网页关闭前确认，并显示进度条

 var _t;
        window.onbeforeunload = function()
        {
            Ext.MessageBox.wait('請稍後...', '消息');
            setTimeout(function(){_t = setTimeout(onunloadcancel, 0)}, 0);
            return "您確定要退出嗎?";
        }


        window.onunloadcancel = function() {
            Ext.MessageBox.hide();
            clearTimeout(_t);
        }