应对可变长度文本的情景，重写viewDidLayoutSubviews方法。

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- (void)viewDidLayoutSubviews
{
    [super viewDidLayoutSubviews];
    
    CGSize size = [_headerView systemLayoutSizeFittingSize:UILayoutFittingCompressedSize];
    CGRect headerFrame = _headerView.frame;
    headerFrame.size = size;
    _headerView.frame = headerFrame;
    [_tableView setTableHeaderView:_headerView];
}