Extjs4 怎么在点击查询按钮后刷新gride.panel?

[net_ph]

第一次用extjs，就是我点击查询按钮后后台重新查询数据传给前台，前台怎么刷新啊？？
function geturl() {
return document.getElementById('url').value;//获取路径
}
function getParams(dnames) {//把传给后台的参数拼到路径里
if (dnames.length == 0) {
return null;
}
pstring = "?";
for (var i = 0; i < dnames.length; i++) {
pstring += '' + dnames[i].id + '=' + dnames[i].value + '&'
}
pstring = pstring.substr(0, pstring.length - 1);
return pstring;
}

var dnames = getParams(document.getElementsByName('bvar'));//通过url把传给后台的参数
var purl = geturl() + dnames;//路径
function getstore(url) {
var store = Ext.create('Ext.data.Store', {
model: 'User',
autoLoad: true,
pageSize: 50,
proxy: {
type: 'ajax',
url: url,
reader: {
type: 'json',
root: 'records',
totalProperty: 'total'
},
extraParams: {
showDel: false
}
}
});
return store
}
var store = getstore(purl);
var ptbar = new Ext.create('Ext.PagingToolbar', {
//pageSize: 50,
store: store,
displayInfo: true,
displayMsg: '显示{0}条到{1}条,共{2}条',
emptyMsg: "没有符合条件的记录"
});
var grid = Ext.create('Ext.grid.Panel', {
height: 400,
renderTo: "grid",
store: store,
columns: arr,
enableColumnMove: false,
constrain: false,
bbar: ptbar
});
store.loadPage(1);





var btn = document.getElementById('select');
btn.onclick = function () {
dnames = getParams(document.getElementsByName('bvar'));//通过url把新的参数传给后台
purl = geturl() + dnames;
store = getstore(purl);
store.loadPage(1);
//这里是我写的但是panel不刷新，不知道这里该怎么写???
}