sdwebimage 缓存策略

常用的下载图片的API是下面这个

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

创建一个operation，这个operation包含一个cache用的operation，图片的缓存策略逻辑就在里面：

__block SDWebImageCombinedOperation *operation = [SDWebImageCombinedOperation new];