首先附上git地址：https://github.com/SDWebImage/SDWebImage

附上SDWebImage類圖：

SDWebImageClassDiagram.png

附上SDWebImage步驟圖：

SDWebImageSequenceDiagram.png

核心模塊三部分：

1.主要調(diào)度類：SDWebImageManager；全部操作都是圍繞它為中心；
2.主要緩存類：SDImageCache；
3.主要下載類：SDWebImageDownload;

次要：

4.和UIView相關(guān)的類，是對UIKit的擴展，只是方便我們調(diào)起Manager里面的功能類；
5.SDImageCacheConfig用于配置緩存；

問題一，SD的大概流程：

1.通過UIImageView+WebCache作為入口，有placeholderImage的情況下，先顯示占位圖；
2.通過系統(tǒng)類NSMapTable判斷當(dāng)前是否有任務(wù)在執(zhí)行；
??3.重要步驟，如圖：

核心方法

問題二，內(nèi)存緩存模塊的實現(xiàn)步驟：

1.SDWebImage實現(xiàn)了磁盤，內(nèi)存雙緩存；
2.內(nèi)存緩存如何實現(xiàn)的：通過SDMemoryCache類來管理；
3.通過shouldUseWeakMemoryCache的Bool值，來判斷是否開啟內(nèi)存緩存；
4.內(nèi)存緩存有可能存在兩份，NSCache和SDMemoryCache，因為NSCache不可控；

問題三，磁盤模塊的實現(xiàn)步驟：

1.創(chuàng)建一個目錄；
2.為每一個緩存文件生成一個MD5文件名；
3.首先檢查內(nèi)存緩存，再檢查磁盤緩存，核心代碼：

- (nullable NSOperation *)queryCacheOperationForKey:(nullable NSString *)key options:(SDImageCacheOptions)options context:(nullable SDWebImageContext *)context done:(nullable SDImageCacheQueryCompletionBlock)doneBlock {
    if (!key) {
        if (doneBlock) {
            doneBlock(nil, nil, SDImageCacheTypeNone);
        }
        return nil;
    }
    
    id<SDImageTransformer> transformer = context[SDWebImageContextImageTransformer];
    if (transformer) {
        // grab the transformed disk image if transformer provided
        NSString *transformerKey = [transformer transformerKey];
        key = SDTransformedKeyForKey(key, transformerKey);
    }
    
    // First check the in-memory cache...
    //首先檢查內(nèi)存緩存…
    UIImage *image = [self imageFromMemoryCacheForKey:key];

    if ((options & SDImageCacheDecodeFirstFrameOnly) && image.sd_isAnimated) {
#if SD_MAC
        image = [[NSImage alloc] initWithCGImage:image.CGImage scale:image.scale orientation:kCGImagePropertyOrientationUp];
#else
        image = [[UIImage alloc] initWithCGImage:image.CGImage scale:image.scale orientation:image.imageOrientation];
#endif
    }

    BOOL shouldQueryMemoryOnly = (image && !(options & SDImageCacheQueryMemoryData));
    if (shouldQueryMemoryOnly) {
        if (doneBlock) {
            doneBlock(image, nil, SDImageCacheTypeMemory);
        }
        return nil;
    }
    
    // Second check the disk cache...
    //第二，檢查磁盤緩存…
    NSOperation *operation = [NSOperation new];
    // Check whether we need to synchronously query disk
    // 1. in-memory cache hit & memoryDataSync
    // 2. in-memory cache miss & diskDataSync
    BOOL shouldQueryDiskSync = ((image && options & SDImageCacheQueryMemoryDataSync) ||
                                (!image && options & SDImageCacheQueryDiskDataSync));
    void(^queryDiskBlock)(void) =  ^{
        if (operation.isCancelled) {
            // do not call the completion if cancelled
            return;
        }
        
        @autoreleasepool {
            NSData *diskData = [self diskImageDataBySearchingAllPathsForKey:key];
            UIImage *diskImage;
            SDImageCacheType cacheType = SDImageCacheTypeNone;
            if (image) {
                // the image is from in-memory cache, but need image data
                diskImage = image;
                cacheType = SDImageCacheTypeMemory;
            } else if (diskData) {
                cacheType = SDImageCacheTypeDisk;
                // decode image data only if in-memory cache missed
                diskImage = [self diskImageForKey:key data:diskData options:options context:context];
                if (diskImage && self.config.shouldCacheImagesInMemory) {
                    NSUInteger cost = diskImage.sd_memoryCost;
                    [self.memCache setObject:diskImage forKey:key cost:cost];
                }
            }
            
            if (doneBlock) {
                if (shouldQueryDiskSync) {
                    doneBlock(diskImage, diskData, cacheType);
                } else {
                    dispatch_async(dispatch_get_main_queue(), ^{
                        doneBlock(diskImage, diskData, cacheType);
                    });
                }
            }
        }
    };
    
    // Query in ioQueue to keep IO-safe
    if (shouldQueryDiskSync) {
        dispatch_sync(self.ioQueue, queryDiskBlock);
    } else {
        dispatch_async(self.ioQueue, queryDiskBlock);
    }
    
    return operation;
}

問題四，下載模塊的實現(xiàn)步驟：

1.SDWebImageDownloader實現(xiàn)了下載管理；圍繞NSURLSession實現(xiàn)的；
2.SDWebImageDownloaderOperation實現(xiàn)了生命周期管理，具體任務(wù)是它來實現(xiàn)的；
3.核心方法SDWebImageDownloader里的：

- (nullable NSOperation<SDWebImageDownloaderOperation> *)createDownloaderOperationWithUrl:(nonnull NSURL *)url
                                                                                  options:(SDWebImageDownloaderOptions)options
                                                                                  context:(nullable SDWebImageContext *)context {
...
}

4.SDWebImageDownloaderConfig里，SDWebImageDownloaderExecutionOrder枚舉，兩種情況：

/// Operation execution order
typedef NS_ENUM(NSInteger, SDWebImageDownloaderExecutionOrder) {
    /**
     * Default value. All download operations will execute in queue style (first-in-first-out).
     */
    SDWebImageDownloaderFIFOExecutionOrder,
    
    /**
     * All download operations will execute in stack style (last-in-first-out).
     */
    SDWebImageDownloaderLIFOExecutionOrder
};

FIFOE為先進先出，LIFOE為后進先出；