关于ExtJs XmlReader的问题

 //在内存中创建一个DOM
        var _xml;
        var str = "<persons><person><name>zxy</name><sex>male</sex></person><person><name>zs</name><sex>female</sex></person></persons>";
        if(window.ActiveXObject){
            var arr = new Array("MSXML2.DOMDocument.5.0","MSXML2.DOMDocument.4.0","MSXML2.DOMDocument.3.0","MSXML2.DOMDocument",
                    "Microsoft.XmlDom");
            for(var i=0;i<arr.length;i++){
                try{
                    _xml = new ActiveXObject(arr[i]);
                    _xml.loadXML(str);
                }catch(e){
                }
            }
        }else{
            _xml = document.implementation.createDocument();
            var parser = new DomParser();
            _xml = parser.parseFromString(str,"text/xml");
        }
       
        alert(_xml.documentElement.nodeName);
        
 //创建一个Store并通过XmlReader,MemoryProxy将DOM载入
        var rc = Ext.data.Record.create([
                                         "name","sex"
                                         ]);
 var _store = new Ext.data.Store({
                proxy:new Ext.data.MemoryProxy(_xml),
                reader:new Ext.data.XmlReader({
                    record:"person"
                       },
                       rc
                        )
            });
        Ext.Msg.alert("消息",_store。getCount());