SDWebImage下载图片后 刷新UI没数据

问题描述：使用SDWebImageDownloader 将图片数组全部下载由于异步操作，图片没下载完全导致传入的数组没有数据。
处理方法：

 dispatch_group_t group = dispatch_group_create();
 for (NSString *urlStr in arr) {
       dispatch_group_enter(group);
       [[SDWebImageDownloader sharedDownloader] downloadImageWithURL:[NSURL URLWithString:urlStr] options:(SDWebImageDownloaderHighPriority) progress:^(NSInteger receivedSize, NSInteger expectedSize) {
                    
        } completed:^(UIImage *image, NSData *data, NSError *error, BOOL finished) {
            if (image) {
                [self.photoArr addObject:image];
            }else{
                UIImage *imagePlaceholder = [UIImage imageNamed:@"加载失败"];
                [self.photoArr addObject:imagePlaceholder];
            }
           dispatch_group_leave(group);
      }];
   }
        // 下载图片完成后, 回到主线
        dispatch_group_notify(group, dispatch_get_main_queue(), ^{
            // 刷新UI
            self.feedPhotoView.photoArr = self.photoArr;
        });