock completed:(SDWebImagePrefetcherCompletionBlock)completionBlock {
    [self cancelPrefetching]; // 先取消图片预取，防止重复操作：Prevent duplicate prefetch request
    self.startedTime = CFAbsoluteTimeGetCurrent();
    self.prefetchURLs = urls;
    self.completionBlock = completionBlock;
    self.progressBlock = progressBlock;

    if (urls.count == 0) {
        if (completionBlock) {
            completionBlock(0,0);
        }
    } else {
        // 循环预取图片：Starts prefetching from the very first image on the list with the max allowed concurrency
        NSUInteger listCount = self.prefetchURLs.count;
        for (NSUInteger i = 0; i < self.maxConcurrentDownloads && self.requestedCount < listCount; i++) {
            [self startPrefetchingAtIndex:i];
        }
    }
}

/**
 *  取消图片预取
 */
- (void)cancelPrefetching {
    self.prefetchURLs = nil;
    self.skippedCount = 0;
    self.requestedCount = 0;
    self.finishedCount = 0;
    [self.manager cancelAll];
}

@end