VisualTreeHelper 向下提取 元素

private ChildType FindVisualChild<ChildType>(DependencyObject obj)
            where ChildType : DependencyObject
        {
            for (int i = 0; i < VisualTreeHelper.GetChildrenCount(obj); i++)
            {
                DependencyObject child = VisualTreeHelper.GetChild(obj, i);
                if (child != null && child is ChildType)
                {
                    return child as ChildType;
                }
                else
                {
                    ChildType childOfChild = FindVisualChild<ChildType>(child);
                    if (childOfChild != null)
                    {
                        return childOfChild;
                    }
 
                }
            }
            return null;
        }