每张图片的下载都有一个Operation操作，操作的创建和加入已在方法

-downloadImageWithURL:options:progress:completed:
-addProgressCallback:completedBlock:forURL:createCallback:

中进行。

操作的初始化方法为：

- (id)initWithRequest:(NSURLRequest *)request
            inSession:(NSURLSession *)session
              options:(SDWebImageDownloaderOptions)options
             progress:(SDWebImageDownloaderProgressBlock)progressBlock
            completed:(SDWebImageDownloaderCompletedBlock)completedBlock
            cancelled:(SDWebImageNoParamsBlock)cancelBlock;

SDWebImageDownloaderOperation遵守 SDWebImageOperation中的协议：

- (void)cancel;

代码注释如下：

/*
 * 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 "SDWebImageDownloaderOperation.h"
#import "SDWebImageDecoder.h"
#import "UIImage+MultiFormat.h"
#import <ImageIO/ImageIO.h>
#import "SDWebImageManager.h"

NSString *const SDWebImageDownloadStartNotification = @"SDWebImageDownloadStartNotification";
NSString *const SDWebImageDownloadReceiveResponseNotification = @"SDWebImageDownloadReceiveResponseNotification";
NSString *const SDWebImageDownloadStopNotification = @"SDWebImageDownloadStopNotification";
NSString *const SDWebImageDownloadFinishNotification = @"SDWebImageDownloadFinishNotification";

@interface SDWebImageDownloaderOperation ()

@property (copy, nonatomic) SDWebImageDownloaderProgressBlock progressBlock;
@property (copy, nonatomic) SDWebImageDownloaderCompletedBlock completedBlock;
@property (copy, nonatomic) SDWebImageNoParamsBlock cancelBlock;

// 是否正在执行
@property (assign, nonatomic, getter = isExecuting) BOOL executing;
// 是否已完成
@property (assign, nonatomic, getter = isFinished) BOOL finished;
// 图片数据
@property (strong, nonatomic) NSMutableData *imageData;

// This is weak because it is injected by whoever manages this session. If this gets nil-ed out, we won't be able to run
// the task associated with this operation
// URL连接会话：因管理此会话的注入所以采用weak修饰。如不为nil，则无法运行此操作关联的任务
@property (weak, nonatomic) NSURLSession *unownedSession;
// This is set if we're using not using an injected NSURLSession. We're responsible of invalidating this one
// URL连接会话：strong修饰，不太理解这两个区别？？？？？？
@property (strong, nonatomic) NSURLSession *ownedSession;
// URL连接会话任务：用于图片下载数据传输。扩展：http://www.cnblogs.com/biosli/p/iOS_Network_URL_Session.html
@property (strong, nonatomic, readwrite) NSURLSessionTask *dataTask;

@property (strong, atomic) NSThread *thread;

#if TARGET_OS_IPHONE && __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_4_0
// app退到后台时，主线程会被暂停。但想在后台完成一个长期任务，须调用UIApplication的beginBackgroundTaskWithExpirationHandler:实例方法,来向 iOS 借点时间
// UIBackgroundTaskIdentifier相当于一个借据，告诉iOS，程序将要借更多的时间来完成Long-Running Task任务
@property (assign, nonatomic) UIBackgroundTaskIdentifier backgroundTaskId;
#endif

@end

@implementation SDWebImageDownloaderOperation {
    size_t width, height; // 图片的宽、高
    UIImageOrientation orientation; // 图片方向
    BOOL responseFromCached; // 是否从缓存中响应
}

@synthesize executing = _executing;
@synthesize finished = _finished;

- (id)initWithRequest:(NSURLRequest *)request
              options:(SDWebImageDownloaderOptions)options
             progress:(SDWebImageDownloaderProgressBlock)progressBlock
            completed:(SDWebImageDownloaderCompletedBlock)completedBlock
            cancelled:(SDWebImageNoParamsBlock)cancelBlock {

    return [self initWithRequest:request
                       inSession:nil
                         options:options
                        progress:progressBlock
                       completed:completedBlock
                       cancelled:cancelBlock];
}

- (id)initWithRequest:(NSURLRequest *)request
            inSession:(NSURLSession *)session
              options:(SDWebImageDownloaderOptions)options
             progress:(SDWebImageDownloaderProgressBlock)progressBlock
            completed:(SDWebImageDownloaderCompletedBlock)completedBlock
            cancelled:(SDWebImageNoParamsBlock)cancelBlock {
    if ((self = [super init])) {
        _request = request;
        _shouldDecompressImages = YES;
        _options = options;
        _progressBlock = [progressBlock copy];
        _completedBlock = [completedBlock copy];
        _cancelBlock = [cancelBlock copy];
        _executing = NO;
        _finished = NO;
        _expectedSize = 0;
        _unownedSession = session;
        responseFromCached = YES; // Initially wrong until `- URLSession:dataTask:willCacheResponse:completionHandler: is called or not called
    }
    return self;
}

/**
 *  复写NSOperation的start方法：没有简单的实现main方法，而是采用更加灵活的start方法，以便自己管理下载的状态，创建了下载所使用的NSURLSession对象
 */
- (void)start {
    // @synchronized 的作用是创建一个互斥锁，保证此时没有其它线程对self对象进行修改。这个是objective-c的一个锁定令牌，防止self对象在同一时间内被其它线程访问，起到线程的保护作用。
    @synchronized (self) {
        // 管理下载状态，如果已取消，则重置当前下载并设置完成状态YES
        if (self.isCancelled) {
            self.finished = YES;
            [self reset];
            return;
        }

#if TARGET_OS_IPHONE && __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_4_0
        Class UIApplicationClass = NSClassFromString(@"UIApplication");
        BOOL hasApplication = UIApplicationClass && [UIApplicationClass respondsToSelector:@selector(sharedApplication)];
        // 是否允许后台执行
        if (hasApplication && [self shouldContinueWhenAppEntersBackground]) {
            __weak __typeof__ (self) wself = self;
            UIApplication * app = [UIApplicationClass performSelector:@selector(sharedApplication)];
            // 标记一个可以在后台长时间运行的后台任务
            self.backgroundTaskId = [app beginBackgroundTaskWithExpirationHandler:^{
                __strong __typeof (wself) sself = wself;

                // 当应用程序留给后台的时间快要结束时（该时间有限），这个block将执行：进行一些清理工作（主线程执行），清理失败会导致程序挂掉
                if (sself) {
                    [sself cancel];

                    // 标记指定的后台任务完成
                    [app endBackgroundTask:sself.backgroundTaskId];
                    // 销毁后台任务标识符
                    sself.backgroundTaskId = UIBackgroundTaskInvalid;
                }
            }];
        }
#endif
        NSURLSession *session = self.unownedSession;
        if (!self.unownedSession) {
            NSURLSessionConfiguration *sessionConfig = [NSURLSessionConfiguration defaultSessionConfiguration];
            sessionConfig.timeoutIntervalForRequest = 15;
            
            /**
             *  Create the session for this task
             *  We send nil as delegate queue so that the session creates a serial operation queue for performing all delegate
             *  method calls and completion handler calls.
             */
            // delegateQueue设置为nil(设置delegate在哪个OperationQueue回调)：以便创建一个用于执行所有委托方法和完成处理程序调用的串行操作队列
            self.ownedSession = [NSURLSession sessionWithConfiguration:sessionConfig
                                                              delegate:self
                                                         delegateQueue:nil];
            session = self.ownedSession;
        }
        
        // 创建数据任务
        self.dataTask = [session dataTaskWithRequest:self.request];
        self.executing = YES;
        self.thread = [NSThread currentThread];
    }
    
    // 启动任务
    [self.dataTask resume];

    if (self.dataTask) {
        if (self.progressBlock) {
            // 刚启动，还不知道图片大小
            self.progressBlock(0, NSURLResponseUnknownLength);
        }
        // 在主线程发送下载开始通知
        dispatch_async(dispatch_get_main_queue(), ^{
            [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStartNotification object:self];
        });
    }
    else {
        if (self.completedBlock) {
            self.completedBlock(nil, nil, [NSError errorWithDomain:NSURLErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Connection can't be initialized"}], YES);
        }
    }

    // 确保后台任务标识符销毁
#if TARGET_OS_IPHONE && __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_4_0
    Class UIApplicationClass = NSClassFromString(@"UIApplication");
    if(!UIApplicationClass || ![UIApplicationClass respondsToSelector:@selector(sharedApplication)]) {
        return;
    }
    if (self.backgroundTaskId != UIBackgroundTaskInvalid) {
        UIApplication * app = [UIApplication performSelector:@selector(sharedApplication)];
        [app endBackgroundTask:self.backgroundTaskId];
        self.backgroundTaskId = UIBackgroundTaskInvalid;
    }
#endif
}

- (void)cancel {
    @synchronized (self) {
        if (self.thread) {
            [self performSelector:@selector(cancelInternalAndStop) onThread:self.thread withObject:nil waitUntilDone:NO];
        }
        else {
            [self cancelInternal];
        }
    }
}

- (void)cancelInternalAndStop {
    if (self.isFinished) return;
    [self cancelInternal];
}

- (void)cancelInternal {
    if (self.isFinished) return;
    [super cancel];
    if (self.cancelBlock) self.cancelBlock();

    if (self.dataTask) {
        [self.dataTask cancel];
        dispatch_async(dispatch_get_main_queue(), ^{
            [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStopNotification object:self];
        });

        // As we cancelled the connection, its callback won't be called and thus won't
        // maintain the isFinished and isExecuting flags.
        if (self.isExecuting) self.executing = NO;
        if (!self.isFinished) self.finished = YES;
    }

    [self reset];
}

- (void)done {
    self.finished = YES;
    self.executing = NO;
    [self reset];
}

/**
 *  重置当前下载
 */
- (void)reset {
    self.cancelBlock = nil;
    self.completedBlock = nil;
    self.progressBlock = nil;
    self.dataTask = nil;
    self.imageData = nil;
    self.thread = nil;
    if (self.ownedSession) {
        [self.ownedSession invalidateAndCancel];
        self.ownedSession = nil;
    }
}

- (void)setFinished:(BOOL)finished {
    [self willChangeValueForKey:@"isFinished"];
    _finished = finished;
    [self didChangeValueForKey:@"isFinished"];
}

- (void)setExecuting:(BOOL)executing {
    [self willChangeValueForKey:@"isExecuting"];
    _executing = executing;
    [self didChangeValueForKey:@"isExecuting"];
}

- (BOOL)isConcurrent {
    return YES;
}

#pragma mark -- NSURLSessionDataDelegate
// 告诉delegate已经接收到服务器的初始应答, 接下来准备数据任务的操作
- (void)URLSession:(NSURLSession *)session
          dataTask:(NSURLSessionDataTask *)dataTask
didReceiveResponse:(NSURLResponse *)response
 completionHandler:(void (^)(NSURLSessionResponseDisposition disposition))completionHandler {
    
    //'304 Not Modified' is an exceptional one
    // HTTP状态码：200+正常成功的；300+重定向；400+请求错误;500+一般时服务端的问题；304 — 未修改，未按预期修改文档。  
    if (![response respondsToSelector:@selector(statusCode)] || ([((NSHTTPURLResponse *)response) statusCode] < 400 && [((NSHTTPURLResponse *)response) statusCode] != 304)) {
        // 预期文件的长度，对于下载来说就是文件的大小
        NSInteger expected = response.expectedContentLength > 0 ? (NSInteger)response.expectedContentLength : 0;
        self.expectedSize = expected;
        if (self.progressBlock) {
            self.progressBlock(0, expected);
        }
        
        self.imageData = [[NSMutableData alloc] initWithCapacity:expected];
        self.response = response;
        dispatch_async(dispatch_get_main_queue(), ^{
            [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadReceiveResponseNotification object:self];
        });
    }
    else {
        NSUInteger code = [((NSHTTPURLResponse *)response) statusCode];
        
        //This is the case when server returns '304 Not Modified'. It means that remote image is not changed.
        //In case of 304 we need just cancel the operation and return cached image from the cache.
        // 返回304状态码时，说明服务器上的图片没有改变。仅需要取消操作，返回缓存中的图片
        if (code == 304) {
            [self cancelInternal];
        } else {
            [self.dataTask cancel];
        }
        dispatch_async(dispatch_get_main_queue(), ^{
            [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStopNotification object:self];
        });
        
        if (self.completedBlock) {
            self.completedBlock(nil, nil, [NSError errorWithDomain:NSURLErrorDomain code:[((NSHTTPURLResponse *)response) statusCode] userInfo:nil], YES);
        }
        [self done];
    }
    
    if (completionHandler) {
        completionHandler(NSURLSessionResponseAllow);
    }
}

// 告诉delegate已经接收到部分数据
- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveData:(NSData *)data {
    // 一张图片的数据是追加到imageData后
    [self.imageData appendData:data];

    if ((self.options & SDWebImageDownloaderProgressiveDownload) && self.expectedSize > 0 && self.completedBlock) {
        // The following code is from http://www.cocoaintheshell.com/2011/05/progressive-images-download-imageio/
        // Thanks to the author @Nyx0uf

        // 已下载图片数据的大小（字节）：Get the total bytes downloaded
        const NSInteger totalSize = self.imageData.length;

        // 更新数据源，须传递所有的数据，而不只是新的字节：Update the data source, we must pass ALL the data, not just the new bytes
        CGImageSourceRef imageSource = CGImageSourceCreateWithData((__bridge CFDataRef)self.imageData, NULL);

        // 首次获得图片的数据时，从图片的数据源中获取图片相关信息：方向、高、宽
        if (width + height == 0) {
            CFDictionaryRef properties = CGImageSourceCopyPropertiesAtIndex(imageSource, 0, NULL);
            if (properties) {
                NSInteger orientationValue = -1;
                
                // 高
                CFTypeRef val = CFDictionaryGetValue(properties, kCGImagePropertyPixelHeight);
                if (val) CFNumberGetValue(val, kCFNumberLongType, &height);
                
                // 宽
                val = CFDictionaryGetValue(properties, kCGImagePropertyPixelWidth);
                if (val) CFNumberGetValue(val, kCFNumberLongType, &width);
                
                // 方向
                val = CFDictionaryGetValue(properties, kCGImagePropertyOrientation);
                if (val) CFNumberGetValue(val, kCFNumberNSIntegerType, &orientationValue);
                
                // CF相关操作注意手动释放
                CFRelease(properties);

                // When we draw to Core Graphics, we lose orientation information,
                // which means the image below born of initWithCGIImage will be
                // oriented incorrectly sometimes. (Unlike the image born of initWithData
                // in didCompleteWithError.) So save it here and pass it on later.
                // 当绘制到Core Graphics时，我们会丢失方向信息，这意味着有时候由initWithCGIImage创建的
                // 图片的方向不对。（不像在connectionDidFinishLoading中用initWithData创建的图片）
                // 所以在这边我们先保存这个信息并在后面使用
                orientation = [[self class] orientationFromPropertyValue:(orientationValue == -1 ? 1 : orientationValue)];
            }

        }

        // 图片还未下载完成
        if (width + height > 0 && totalSize < self.expectedSize) {
            // 使用现有的数据创建图片对象，如果数据中存有多张图片，则取第一张：Create the image
            CGImageRef partialImageRef = CGImageSourceCreateImageAtIndex(imageSource, 0, NULL);

#ifdef TARGET_OS_IPHONE
            // Workaround for iOS anamorphic image
            if (partialImageRef) {
                const size_t partialHeight = CGImageGetHeight(partialImageRef);
                CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
                CGContextRef bmContext = CGBitmapContextCreate(NULL, width, height, 8, width * 4, colorSpace, kCGBitmapByteOrderDefault | kCGImageAlphaPremultipliedFirst);
                CGColorSpaceRelease(colorSpace);
                
                if (bmContext) {
                    CGContextDrawImage(bmContext, (CGRect){.origin.x = 0.0f, .origin.y = 0.0f, .size.width = width, .size.height = partialHeight}, partialImageRef);
                    CGImageRelease(partialImageRef);
                    partialImageRef = CGBitmapContextCreateImage(bmContext);
                    CGContextRelease(bmContext);
                }
                else {
                    CGImageRelease(partialImageRef);
                    partialImageRef = nil;
                }
            }
#endif

            if (partialImageRef) {
                // 对图片进行缩放、解码操作：按道理说这里比较耗资源，为啥不添加自动释放池？
                UIImage *image = [UIImage imageWithCGImage:partialImageRef scale:1 orientation:orientation];
                NSString *key = [[SDWebImageManager sharedManager] cacheKeyForURL:self.request.URL];
                UIImage *scaledImage = [self scaledImageForKey:key image:image];
                if (self.shouldDecompressImages) {
                    image = [UIImage decodedImageWithImage:scaledImage];
                }
                else {
                    image = scaledImage;
                }
                CGImageRelease(partialImageRef);
                // 确保下载完的回调在主线程中执行
                dispatch_main_sync_safe(^{
                    if (self.completedBlock) {
                        self.completedBlock(image, nil, nil, NO);
                    }
                });
            }
        }

        CFRelease(imageSource);
    }

    if (self.progressBlock) {
        self.progressBlock(self.imageData.length, self.expectedSize);
    }
}

// 告诉delegate是否把response存储到缓存中
- (void)URLSession:(NSURLSession *)session
          dataTask:(NSURLSessionDataTask *)dataTask
 willCacheResponse:(NSCachedURLResponse *)proposedResponse
 completionHandler:(void (^)(NSCachedURLResponse *cachedResponse))completionHandler {

    responseFromCached = NO; // 如果这个代理方法被调用，说明缓存中没有对改response进行缓存：If this method is called, it means the response wasn't read from cache
    NSCachedURLResponse *cachedResponse = proposedResponse;

    if (self.request.cachePolicy == NSURLRequestReloadIgnoringLocalCacheData) {
        // Prevents caching of responses
        cachedResponse = nil;
    }
    if (completionHandler) {
        completionHandler(cachedResponse);
    }
}

#pragma mark -- NSURLSessionTaskDelegate
// 告诉delegate，task已经完成：即一张图片已完整下载完这个task完成时，对图片进行处理后回调
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error {
    @synchronized(self) {
        self.thread = nil;
        self.dataTask = nil;
        dispatch_async(dispatch_get_main_queue(), ^{
            [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStopNotification object:self];
            if (!error) {
                [[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadFinishNotification object:self];
            }
        });
    }
    
    if (error) {
        if (self.completedBlock) {
            self.completedBlock(nil, nil, error, YES);
        }
    } else {
        SDWebImageDownloaderCompletedBlock completionBlock = self.completedBlock;
        
        if (![[NSURLCache sharedURLCache] cachedResponseForRequest:_request]) {
            responseFromCached = NO;
        }
        
        if (completionBlock) {
            if (self.options & SDWebImageDownloaderIgnoreCachedResponse && responseFromCached) {
                completionBlock(nil, nil, nil, YES);
            } else if (self.imageData) {
                UIImage *image = [UIImage sd_imageWithData:self.imageData];
                NSString *key = [[SDWebImageManager sharedManager] cacheKeyForURL:self.request.URL];
                image = [self scaledImageForKey:key image:image];
                
                // Gif图片不要解码：Do not force decoding animated GIFs
                if (!image.images) {
                    if (self.shouldDecompressImages) {
                        image = [UIImage decodedImageWithImage:image];
                    }
                }
                if (CGSizeEqualToSize(image.size, CGSizeZero)) {
                    // 空图片
                    completionBlock(nil, nil, [NSError errorWithDomain:SDWebImageErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Downloaded image has 0 pixels"}], YES);
                }
                else {
                    completionBlock(image, self.imageData, nil, YES);
                }
            } else {
                completionBlock(nil, nil, [NSError errorWithDomain:SDWebImageErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Image data is nil"}], YES);
            }
        }
    }
    
    self.completionBlock = nil;
    [self done];
}

// 告诉delegate, task已收到授权：处理服务器返回的证书, 需要在该方法中告诉系统是否需要安装服务器返回的证书
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didReceiveChallenge:(NSURLAuthenticationChallenge *)challenge completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition disposition, NSURLCredential *credential))completionHandler {
    
    NSURLSessionAuthChallengeDisposition disposition = NSURLSessionAuthChallengePerformDefaultHandling;
    __block NSURLCredential *credential = nil;
    
    // 判断服务器返回的证书是否是服务器信任的
    if ([challenge.protectionSpace.authenticationMethod isEqualToString:NSURLAuthenticationMethodServerTrust]) {
        if (!(self.options & SDWebImageDownloaderAllowInvalidSSLCertificates)) {
            // 默认处理方式
            disposition = NSURLSessionAuthChallengePerformDefaultHandling;
        } else {
            // 使用指定证书
            credential = [NSURLCredential credentialForTrust:challenge.protectionSpace.serverTrust];
            disposition = NSURLSessionAuthChallengeUseCredential;
        }
    } else {
        if ([challenge previousFailureCount] == 0) {
            if (self.credential) {
                credential = self.credential;
                disposition = NSURLSessionAuthChallengeUseCredential;
            } else {
                disposition = NSURLSessionAuthChallengeCancelAuthenticationChallenge;
            }
        } else {
            disposition = NSURLSessionAuthChallengeCancelAuthenticationChallenge;
        }
    }
    
    // 安装证书
    if (completionHandler) {
        completionHandler(disposition, credential);
    }
}

#pragma mark Helper methods

+ (UIImageOrientation)orientationFromPropertyValue:(NSInteger)value {
    switch (value) {
        case 1:
            return UIImageOrientationUp;
        case 3:
            return UIImageOrientationDown;
        case 8:
            return UIImageOrientationLeft;
        case 6:
            return UIImageOrientationRight;
        case 2:
            return UIImageOrientationUpMirrored;
        case 4:
            return UIImageOrientationDownMirrored;
        case 5:
            return UIImageOrientationLeftMirrored;
        case 7:
            return UIImageOrientationRightMirrored;
        default:
            return UIImageOrientationUp;
    }
}

/**
 *  放缩图片：根据图片的scale 或 图片组 重新计算返回图片
 *
 *  @param key   key
 *  @param image image
 *
 *  @return UIImage
 */
- (UIImage *)scaledImageForKey:(NSString *)key image:(UIImage *)image {
    return SDScaledImageForKey(key, image);
}

/**
 *  App在后台运行，相关任务是否继续执行
 *
 *  @return <#return value description#>
 */
- (BOOL)shouldContinueWhenAppEntersBackground {
    return self.options & SDWebImageDownloaderContinueInBackground;
}

@end