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,
请关闭不使用的窗口.'");
    }

}