Ext.Net 1.x_Ext.Net.TabPanel控制最多打开N个页面

var addTab = function (tabPanel, id, url, caption) {    if (tabPanel.items.getCount() < 3) {        var tab = tabPanel.getComponent(id);        if (!tab) {            tab = tabPanel.add({                id: id,                title: caption,                closable: true,                autoLoad: {                    showMask: true,                    url: url,                    mode: "iframe",                    maskMsg: "Loading " + url + "..."                }            });            tab.on("activate", function () {                var item = MenuPanel1.menu.items.get(id + "_item");                if (item) {                    MenuPanel1.setSelection(item);                }            }, this);        }        tabPanel.setActiveTab(tab);    }    else {        Ext.Msg.alert("提示", "'最多支持同时打开3个窗口,</br>请关闭不使用的窗口.'");    }}