寫在前面

SDWebImage是一個強大的圖片下載庫，提供的主要功能有：圖片異步下載，圖片緩存，圖片解碼以及其他確保程序健壯性的功能。其Github地址戳這里。

SDWebImage Class Diagram UML類圖

SDWebImage官方提供了這個開源庫的類圖。

SDWebImageClassDiagram.png

SDWebImage Sequence Diagram 流程圖

官方提供的流程圖如下：

SDWebImageSequenceDiagram.png

清晰明了。

源碼分析

SDWebImageManager

SDWebImageManager是封裝好的一個單例，通過

+ (nonnull instancetype)sharedManager;

方法獲取。
SDWebImageManager是用于支撐WebCache分類（如UIImageView+WebCache）實現的一個類，并且連接了SDWebImageDownloader異步下載器和SDImageCache緩存模塊。
因此，在SDWebImageManager中封裝了以下幾個重要屬性：

//代理
@property (weak, nonatomic, nullable) id <SDWebImageManagerDelegate> delegate;
//緩存類
@property (strong, nonatomic, readonly, nullable) SDImageCache *imageCache;
//異步下載器
@property (strong, nonatomic, readonly, nullable) SDWebImageDownloader *imageDownloader;

代理對象中有兩個重要方法(但是是@optional的)在下面的代碼中將會使用到：

/**
 * 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;

• 第一個方法用于詢問代理在查找不到緩存的情況下是否需要根據url下載圖片，默認返回YES。如果返回NO，即使緩存未命中，也不執行下載操作。
• 第二個方法用于詢問代理是否需要對下載的圖像進行transform操作，然后緩存transform之后的圖片。如果代理實現了這個方法，則需要返回一張圖片。

WebCache分類中的sd_setImageWithURL最終都會調用SDWebImageManager類中的

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

進行圖片的請求。
代碼注釋中有對該方法中傳入的幾個參數做出的說明，其中需要注意傳入的SDInternalCompletionBlock 參數的作用。

/**
 * 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是一個枚舉類型，里面存放了一些用戶可以自定義的圖片下載/緩存選項，在代碼中有用到的話再專門解釋其含義。

loadImageWithURL()方法

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

    // 將url設置為nil繼續執行后續操作，防止程序崩潰
    if (![url isKindOfClass:NSURL.class]) {
        url = nil;
    }
    //初始化operation
    __block SDWebImageCombinedOperation *operation = [SDWebImageCombinedOperation new];
    __weak SDWebImageCombinedOperation *weakOperation = operation;

    BOOL isFailedUrl = NO;
    //2. 判斷url是否在failedURLs中
    if (url) {
        @synchronized (self.failedURLs) {// 加了同步鎖，保證集合類使用的線程安全
            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;
    }
    //將operation添加到數組中，任務完成后將被移除（后面會提到）
    @synchronized (self.runningOperations) {
        [self.runningOperations addObject:operation];
    }
    //3. 根據url獲取key
    NSString *key = [self cacheKeyForURL:url];
   //4. 請求緩存
    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;
        }
        //需要更新已在緩存中的圖片 || 未獲取到緩存圖片
        if ((!cachedImage || options & SDWebImageRefreshCached) && (![self.delegate respondsToSelector:@selector(imageManager:shouldDownloadImageForURL:)] || [self.delegate imageManager:self shouldDownloadImageForURL:url])) {
            //4.2 options 中的 SDWebImageRefreshCached位為1 || 未獲取到緩存圖片
            //用于處理來自同一個url的圖片，但是服務器中的圖片已經更新，因此需要強制下載并更新緩存圖片。
            if (cachedImage && options & SDWebImageRefreshCached) {
                //緩存中已存在舊圖片，執行回調通知。
                [self callCompletionBlockForOperation:weakOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
            }
            //繼續執行下載任務
            
            SDWebImageDownloaderOptions downloaderOptions = 0;
            //省略初始化downloaderOptions代碼
            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開啟下載圖片任務
            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) {
                    // 如果任務被取消，什么都不做
                } else if (error) {
                    //error handling 錯誤處理
                    /*執行回調將error傳出并將url放入failedURLs數組中*/
                    //代碼省略
                }
                else {
                    if ((options & SDWebImageRetryFailed)) {
                        /*options的SDWebImageRetryFailed位為1*/
                        /*默認情況下當url無法下載時，會添加到failedURLs數組中防止反復嘗試通過該url下載圖片，而如果該位為1，則該機制失效。*/
                        @synchronized (self.failedURLs) {
                            [self.failedURLs removeObject:url];
                        }
                    }
                    //是否需要存儲在磁盤上                    
                    BOOL cacheOnDisk = !(options & SDWebImageCacheMemoryOnly);

                    if (options & SDWebImageRefreshCached && cachedImage && !downloadedImage) {
                        // Image refresh hit the NSURLCache cache, do not call the completion block
                        //圖片刷新命中NSURLCache的情況 && downloadedImage為空
                    } else if (downloadedImage && (!downloadedImage.images || (options & SDWebImageTransformAnimatedImage)) && [self.delegate respondsToSelector:@selector(imageManager:transformDownloadedImage:withURL:)]) {
                        //未命中NSURLCache且downloadedImage不為空
                        //options的SDWebImageTransformAnimatedImage位為1情況
                        //此時說明圖片需要執行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.
                           //此時調用代理方法獲取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
                                //緩存圖片
                                [self.imageCache storeImage:transformedImage imageData:(imageWasTransformed ? nil : downloadedData) forKey:key toDisk:cacheOnDisk completion:nil];
                            }
                            //執行回調
                            [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:transformedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];
                        });
                    } else {


                        if (downloadedImage && finished) {
                            //一般情況：即以上出現的options位為0且獲取到下載的圖片
                            //根據cacheOnDisk緩存downloadedImage
                            
                            [self.imageCache storeImage:downloadedImage imageData:downloadedData forKey:key toDisk:cacheOnDisk completion:nil];
                        }
                        //執行回調
                        [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:downloadedImage data:downloadedData error:nil cacheType:SDImageCacheTypeNone finished:finished url:url];
                    }
                }

                if (finished) {
                    //通過原子操作移除下載
                    [self safelyRemoveOperationFromRunning:strongOperation];
                }
            }];
            //給cancelBlock輔助，注意這段代碼不是在這里執行的
            operation.cancelBlock = ^{
                [self.imageDownloader cancel:subOperationToken];
                __strong __typeof(weakOperation) strongOperation = weakOperation;
                [self safelyRemoveOperationFromRunning:strongOperation];
            };
        }else if (cachedImage) {
            //4.3 請求到緩存圖片 && !SDWebImageRefreshCached
            //直接回調，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 圖片沒有在緩存中 && 代理沒有允許下載操作
            __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;
}

方法內部的注釋詳細描述了SDWebImageManager的loadImageWithURL工作流程。概括起來其實很簡單：首先判斷緩存中是否能獲取到圖片，如果沒獲取到則使用異步下載器下載，然后使用緩存類緩存，執行回調返回圖片。如果緩存中能夠獲取到，執行回調返回緩存中的圖片。

下一篇文章將著重分析SDWebImage的緩存實現。