roleData.getAt(0).data['USER_NAME'] 为空或不是对象

grid.addListener('rowdblclick', onRowDoubleClick)

var onRowDbClick = function(grid, rowIndex, e){
                             var selectionModel = grid.getSelectionModel();    
                                   var record = selectionModel.getSelected();
                                   alert(record.data['USER_PWD']);
                                   selectId=record.data['OID'];
                  
                //var selectId = this.ds.data.items[rowIndex].id;
                var roleData = new Ext.data.Store({
                    proxy : new Ext.data.HttpProxy({url : 'http://localhost:7788/WebWork/extTest.action?oid=' + selectId}),
                    reader : new Ext.data.JsonReader({}, ['OID', 'USER_NAME', 'USER_PWD']),
                    remoteSort : false
                });
                roleData.on('load', function() {
                    updateId = roleData.getAt(0).data['USER_NAME'];
                    alert(updateId);
                });
                roleData.load();
            }

roleData.getAt(0).data['USER_NAME']  为空或不是对象
why?????
好像是roleData还没有加载，如果我把roleData放在最外面，则通过函数调用能得到
roleData.getCount()的值，说明在最外面能加载，请教，我是最近才接触ext的

郁闷~~~~竟然没人回复！！