extjs动态树刷新问题！

我现在有一颗树！我想当刷新时保存当前的展开状态！高手指点一下！
Ext.onReady(function(){
    var Tree = Ext.tree;
    var tree = new Tree.TreePanel({
        el:'tree-div1',
        rootVisible:true,
        border:false,
        id:'calmtree',
        autoScroll:false,
        animate:true,
        enableDD:true,
        containerScroll: true,
        height:'auto',
        loader: new Tree.TreeLoader({
         listeners: {
            "loadexception": function(loader, node, response) {
                node.loaded = false;
                node.reload.defer(10, node);
            }
        }
        }),
        tools : [{
            id : 'refresh',
              handler : function() {
              var tree = Ext.getCmp('calmtree1');
              tree.root.reload();
              }
              }]
    });    // set the root node
    var root = new Tree.AsyncTreeNode({
        text: "<a onclick=\"openPage('root/summary.html','root/rootmenu.jsp')\">Root</a>",
        draggable:false,
        cls:'root',
        id:'root',
        qtip:'root',
        flag:''
    });
    tree.setRootNode(root);
    tree.on('beforeload', 
         function(node)
         { 
     tree.loader.dataUrl='loadTree.do?paramEntity.nodeId='+node.id+'&paramEntity.nodeName='+node.attributes.qtip+'&paramEntity.flag='+node.attributes.flag+'&paramEntity.soluFlag='+node.attributes.soluFlag;
         });
         tree.render();
});
function openPage(mainFrame,menuFrame)
{
parent.frames['mainFrame'].document.write("<html><head><link rel='stylesheet' type='text/css' href='css/main.css'/></head><body class='mainbg'><font style='font-size:12px'><br/>&nbsp;&nbsp;&nbsp;&nbsp;数据加载中，请稍后...</font></body></html>");
parent.frames['mainFrame'].document.location.href = mainFrame;
window.parent.document.getElementById('navigate').src= menuFrame;

}