怎么实现google地图拖拽的时候旁边的文本框中的经纬值也跟着变化 _JavaScript

如何实现google地图拖拽的时候旁边的文本框中的经纬值也跟着变化
如题，先粘上代码吧、
C# code

<html> 
<head>
<title></title>
 <meta name="viewport" content="initial-scale=1.0, user-scalable=no" />
<script src="http://maps.google.com/maps?file=api&amp;v=2&amp;sensor=false&amp;key=ABQIAAAAn8dF06B-QpniAeXC-bdCVBSCooeQ7x8_7WnK9Fc_vSUhGs5F4BSMl23EayM7v4EESxLBoX22Z0GQ3Q" type="text/javascript"></script>
 <script type="text/javascript">

     // 矩形是在地图上勾勒经纬边界的简单叠加层，其边界已指定
     // 粗细和颜色，并（可选）可以使用半透明背景颜色.
     function Rectangle(bounds, opt_weight, opt_color) {
         this.bounds_ = bounds;
         this.weight_ = opt_weight || 2;
         this.color_ = opt_color || "#FF0000";
     }
     Rectangle.prototype = new GOverlay();

     // 创建表示此矩形的 DIV.
     Rectangle.prototype.initialize = function (map) {
         // 创建表示我们的矩形的 DIV
         var div = document.createElement("div");
         div.style.border = this.weight_ + "px solid " + this.color_;
         div.style.position = "absolute";

         // 我们的矩形相对于地图是平面，所以将其添加到MAP_PANE 面板，
         // 与地图本身的绘制顺序相同（即在标记阴影下方）
         map.getPane(G_MAP_MAP_PANE).appendChild(div);

         this.map_ = map;
         this.div_ = div;
     }

     // 从地图面板删除 DIV
     Rectangle.prototype.remove = function () {
         this.div_.parentNode.removeChild(this.div_);
     }

     // 将我们的数据复制到新的矩形
     Rectangle.prototype.copy = function () {
         return new Rectangle(this.bounds_, this.weight_, this.color_,
                           this.backgroundColor_, this.opacity_);
     }

     // 基于当前投影和缩放级别重新绘制矩形
     Rectangle.prototype.redraw = function (force) {
         // We only need to redraw if the coordinate system has changed
         if (!force) return;

         // 计算边界两个对角的 DIV 坐标，获取矩形的尺寸和位置
         var c1 = this.map_.fromLatLngToDivPixel(this.bounds_.getSouthWest());
         var c2 = this.map_.fromLatLngToDivPixel(this.bounds_.getNorthEast());

         // 现在基于边界的 DIV 坐标放置 DIV
         this.div_.style.width = Math.abs(c2.x - c1.x) + "px";
         this.div_.style.height = Math.abs(c2.y - c1.y) + "px";
         this.div_.style.left = (Math.min(c2.x, c1.x) - this.weight_) + "px";
         this.div_.style.top = (Math.min(c2.y, c1.y) - this.weight_) + "px";
     }


     function initialize() {
         if (GBrowserIsCompatible()) {
             var map = new GMap2(document.getElementById("map_canvas"));
             map.setCenter(new GLatLng(39.917, 116.397), 14);
             map.addControl(new GSmallMapControl());
             map.addControl(new GMapTypeControl());

             // 在地图中心显示矩形，大小约为主地图的四分之一
             var result = document.getElementById("bound");
             var bounds = map.getBounds();
             result.value = bounds;
             var southWest = bounds.getSouthWest();
             var northEast = bounds.getNorthEast();
             var lngDelta = northEast.lng() - southWest.lng();
             var latDelta = northEast.lat() - southWest.lat();
             var rectBounds = new GLatLngBounds(
            new GLatLng(southWest.lat() + latDelta,
                        southWest.lng() + lngDelta),
            new GLatLng(northEast.lat() - latDelta,
                        northEast.lng() - lngDelta));
             map.addOverlay(new Rectangle(rectBounds));
         }
     }

</script> 
</head>
<body onload="initialize()" onunload="GUnload()">
            <div id="map_canvas" style="width:50%; height:500px;float:left;"></div><br />
            <input type="text" id="bound" name="bound" value=""  />
</body> 
</html>
参考网址：http://gcmd.nasa.gov/KeywordSearch/PageForward.do?ForwardPage=advancedSearch.jsp&KeywordPath=Parameters&Portal=GCMD&MetadataType=0&homepg