请大侠帮忙加一个事件！


现在，我已经能弹出事件了，代码如下：/*
 * Ext JS Library 2.2
 * Copyright(c) 2006-2008, Ext JS, LLC.
 * [email protected]
 * 
 * http://extjs.com/license
 */
 Ext.BLANK_IMAGE_URL="s.gif";var root = new Ext.tree.AsyncTreeNode({
text: 'C:',
draggable:false,
id:'\\'
});Ext.onReady(function(){
    // shorthand
    var Tree = Ext.tree;
    
    var tree = new Tree.TreePanel({
        el:'tree-div',
        useArrows:true,
        autoScroll:true,
        animate:true,
        enableDD:true,
        containerScroll: true,        // auto create TreeLoader
        dataUrl: 'get-nodes.php',        root: root,
tbar : [
{
     id: 'sortSelect',
        xtype: 'combo',
typeAhead: true,
triggerAction: 'all',
width: 100,
editable: false,
mode: 'local',
displayField: 'name',
valueField: 'name',
lazyInit: false,
value: 'name',
store: new Ext.data.SimpleStore({
fields: ['id', 'name'],
data : [['0', 'C:\\'],['1', 'D:\\'],['2', 'E:\\']]
}),
listeners : {
'select' : function(combo, record, index) {
typename = record.data.name;
alert(typename);
}}
}]    });
    tree.on('checkchange', function(node, checked) {      
        node.expand();      
        node.attributes.checked = checked;      
        node.eachChild(function(child) {      
            child.ui.toggleCheck(checked);      
            child.attributes.checked = checked;  
            child.fireEvent('checkchange', child, checked);      
        });      
    }, tree);         // render the tree
    tree.render();
    tree.getRootNode().expand();});但我想选择C盘时，就把根目录设为'C:\',选D盘时，就把根目录设为'D:\'，也就是把根目录设为我alert出来的值，求
达人指教啊