AJAX实现loading….效果

<script type="text/javascript"> var xmlHttp; //创建一个xmlHttpRequest对象 function createXMLHttpRequest() {     if (window.ActiveXObject) {         xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");     }     else if (window.XMLHttpRequest) {         xmlHttp = new XMLHttpRequest();     } } function handleStateChange(){        var img = "<img src='/images/green/loading.gif' style='margin-top:130px; margin-left:250px'>";     if(xmlHttp.readystate == 4){   //表示请求状态 4为完成            if(xmlHttp.status == 200){ //http状态指示码，200表示ok                      //将服务器返回信息作为文本插入到DIV容器中。                document.getElementById("data").innerHTML = xmlHttp.responseText;           }     } else document.getElementById("data").innerHTML = img; //若响应未完成的话，则显示loading.. }        function showData(entityName){          var url = "/greengate.action?doing=listResult&entity_name="+entityName;       createXMLHttpRequest();          //请求状态改变事件触发handleStateChange功能       xmlHttp.onreadystatechange = handleStateChange;         xmlHttp.open("GET",url);       xmlHttp.send(null);            } </script>