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

附上SDWebImage类图：

SDWebImageClassDiagram.png

附上SDWebImage步骤图：

SDWebImageSequenceDiagram.png

核心模块三部分：

1.主要调度类：SDWebImageManager；全部操作都是围绕它为中心；
2.主要缓存类：SDImageCache；
3.主要下载类：SDWebImageDownload;

次要：

4.和UIView相关的类，是对UIKit的扩展，只是方便我们调起Manager里面的功能类；
5.SDImageCacheConfig用于配置缓存；

问题一，SD的大概流程：

1.通过UIImageView+WebCache作为入口，有placeholderImage的情况下，先显示占位图；
2.通过系统类NSMapTable判断当前是否有任务在执行；
⭐️3.重要步骤，如图：

核心方法

问题二，内存缓存模块的实现步骤：

1.SDWebImage实现了磁盘，内存双缓存；
2.内存缓存如何实现的：通过SDMemoryCache类来管理；
3.通过shouldUseWeakMemoryCache的Bool值，来判断是否开启内存缓存；
4.内存缓存有可能存在两份，NSCache和SDMemoryCache，因为NSCache不可控；

问题三，磁盘模块的实现步骤：

1.创建一个目录；
2.为每一个缓存文件生成一个MD5文件名；
3.首先检查内存缓存，再检查磁盘缓存，核心代码：

- (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...
    //首先检查内存缓存…
    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，检查磁盘缓存…
    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;
}

问题四，下载模块的实现步骤：

1.SDWebImageDownloader实现了下载管理；围绕NSURLSession实现的；
2.SDWebImageDownloaderOperation实现了生命周期管理，具体任务是它来实现的；
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枚举，两种情况：

/// 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为先进先出，LIFOE为后进先出；