SDWebImage从缓存中获取图片

  if ([[SDImageCache sharedImageCache] imageFromKey:sort.imageUrl]) {
        [cell.photoImageView setImage:[[SDImageCache sharedImageCache] imageFromKey:sort.imageUrl]];

    } else {

        [cell.photoImageView setImageWithURL:[NSURL URLWithString:sort.imageUrl]];

    }