#pragma mark 更新单个cell的方法
NSIndexPath *indexPath = [NSIndexPath indexPathForRow:row inSection:0];
[self.tableView reloadRowsAtIndexPaths:@[indexPath] withRowAnimation:UITableViewRowAnimationFade];
#pragma mark 直接获取对应位置的cell,对cell进行操作
NSIndexPath *indexPath = [NSIndexPath indexPathForRow:12 inSection:0];
ImageAttachmentCell *cell = (ImageAttachmentCell *)[self.tableView cellForRowAtIndexPath:indexPath];