SDWebImage获取到图片大小计算

completed 的block回调中有图片对象

[self.bottomImgView sd_setImageWithURL:[NSURL URLWithString:@"...."] placeholderImage:[UIImage imageNamed:@""] options:(SDWebImageRetryFailed) completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, NSURL *imageURL) {
  double imgH = self.bottomImgView.width*image.size.height/image.size.width;
  [self.bottomImgView mas_updateConstraints:^(MASConstraintMaker *make) {
    make.height.mas_equalTo(imgH);
  }];
}];