2.2. Basic XUL Concepts

You have already seen many of XUL's basic features at work. When you load the example in the previous example, the browser identifies it as a XUL file, parses the data, creates a new window and draws the button widget, and executes the function you've defined when the button is clicked.

These activities are part of the basic and often transparent interaction between your application files and Mozilla. However, the format of your XUL files, their syntax and namespace, the XUL layout, and the windowing system are all basic to successful XUL programming.

2.2.1. The XUL File Format

A XUL file is a simple text file that contains proper XML syntax and has a .xul file extension. Mozilla expects to draw UI widgets when it encounters a file with a .xul extension or when it encounters the XUL namespace in other markup files that it recognizes, including HTML and XML.

The MIME type registered for XUL files is application/vnd.mozilla.xul+xml. When editing and using XUL files locally, you shouldn't need to worry about setting this on your computer; however, sometimes you may need to set the MIME type, such as when you host XUL files on a server. Chapter 12 provides additional information about how you can set the correct file type for your system.

2.2.2. Conventions

XUL has to follow certain conventions (as does XHTML or any other XML-based file) in order to be valid. Mozilla generates an error when it encounters an invalid XUL file.

The first thing required in a XUL document is the XML declaration.

<?xml version="1.0"?>

Any comments used to introduce your file can begin on the line after the declaration. Comments in XUL follow the same format used in HTML and XML, delimited by <!-- and -->.

All tag sets must be closed. Empty tags are allowed for some elements, such as the <label> element, that are used without nested elements or content. Note that a trailing slash at the end of the tag is required to denote an empty element.

<label value="Getting Started" />

Another thing to remember is that XUL is case-sensitive. Closing a XUL <window> tag with </Window> renders it invalid.

These conventions ensure that the rendering engine can parse the XUL file successfully and display the elements defined there. Mozilla does not validate XML files, such as XUL, and it does not resolve externally parsed entities, but it does check for document well-formedness.

Following the XML specification, Mozilla ignores well-formed tags that it does not recognize, which can give your applications extra flexibility, particularly as you begin to use technologies such as XBL. But it can also make debugging a little more difficult, as when you create an element named <botton> and don't see why your XUL button doesn't have the typical borders or three-dimensional style.

A good practice to follow when creating XUL files is to use comments, copious whitespace, indentations (but not tabbed indentations where you can avoid them), and XUL widgets you are familiar with.

2.2.3. The XUL Namespace

Like other markup vocabularies, XUL uses a namespace declaration to define the particular elements that may be included in a valid file. Example 2-2 shows a sample of the required namespace declaration. The namespace is an attribute of the root window element. The lack of any suffix on the XML namespace declaration (i.e., xmlns:xul) indicates that XUL is the default namespace for this file.

If you want to include XUL content in documents that use other types of markup, you need to declare more than one namespace. Common namespace declarations for getting other language elements into your code include HTML and RDF, but you can invent your own as well. If you wanted to put the button from Example 2-1 into a vanilla XML file, for example, you could place it into an XML document by using the xmlns:xul attribute, as shown in Example 2-3.

This file has three types of content: XUL, HTML, and customized markup called flies. When you use mixed namespaces, you have to prefix the XUL elements with xul: to distinguish them from markup in other namespaces, as with the xul:box and xul:button shown in Example 2-3.

2.2.4. Basic XUL Layout

Example 2-1 features some very common XUL elements. In this section, each element is dissected to show what it does and how it interacts with other elements. The <window> element is the root of individual primary XUL documents (in contrast to dialogs that pop up from windows, which can use <dialog> as the root, and XUL documents loaded within other XUL containers, which can use <page>).

As in HTML, the root element defines the document into which all elements are drawn, but in XUL, that document is a piece of an application interface and not a web page. We'll have more to say about the window and some of its features in the second example.

A <box> element that contains a <button> is inside the window in Example 2-1. Although you can use attributes on the window element to lay out and position window children, it's never a bad idea to use the <box> as a container, particularly when you add new layout to your document, such as rows of buttons, grids, tabs, or other elements that need to be arranged precisely within the space of the window. The box is the basic element for layout in XUL.

The align attribute on the box specifies that the children do not stretch and center themselves in the middle of the available space. If the box was omitted and there were multiple children of the root window, they would be laid out vertically by default, one under the other. This setting can be overridden by adding the orient attribute to <window> and giving it a value of "horizontal."

2.2.5. Using XUL Windows

The foundation of an XPFE application is a window. Each XUL document has to have at least one XUL <window> element, and it must be the root of the document -- the surrounding, outermost element in the XML document, set apart from the XML declaration itself and other processing "preambles." A basic window with no content looks like this:

<?xml version="1.0"?>
<!DOCTYPE window>

Commonly, an application has more than one window, with a number of dialogs and secondary windows. Each window is also contained within a <window> element (though recent additions to the XUL specification include the dialog and page elements, which are derived from window and can be used in its place as root elements in your XUL files).

As your application becomes more complex, you need a way to keep track of the windows and ensure that they can communicate with one another. In Mozilla, there is a way to do this by using the type attribute identifier, which allows you to use special window-opening functions like toOpenWindowByType( ) to manage particular window types. Window features

An id attribute is present on the <window> element. Using this attribute is not necessary to run the windows system, but it is a good idea to give each window a unique identifier because it makes nodes easier to find from script (see the DOM method getElementByID in Chapter 5 for information about how to get elements by identifier). This is how to set up an ID attribute:


Load event handlers such as onload and onunload are useful and necessary if you want to add behavior to a window, pass input to it, or manipulate its content depending on context:

    onload="startUp( )"
    onunload="shutdown( )"
    onclose="onClose( )">

When you load a XUL file that begins in this way, the event handler attributes onload and onunload carry out the functions listed as values (startUp( ) and shutdown( )). In addition, Mozilla provides an onclose event handler that intercepts the upcoming window closure to carry out any extra processing you need. The close event is fired before the unload event, so you can stop the window from closing in the onclose event handler if necessary. To stop window closure, the close event must return false.

Additional handlers are available for dialog windows. They are listed and their use is outlined in the section Section 3.2 in Chapter 3. Window properties

The window declaration is expanding, but there is still plenty of room for more features. In addition to the attributes -- the event handlers, the ID, and the namespace that appear within the <window> tag itself -- a XUL window also has all of the properties of the DOM window object from HTML. These properties are listed below, along with additional properties for application specific tasks.

Special properties of the XUL window object include:

<browser type="content-primary" ...>

Subsequently, you can access and manipulate the content.

window.content.focus( );

window.sizeToContent( )

This property is used to ensure intrinsic sizing, which is important in XUL application development, especially in dialog windows. Intrinsic sizing ensures that the window adapts and morphs to fit the content. This is preferable to constraining your window with a fixed width and height when the onload handler anticipates changeable content, depends on context, or takes input from another window. The colorpicker in the Mozilla Editor, for example, uses this function to make sure that the window displaying the chosen palette shrinks to fit that palette:

function ChangePalette(palette)
  gDialog.ColorPicker.setAttribute("palettename", palette);
  window.sizeToContent( );
} Window behavior

Mozilla supports the standard window.open JavaScript function, which has its origins in the world of browser scripting and the launching of new browser windows. Mozilla extends the function to provide some features for application development. It also provides the window.openDialog function for opening windows in the XPFE scripting environment. The latter function has become the more commonly used method to open a new XUL window, although the two are interchangeable.

The usage of window.open is:

window.open (url, name, features);

window.openDialog extends this functionality with a new argument list passed to it, which is optional and can be any number of arguments as needed:

window.openDialog (url, type, features, argument1, argument2);

Here is a list of some of the features of a XUL window opened using window.openDialog:

A comma delimits the features list and the entire list must be in quotes. The script that handles the new window accesses the arguments list:


The window created in this example will be modal and use the message that was passed to it in the variable message. By default, Mozilla assumes that the chrome feature is on when you use either window.open or window.openDialog in a chrome environment, and creates a new window in the window hierarchy.