1、来到SDWebImageDownloader.h，SDWebImageDownloaderOptions

2、全局搜索SDWebImageDownloaderUseNSURLCache

2.1、用户设置了SDWebImageDownloaderUseNSURLCache缓存策略，使用默认缓存NSURLRequestUseProtocolCachePolicy；没设置，忽略缓存NSURLRequestReloadIgnoringLocalCacheData，避免NSURLCache再缓存一次。

    NSURLRequestCachePolicy cachePolicy = options & SDWebImageDownloaderUseNSURLCache ? NSURLRequestUseProtocolCachePolicy : NSURLRequestReloadIgnoringLocalCacheData;

    if (options & SDWebImageRefreshCached) downloaderOptions |= SDWebImageDownloaderUseNSURLCache;

2.3、cachedResponse置为nil，避免进行本地缓存

- (void)URLSession:(NSURLSession *)session
          dataTask:(NSURLSessionDataTask *)dataTask
 willCacheResponse:(NSCachedURLResponse *)proposedResponse
 completionHandler:(void (^)(NSCachedURLResponse *cachedResponse))completionHandler {
    
    NSCachedURLResponse *cachedResponse = proposedResponse;

    if (!(self.options & SDWebImageDownloaderUseNSURLCache)) {
        // Prevents caching of responses
        cachedResponse = nil;
    }
    if (completionHandler) {
        completionHandler(cachedResponse);
    }
}

3、全局搜索SDWebImageDownloaderUseNSURLCache

    if (cachedImage && options & SDWebImageRefreshCached) {
        // force progressive off if image already cached but forced refreshing
        downloaderOptions &= ~SDWebImageDownloaderProgressiveLoad;
        // ignore image read from NSURLCache if image if cached but force refreshing
        downloaderOptions |= SDWebImageDownloaderIgnoreCachedResponse;
    }

        if (self.options & SDWebImageDownloaderIgnoreCachedResponse) {
            // Grab the cached data for later check
            NSURLCache *URLCache = session.configuration.URLCache;
            if (!URLCache) {
                URLCache = [NSURLCache sharedURLCache];
            }
            NSCachedURLResponse *cachedResponse;
            // NSURLCache's `cachedResponseForRequest:` is not thread-safe, see https://developer.apple.com/documentation/foundation/nsurlcache#2317483
            @synchronized (URLCache) {
                cachedResponse = [URLCache cachedResponseForRequest:self.request];
            }
            if (cachedResponse) {
                self.cachedData = cachedResponse.data;
            }
        }

                /**  if you specified to only use cached data via `SDWebImageDownloaderIgnoreCachedResponse`,
                 *  then we should check if the cached data is equal to image data
                 */
                if (self.options & SDWebImageDownloaderIgnoreCachedResponse && [self.cachedData isEqualToData:imageData]) {
                    self.responseError = [NSError errorWithDomain:SDWebImageErrorDomain code:SDWebImageErrorCacheNotModified userInfo:nil];
                    // call completion block with not modified error
                    [self callCompletionBlocksWithError:self.responseError];
                    [self done];
                } else {
                    // decode the image in coder queue
                    dispatch_async(self.coderQueue, ^{
                        @autoreleasepool {
                            UIImage *image = SDImageLoaderDecodeImageData(imageData, self.request.URL, [[self class] imageOptionsFromDownloaderOptions:self.options], self.context);
                            CGSize imageSize = image.size;
                            if (imageSize.width == 0 || imageSize.height == 0) {
                                [self callCompletionBlocksWithError:[NSError errorWithDomain:SDWebImageErrorDomain code:SDWebImageErrorBadImageData userInfo:@{NSLocalizedDescriptionKey : @"Downloaded image has 0 pixels"}]];
                            } else {
                                [self callCompletionBlocksWithImage:image imageData:imageData error:nil finished:YES];
                            }
                            [self done];
                        }
                    });
                }

• SDWebImageErrorCacheNotModified 错误码

    if (statusCode == 304 && !self.cachedData) {
        valid = NO;
        self.responseError = [NSError errorWithDomain:SDWebImageErrorDomain code:SDWebImageErrorCacheNotModified userInfo:nil];
    }