写在前面

SDWebImage是一个强大的图片下载库，提供的主要功能有：图片异步下载，图片缓存，图片解码以及其他确保程序健壮性的功能。其Github地址戳这里。

SDWebImage Class Diagram UML类图

SDWebImage官方提供了这个开源库的类图。

SDWebImageClassDiagram.png

SDWebImage Sequence Diagram 流程图

官方提供的流程图如下：

SDWebImageSequenceDiagram.png

清晰明了。

源码分析

SDWebImageManager

SDWebImageManager是封装好的一个单例，通过

+ (nonnull instancetype)sharedManager;

方法获取。
SDWebImageManager是用于支撑WebCache分类（如UIImageView+WebCache）实现的一个类，并且连接了SDWebImageDownloader异步下载器和SDImageCache缓存模块。
因此，在SDWebImageManager中封装了以下几个重要属性：

//代理
@property (weak, nonatomic, nullable) id <SDWebImageManagerDelegate> delegate;
//缓存类
@property (strong, nonatomic, readonly, nullable) SDImageCache *imageCache;
//异步下载器
@property (strong, nonatomic, readonly, nullable) SDWebImageDownloader *imageDownloader;

代理对象中有两个重要方法(但是是@optional的)在下面的代码中将会使用到：

/**
 * 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.
 */
- (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.
 */
- (nullable UIImage *)imageManager:(nonnull SDWebImageManager *)imageManager transformDownloadedImage:(nullable UIImage *)image withURL:(nullable NSURL *)imageURL;

• 第一个方法用于询问代理在查找不到缓存的情况下是否需要根据url下载图片，默认返回YES。如果返回NO，即使缓存未命中，也不执行下载操作。
• 第二个方法用于询问代理是否需要对下载的图像进行transform操作，然后缓存transform之后的图片。如果代理实现了这个方法，则需要返回一张图片。

WebCache分类中的sd_setImageWithURL最终都会调用SDWebImageManager类中的

- (nullable id <SDWebImageOperation>)loadImageWithURL:(nullable NSURL *)url
 options:(SDWebImageOptions)options :(nullable SDWebImageDownloaderProgressBlock) progressBlock completed:(nullable SDInternalCompletionBlock)completedBlock;

进行图片的请求。
代码注释中有对该方法中传入的几个参数做出的说明，其中需要注意传入的SDInternalCompletionBlock 参数的作用。

/**
 * 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 fifth 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
 */

SDWebImageOptions是一个枚举类型，里面存放了一些用户可以自定义的图片下载/缓存选项，在代码中有用到的话再专门解释其含义。

loadImageWithURL()方法

- (nullable id <SDWebImageOperation>)loadImageWithURL:(nullable NSURL *)url
 options:(SDWebImageOptions)options :(nullable SDWebImageDownloaderProgressBlock) progressBlock completed:(nullable SDInternalCompletionBlock)completedBlock{
    // 1. 判断传入的url合法性
    if ([url isKindOfClass:NSString.class]) {
        url = [NSURL URLWithString:(NSString *)url];
    }

    // 将url设置为nil继续执行后续操作，防止程序崩溃
    if (![url isKindOfClass:NSURL.class]) {
        url = nil;
    }
    //初始化operation
    __block SDWebImageCombinedOperation *operation = [SDWebImageCombinedOperation new];
    __weak SDWebImageCombinedOperation *weakOperation = operation;

    BOOL isFailedUrl = NO;
    //2. 判断url是否在failedURLs中
    if (url) {
        @synchronized (self.failedURLs) {// 加了同步锁，保证集合类使用的线程安全
            isFailedUrl = [self.failedURLs containsObject:url];
        }
    }

    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;
    }
    //将operation添加到数组中，任务完成后将被移除（后面会提到）
    @synchronized (self.runningOperations) {
        [self.runningOperations addObject:operation];
    }
    //3. 根据url获取key
    NSString *key = [self cacheKeyForURL:url];
   //4. 请求缓存
    operation.cacheOperation = [self.imageCache queryCacheOperationForKey:key done:^(UIImage *cachedImage, NSData *cachedData, SDImageCacheType cacheType) {
        if (operation.isCancelled) {
            //4.1 操作已取消
            [self safelyRemoveOperationFromRunning:operation];
            return;
        }
        //需要更新已在缓存中的图片 || 未获取到缓存图片
        if ((!cachedImage || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) {
            //4.2 options 中的 SDWebImageRefreshCached位为1 || 未获取到缓存图片
            //用于处理来自同一个url的图片，但是服务器中的图片已经更新，因此需要强制下载并更新缓存图片。
            if (cachedImage && options & SDWebImageRefreshCached) {
                //缓存中已存在旧图片，执行回调通知。
                [self callCompletionBlockForOperation:weakOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
            }
            //继续执行下载任务
            
            SDWebImageDownloaderOptions downloaderOptions = 0;
            //省略初始化downloaderOptions代码
            if (cachedImage && options & SDWebImageRefreshCached) {
                //更新downloaderOptions
                // 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;
            }
            //使用更新的downloaderOptions开启下载图片任务
            SDWebImageDownloadToken *subOperationToken = [self.imageDownloader downloadImageWithURL:url options:downloaderOptions progress:progressBlock completed:^(UIImage *downloadedImage, NSData *downloadedData, NSError *error, BOOL finished) {
                __strong __typeof(weakOperation) strongOperation = weakOperation;
                if (!strongOperation || strongOperation.isCancelled) {
                    // 如果任务被取消，什么都不做
                } else if (error) {
                    //error handling 错误处理
                    /*执行回调将error传出并将url放入failedURLs数组中*/
                    //代码省略
                }
                else {
                    if ((options & SDWebImageRetryFailed)) {
                        /*options的SDWebImageRetryFailed位为1*/
                        /*默认情况下当url无法下载时，会添加到failedURLs数组中防止反复尝试通过该url下载图片，而如果该位为1，则该机制失效。*/
                        @synchronized (self.failedURLs) {
                            [self.failedURLs removeObject:url];
                        }
                    }
                    //是否需要存储在磁盘上                    
                    BOOL cacheOnDisk = !(options & SDWebImageCacheMemoryOnly);

                    if (options & SDWebImageRefreshCached && cachedImage && !downloadedImage) {
                        // Image refresh hit the NSURLCache cache, do not call the completion block
                        //图片刷新命中NSURLCache的情况 && downloadedImage为空
                    } else if (downloadedImage && (!downloadedImage.images || (options & SDWebImageTransformAnimatedImage)) && [self.delegate respondsToSelector:@selector(imageManager:transformDownloadedImage:withURL:)]) {
                        //未命中NSURLCache且downloadedImage不为空
                        //options的SDWebImageTransformAnimatedImage位为1情况
                        //此时说明图片需要执行transform操作
                       dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
                            //Allows to transform the image immediately after it has been downloaded and just before to cache it on disk and memory.
                           //此时调用代理方法获取transformed image
                            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
                                //缓存图片
                                [self.imageCache storeImage:transformedImage imageData:(imageWasTransformed ? nil : downloadedData) forKey:key toDisk:cacheOnDisk completion:nil];
                            }
                            //执行回调
                            [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:transformedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];
                        });
                    } else {


                        if (downloadedImage && finished) {
                            //一般情况：即以上出现的options位为0且获取到下载的图片
                            //根据cacheOnDisk缓存downloadedImage
                            
                            [self.imageCache storeImage:downloadedImage imageData:downloadedData forKey:key toDisk:cacheOnDisk completion:nil];
                        }
                        //执行回调
                        [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:downloadedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];
                    }
                }

                if (finished) {
                    //通过原子操作移除下载
                    [self safelyRemoveOperationFromRunning:strongOperation];
                }
            }];
            //给cancelBlock辅助，注意这段代码不是在这里执行的
            operation.cancelBlock = ^{
                [self.imageDownloader cancel:subOperationToken];
                __strong __typeof(weakOperation) strongOperation = weakOperation;
                [self safelyRemoveOperationFromRunning:strongOperation];
            };
        }else if (cachedImage) {
            //4.3 请求到缓存图片 && !SDWebImageRefreshCached
            //直接回调，移除操作
            __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 {
            // 4.4 图片没有在缓存中 && 代理没有允许下载操作
            __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;
}

方法内部的注释详细描述了SDWebImageManager的loadImageWithURL工作流程。概括起来其实很简单：首先判断缓存中是否能获取到图片，如果没获取到则使用异步下载器下载，然后使用缓存类缓存，执行回调返回图片。如果缓存中能够获取到，执行回调返回缓存中的图片。

下一篇文章将着重分析SDWebImage的缓存实现。