有时候需要知道图片的宽高去设置相应的位置，SDWebImage里面的方法就可以直接得到：

[[SDWebImageDownloader sharedDownloader]downloadImageWithURL:[NSURL URLWithString:urlString] options:SDWebImageDownloaderUseNSURLCache progress:^(NSInteger receivedSize, NSInteger expectedSize) {
                         
                     } completed:^(UIImage *image, NSData *data, NSError *error, BOOL finished) {
                        //这边就能拿到图片了

 _width = image.width;
  _height = image.height;
                     }];