1. sd_cancelCurrentImageLoad2.[SDWebImageManager.sharedManager downloadImageWithURL:url ]
self.connection = [[NSURLConnection alloc] initWithRequest:self.request delegate:self]
SDWebImageManager
1. queryDiskCacheForKey2. self.imageDownloader downloadImageWithURL
SDWebImageDownloader
SDWebImageDownloaderOperation
1. addProgressCallback2. [slef.operation alloc] initWithRequest:request]3. [wself.downloadQueue addOperation:operation];
UIImageView
