请教 Ext GridPanel 怎么刷新 ？


<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<link rel="stylesheet" type="text/css" href="extjs/resources/css/ext-all.css" />
<script type="text/javascript" src="extjs/adapter/ext/ext-base.js"></script>
<script type="text/javascript" src="extjs/ext-all.js"></script>
<script>
Ext.onReady(function(){
    var ds = new Ext.data.Store({
        autoLoad:true,
        remoteSort: true,
        proxy: new Ext.data.HttpProxy({url:"test1.asp"}),
        reader: new Ext.data.JsonReader({
            totalProperty: "totalProperty",
            root: "results"
        },[
          {name:"tablename",mapping:'tablename'},
          {name:"cname",mapping:'cname'},
          {name:'ncreatedate',mapping:'ncreatedate'},
          {name:'re',mapping:'re'},
          {name:'nuser',mapping:'nuser'}
        ])
    });
ds.load();
oGrid = new Ext.grid.GridPanel({
renderTo:"pad",
monitorResize:true,
autoHeight:true,
columns:[
{header:"tablename",dataIndex:"tablename"},
{header:"cname",dataIndex:"cname"},
{header:"ncreatedate",dataIndex:"ncreatedate"},
{header:"re",dataIndex:"re"},
{header:"nuser",dataIndex:"nuser"}
],
store:ds
});
ds.load();});function doit(){
oGrid.store.baseParams = {params:{a:'xx'}}
oGrid.store.load({callback:function(r){
alert(r[0].get('ncreatedate'));
}})
}
</script>
<div id="pad"></div>
<input type="button" value="click" onclick="doit()">