JavaScript实现div相对于input的绝对定位[实例：C#用户控件dropDownList]

<%@ Register TagPrefix="iewc" Namespace="Microsoft.Web.UI.WebControls" Assembly="Microsoft.Web.UI.WebControls" %>
<%@ Control Language="c#" AutoEventWireup="false" Codebehind="SelUnit.ascx.cs" Inherits="DXK.Web.component.SelUnit" TargetSchema="http://schemas.microsoft.com/intellisense/ie5"%>
<script type="text/javascript">
    //Sel_Text.onclick = function()
    function clickSel(obj)
    {    
        if(divTree.style.display == 'none')
        {
            divTree.style.display = '';    
            divTree.style.top = findOffsetTop(obj) + obj.offsetHeight;            
            divTree.style.left = findOffsetLeft(obj);
        }
        else
        {
            divTree.style.display = 'none';            
        }
    }
    document.onclick = function()
    {
        var the_obj = event.srcElement;
        if ((the_obj.id.indexOf('_Sel_Text') == -1) && (get_ElementById(the_obj, 'divTree') == null))
        {
            if(divTree.style.display == '')
                divTree.style.display = 'none';    
        }
    }
    function get_ElementById(the_ele,the_id)
    {
        the_id = the_id.toLowerCase();
        if(the_ele.id.toLowerCase()==the_id)return the_ele;
        while(the_ele=the_ele.offsetParent){
            if(the_ele.id.toLowerCase()==the_id)return the_ele;
        }
        return(null);
    }
    function dodblclick(objTV, UCTag) 
    { 
        if(objTV.getTreeNode(objTV.clickedNodeIndex)!=undefined)
        {
            document.getElementById(UCTag+'_Sel_Text').value = objTV.getTreeNode(objTV.clickedNodeIndex).getAttribute("Text");
            document.getElementById(UCTag+'_Sel_Value').value = objTV.getTreeNode(objTV.clickedNodeIndex).getAttribute("NodeData");    
            divTree.style.display = 'none';    
        }
    } 
    function findOffsetTop(o)
    {
        var t = 0;
        if (o.offsetParent)
        {
            while (o.offsetParent)
            {
                t += o.offsetTop;
                o  = o.offsetParent;
            }
        }
        return t;
    }
    function findOffsetLeft(o)
    {
        var t = 0;
        if (o.offsetParent)
        {
            while (o.offsetParent)
            {
                t += o.offsetLeft;
                o  = o.offsetParent;
            }
        }
        return t;
    }
</script>
<input id="Sel_Text" style="BACKGROUND-POSITION-X: right; BACKGROUND-IMAGE: url(../images/seltip.jpg); WIDTH: 150px; CURSOR: pointer; BACKGROUND-REPEAT: no-repeat"
    readOnly type="text" runat="server"> <INPUT id="Sel_Value" type="hidden" runat="server">
<DIV id="divTree" style="BORDER-RIGHT: black 1px groove; BORDER-TOP: black 1px groove; DISPLAY: none; OVERFLOW: auto; BORDER-LEFT: black 1px groove; WIDTH: 240px; BORDER-BOTTOM: black 1px groove; POSITION: absolute; HEIGHT: 350px"><iewc:treeview id="tvUnitList" runat="server" BorderWidth="0px" BorderStyle="None"
        Indent="20" SelectExpands="True" ShowToolTip="False"></iewc:treeview></DIV>

.cs后台代码
  1using System;
  2    using System.Data;
  3    using System.Drawing;
  4    using System.Web;
  5    using System.Web.UI.WebControls;
  6    using System.Web.UI.HtmlControls;
  7    using Microsoft.Web.UI.WebControls;
  8
  9    /**//// <summary>
 10    ///        SelUnit 的摘要说明。
 11    /// </summary>
 12    public class SelUnit : System.Web.UI.UserControl
 13    {
 14        protected System.Web.UI.HtmlControls.HtmlInputHidden Sel_Value;
 15        protected System.Web.UI.HtmlControls.HtmlInputText Sel_Text;
 16        protected Microsoft.Web.UI.WebControls.TreeView tvUnitList;
 17
 18        Property#region Property
 19        /**//// <summary>
 20        /// [只读]
 21        /// </summary>
 22        public string SelectedText
 23        {            
 24            get{return this.Sel_Text.Value;}            
 25        }
 26        /**//// <summary>
 27        /// [只读]
 28        /// </summary>
 29        public string SelectedValue
 30        {
 31            get{return this.Sel_Value.Value;}
 32        }
 33        public string SelectedIndex
 34        {
 35            get{return this.tvUnitList.SelectedNodeIndex;}
 36            set{this.tvUnitList.SelectedNodeIndex = value;}
 37        }
 38        public string CssClass
 39        {
 40            get{return this.Sel_Text.Attributes["class"];}
 41            set{this.Sel_Text.Attributes.Add("class", value);}
 42        }
 43        public string Width
 44        {
 45            get{return this.Sel_Text.Attributes["width"];}
 46            set{this.Sel_Text.Attributes.Add("width", value);}
 47        }
 48        public bool AutoPostBack
 49        {
 50            get{return this.tvUnitList.AutoPostBack;}
 51            set{this.tvUnitList.AutoPostBack = value;}
 52        }
 53        private bool _Enabled = true;
 54        public bool Enabled
 55        {
 56            get{return this._Enabled;}
 57            set{this._Enabled = value;}
 58        }
 59        #endregion
 60
 61        private void Page_Load(object sender, System.EventArgs e)
 62        {
 63            // 在此处放置用户代码以初始化页面
 64            Response.Buffer=true;   
 65            Response.ExpiresAbsolute=System.DateTime.Now.AddSeconds(-1);   
 66            Response.Expires=0;   
 67            Response.CacheControl="no-cache";   
 68
 69            if(!Page.IsPostBack)
 70            {
 71                string strUserUnit = "z"; //默认为全省
 72                if(Session["userinfo"] != null)
 73                {
 74                    DataTable dtUserInfo = (DataTable)Session["userinfo"];                                    
 75                    if(dtUserInfo.Rows.Count > 0)                        
 76                        strUserUnit = dtUserInfo.Rows[0]["user_unit"].ToString().Trim();
 77                }
 78                
 79                this.tvUnitList.Nodes.Clear();
 80                this.InitStatUnitTree(ref this.tvUnitList, strUserUnit, false);
 81                    
 82            }            
 83            if(this._Enabled)
 84            {
 85                this.Sel_Text.Attributes.Add("onclick","javascript:clickSel(this);");
 86                this.tvUnitList.Attributes.Add("ondblclick","javascript:dodblclick(this,'" + this.UniqueID + "');");
 87            }
 88        }
 89
 90        /**//// <summary>
 91        /// 初始化单位列表,TreeView
 92        /// </summary>
 93        /// <param name="tv"></param>
 94        /// <param name="strUserUnit"></param>
 95        /// <param name="ifCheckBox"></param>
 96        private void InitStatUnitTree(ref TreeView tv,string strUserUnit, bool ifCheckBox)
 97        {
 98
 99TreeNode tn                = null;
100            TreeNode tnChild        = null;
101            TreeNode tnGrandChild    = null;
102            //TreeNode root = new TreeNode();
103            //root.Text = "单位列表";
104            //tv.Nodes.Add(root);    
105foreach(DataRow drUnit in dtUnit.Rows)
106{
107 tn = DXK.Com.UICommon.tnGetTreeNode("mykey","myvalue");
108                    tn.CheckBox = ifCheckBox;                    
109                    tv.Nodes.Add(tn);
110}
111
112
113/**//// <summary>
114        /// 返回TreeNode
115        /// </summary>
116        /// <param name="strText"></param>
117        /// <param name="strData"></param>
118        /// <returns></returns>
119        public static TreeNode tnGetTreeNode(string strText,string strData)
120        {
121            TreeNode tn = new TreeNode();
122            tn.CheckBox = true;
123            tn.Text = strText;
124            tn.NodeData = strData;
125            return tn;
126        }
127}
128        }