支持自定义回调函数的异步调用

    做一个最简单的POST方式异步调用的请求，支持自定义的回调函数，该回调函数获取异步请求返回的XMLDOM对象，代码如下：

function  postRequest(url,parameters,callBack){
     var  xmlHttp  =  getXmlHttp();  // create xmlHttpRequest
     if (xmlHttp  != null ){
        xmlHttp.onreadystatechange  =   function (){
             if (xmlHttp.readyState  ==   4 ){
                 if (xmlHttp.status  ==   200 ){
                    xmlDom  =  getXmlDom(xmlHttp.responseText);//create xmlDom
                     if (xmlDom  !=   null ){
                        eval(callBack(xmlDom));
                    }
                }
            }
        }
        xmlHttp.open( ' Post ' ,url, true );
        xmlHttp.setRequestHeader( " Content-Length " ,parameters.length);
        xmlHttp.setRequestHeader( " Content-Type " , " application/x-www-form-urlencoded " );
        xmlHttp.send(parameters);
    }
}