XUL-Node - server-side XUL for Perl


  use XUL::Node;

  # creating
  $window = Window(                            # window with a header,
     HTML_H1(textNode => 'a heading'),         # a label, and a button
     $label = Label(FILL, value => 'a label'),
     Button(label => 'a button'),

  # attributes
  $label->value('a value');
  print $label->flex;

  # compositing
  print $window->child_count;                  # prints 2
  $window->Label(value => 'another label');    # add a label to window
  $window->add_child(Label);                   # same but takes child as param
  $button = $window->get_child(1);             # navigate the widget tree
  $window->add_child(Label, 0);                # add a child at an index

  # events
  $window->Button(Click => sub { $label->value('clicked!') });
     MenuPopup(map { MenuItem(label => "item #$_", ) } 1..10),
     Select => sub { $label->value(shift->selectedIndex) },

  # destroying
  $window->remove_child($button);              # remove child widget
  $window->remove_child(1);                    # remove child by index


XUL-Node is a rich user interface framework for server-based Perl applications. It includes a server, a UI framework, and a Javascript XUL client for the Firefox web browser. Perl applications run inside a POE server, and are displayed in a remote web browser.

The goal is to provide Perl developers with the XUL/Javascript development model, but with two small differences:

  • Make it Perl friendly

  • Allow users to run the application on remote servers, so clients only require Firefox, while the Perl code runs on the server

XUL-Node works by splitting each widget into two: a server half, and a client half. The server half sends DOM manipulation commands, and the client half sends DOM events. A small Javascript client library takes care of the communications.

The result is an application with a rich user interface, running in Firefox with no special security permissions, built in 100% pure Perl.


Programming in XUL-Node feels very much like working in a desktop UI framework such as PerkTk or WxPerl. You create widgets, arrange them in a composition tree, configure their attributes, and listen to their events.

Web development related concerns are pushed from user code into the framework- no need to worry about HTTP, parameter processing, saving state, and all those other things that make it so hard to develop a high-quality web application.

Welcome to XUL

XUL is an XML-based User interface Language. XUL-Node exposes XUL to the Perl developer. You need to know the XUL bindings to use XUL-Node. Fortunately, these can be learned in minutes. For a XUL reference, see XUL Planet (

Hello World

We start with the customary Hello World:

  package XUL::Node::Application::HelloWorld;
  use XUL::Node;
  use base 'XUL::Node::Application';

  sub start { Window(Label(value => 'Hello World!')) }


This is an application package, which creates a window with a label as its single child.


To create an application:

  • Subclass XUL::Node::Application.

  • Name your application package under XUL::Node::Application. E.g. XUL::Node::Application::MyApp.

  • Implement one template method, start().

In start() you must create at least one window, if you want the UI to show. The method is run once per application per session. This is where you create widgets and attach event listeners.

XUL-Node comes with 14 example applications in the XUL::Node::Application namespace.

Applications are launched by starting the server and pointing Firefox at a URL. You start the server with the command:


Run it with the option --help for usage info. This will start a XUL-Node server on the default server root and port you defined when running the Makefile.PL script.

You can then run the application from Firefox, by constructing a URL so:


  SERVER       server name
  PORT         HTTP port configured when starting server
  APPLICATION  application name, if none given, runs HelloWorld
  DEBUG        0 or 1, default is 0, turn on client debug info

The application name is the last part of its package name. So the package XUL::Node::Application::PeriodicTable can be run using the application name PeriodicTable. All applications must exist under @INC, under the namespace XUL::Node::Application.

So for example, to run the splitter example on a locally installed server, you would browse to:


The installation also creates an index page, providing links to all examples. By default it will be available at:



To create a UI, you will want your start() method to create a window with some widgets in it. Widgets are created by calling a function or method named after their tag:

  $button = Button;                           # orphan button with no label
  $box->Button;                               # another, but added to a box
  $widget = XUL::Node->new(tag_name => $tag); # using dynamic tag

After creating a widget, you must add it to a parent. The widget will show when there is a containment path between it and a window. There are 3 ways to parent widgets:

  $parent->add_child($button);                # using add_child
  $parent->Button(label => 'hi!');            # create and add in one shot
  Box(style => 'color:red', $label);          # add in parent constructor

Widgets have attributes. These can be set in the constructor, or via get/set methods:

  $button->value('a button');
  print $button->value;                       # prints 'a button'

Widget can be removed from the document by calling the remove_child() method on their parent. The only parameter is a widget, or an index of a widget. For example:


You can configure all attributes, event handlers, and children of a widget, in the constructor. There are also constants for commonly used attributes. This allows for some nice code:

        Columns(Column(FLEX), Column(FLEX)),
              Button(label => "cell 1"),
              Button(label => "cell 2"),
              Button(label => "cell 3"),
              Button(label => "cell 4"),

Check out the XUL references ( for an explanation of available widget attributes.


Widgets receive events from their client halves, and pass them on to attached listeners in the application. You attach a listener to a widget so:

  # listening to existing widget
  $button->attach(Change => sub { print 'clicked!' });

  # listening to widget in constructor
  TextBox(Change => sub { print shift->value });

You attach events by providing an event name and a listener. Possible event names are Click, Change, Select, and Pick. Different widgets fire different events. These are listed in XUL::Node::Event.

Listener are callbacks that receive a single argument: the event object (XUL::Node::Event). You can query this object for information about the event: name, source, and depending on the event type: checked, value, color, and selectedIndex.

Here is an example of listening to the Select event of a list box:

        $label = Label(value => 'select item from list'),
        ListBox(FILL, selectedIndex => 2,
           (map { ListItem(label => "item #$_") } 1..10),
           Select => sub {
                 ("selected item #${\( shift->selectedIndex + 1 )}");

Images and Other Resources

When XUL-Node is installed, a server root directory is created at a user-specified location. By default it is C:\Perl\xul-node on Win32, and /usr/local/xul-node elsewhere.

You place images and other resources you want to make available via HTTP under the directory:


The example images are installed under:


You can access them from your code by pointing at the file:

     label => 'a button',
     image => 'images/button_image.png',

Any format Firefox supports should work.

XUL-Node API vs. the Javascript XUL API

The XUL-Node API is different in the following ways:

  • Booleans are Perl booleans.

  • There is no difference between attributes, properties, and methods. They are all attributes.

  • There exist constants for common attribute key/value pairs. See XUL::Node::Constants.


XUL-Node acts as a very thin layer between your Perl application, and the Firefox web browser. All it does is expose the XUL API in Perl, and provide the server so you can actually use it. Thus it is very small.

It does this using the Half Object pattern ( XUL elements have a client half (the DOM element in the document), but also a server half, represented by a XUL::Node object. User code calls methods on the server half, and listens for events. The server half forwards them to the client, which runs them on the displayed DOM document.

The Wire Protocol

Communications is done through HTTP POST, with an XML message body in the request describing the event, and a response composed of a list of DOM manipulation commands.

Here is a sample request, showing a boot request for the HelloWorld application:


Here is a request describing a selection event in a MenuList:


Here is the response to the HelloWorld boot request. The 1st line of the boot response is the session ID created by the server.

  Li6iZ6soj4JqwnkDUmmXsw, 0)
  E2.set(sizeToContent, 1), E2)
  E1.set(value, Hello World!)

Each command in a response is built of the widget ID, the attribute/property/method name, and an argument list.

The Server

The server uses POE::Component::HTTPServer. It configures a handler that forwards requests to the session manager. The session manager creates or retrieves the session object, and gives it the request. The session runs user code, and collects any changes done by user code to the DOM tree. These are sent to the client.

Aspects are used by XUL::Node::ChangeManager to listen to DOM state changes, and record them for passing on to the client.

The XUL::Node::EventManager keeps a weak list of all widgets, so they can be forwarded events, as they arrive from the client.

A time-to-live timer is run using POE, so that sessions will expire after 10 minutes of inactivity.

The Client

The client is a small Javascript library which handles:

  • Client/server communications, using Firefox XMLHTTPRequest.

  • Running commands as they are received from the server.

  • Unifying attributes/properties/methods, so they all seem like attributes to the XUL-Node developer.


  Box, HBox, VBox, Label, Button, TextBox, TabBox, Grid, CheckBox,
  GroupBox, MenuList, ListBox, Splitter, Deck, Spacer, HTML_Pre,
  HTML_H1-4, HTML_A, HTML_Div, ColorPicker, Description, Image, Stack,
  Radio, ProgressMeter, ArrowScrollBox, ToolBox, ToolBar,
  ToolBarSeperator, MenuBar, Menu, StatusBar.


  Window, Box, HBox, VBox, Label, Button, TextBox, TabBox, Tabs,
  TabPanels, Tab, TabPanel Grid, Columns, Column, Rows, Row, CheckBox,
  Seperator, Caption, GroupBox, MenuList MenuPopup, MenuItem, ListBox,
  ListItem, Splitter, Deck, Spacer, HTML_Pre, HTML_H1 HTML_H2, HTML_H3,
  HTML_H4, HTML_A, HTML_Div, ColorPicker, Description, Image ListCols,
  ListCol, ListHeader, ListHead, Stack, RadioGroup, Radio, Grippy
  ProgressMeter, ArrowScrollBox, ToolBox, ToolBar, ToolBarSeperator,
  ToolBarButton MenuBar, Menu, MenuSeparator, StatusBarPanel, StatusBar.


  • Some widgets are not supported yet: tree, popup, and multiple windows

  • Some widget features are not supported yet:

      * multiple selections
      * node disposal
      * color picker will not fire events if type is set to button
      * equalsize attribute will not work
      * menus with no popups may not show

See the TODO file included in the distribution for more information.


XUL::Node::Event presents the list of all possible events. has a good XUL reference. is the browser home page.


None known so far. If you find any bugs or oddities, please do inform the author.


Ran Eilam <>


Copyright 2003-2004 Ran Eilam. All rights reserved.

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.