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.[@more@]

2.2 基本XUL概念
你已经看到了XUL的基本功能是如何工作的了。当你再上一个例子中加载文件的时候，浏览器识别出这是一个XUL文件之后，就解析了数据，创建了一个新窗口，并画了一个按钮组件，并且在按钮被点击以后执行了你指定的函数。

这些行为都是基本功能的一部分，提供了你和浏览器之间透明的交互作用。但是，xul文件的格式，语法以及名称空间，xul布局元素以及这个窗口系统都是成功的xul编程的基础。

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.

2.2.1 xul文件格式

一个xul文件只是一个简单的包含了正确的xml语法的后缀为xul的文本文件。当Mozilla碰到一个xul文件或者在其他标记式文件中碰到xul名称空间下的元素时，他会尝试去描绘一个用户界面组建，这里所说的标记式文件包括xml和html。

xul文件的MIME类型是application/vnd.mozilla.xul+xml。当在本地编辑和使用xul文件时，无需设置mime类型。但是有时候你会需要设置mime类型，比如把xul文件放在服务器上的时候。第十二章具体给出了如何在你的系统上正确设置xul的mime类型。

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.



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 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.



Another thing to remember is that XUL is case-sensitive. Closing a XUL tag with 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 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.

2.2.2 惯例

为了编写合格的XUL文件，必须遵从一些惯例，否则Mozilla就会报错。首先是XML声明，其他的内容都必须在这一句后面。注释的格式和HTML以及其他xml文件一样，以<!--开始，-->结束。所有的标签必须闭合，如果是单个非嵌套标签，则以/>方式结束。另外就是xul的标签名必须严格按照正确的大小写拼写。

这些惯例确保了渲染引擎可以成功的解析xul文件并且显示文件中所定义的元素。Mozilla并不校验xml文件，而且也不解析外部实体，但是它确实检查文件的格式完整性。

遵循XML规范，Mozilla会忽略那些它并不认识但格式良好的标签，这些标签可能会给你的应用程序增加额外的功能，尤其是当你开始使用XBL这样的技术时更是如此。但是这样做也会使程序调试的难度有所增加，比如你可能会奇怪为什么你定义的按钮标签不具有通常按钮所具有的边框和立体外观。

一个比较好的做法是在建立xul文件的时候使用注释，大量的空白和缩进（但是能不用tab键的时候就尽量不要用），以及你所熟悉的xul组件。

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.

Example 2-2. The XUL namespace declaration

xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
Illustrating the XUL namespace


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.

Example 2-3. Mixed namespaces in an XML document

xmlns:flies="http://www.flies.com/come.fly.with.me.xml#"
xmlns:html="http://www.w3.org/1999/xhtml"
xmlns:xul="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">








2.2.3 XUL名称空间

像其他标记式语言一样，xul使用名称空间来声明那些可以在合法的文件中出现的元素。例子2-2展示了一个需要名称空间声明的情况。名称空间是根窗口的一个属性，如果名称空间声明中没有任何后缀，则表示这个名称空间是这个xul文件的默认名称空间。

如果你想在使用其他标记元素的文档中包含xul内容，你就需要声明不止一个名称空间。比较常见的声明可以用来把包括HTML，RDF在内的其他语言内容包含到你的代码中，但是你也可以定义自己的标记语言。例如，如果你想把例子2-1中的按钮放到你的xml文件中，你就可以像例子2-3那样，使用一个xmlns:xul属性来达到目的。

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 element is the root of individual primary XUL documents (in contrast to dialogs that pop up from windows, which can use

as the root, and XUL documents loaded within other XUL containers, which can use ).

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 element that contains a 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 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 and giving it a value of "horizontal."

2.2.4 基本XUL布局

例子2-1只是使用了几个非常基本的xul元素。本节我们会讨论每一个元素，说明他们怎么和其他元素进行交互作用的。元素是一个独立存在的XUL文件的根元素（与此不同的是，定义对话框的时候

元素则是根元素，而如果xul被其他xul容器装入的时候，则可以使用元素作为根元素）。

就像在HTML中一样，根元素定义了所有其他元素将怎样被渲染，但是在xul中，一个XUL文件只是程序界面的一部分，而不是一个web页面，所以我们将会在第二个例子中详细的介绍元素及其特性。

在例子2-1中我们用了一个元素来包含一个

元素，尽管可以使用window元素的属性来指定这些子元素的布局和位置，使用box元素总不是一个坏主意，尤其是当需要在文件中添加其他元素的时候，比如一行按钮，表格，标签或者其他需要在window元素内精确定位和摆放的元素。总之，box是xul中用于布局的基本元素。

box的align属性指定了它的子元素不要拉伸以适应box的大小或者居于box的中心位置。如果没有box元素，而window又有多个直接子元素，那么他们默认会从上到下竖向排列。当然这个也可以通过把window元素的orient属性设定为"horizontal"来改变。

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 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:



xmlns:html="http://www.w3.org/1999/xhtml"
xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">


Commonly, an application has more than one window, with a number of dialogs and secondary windows. Each window is also contained within a 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.

Tip

As with any existing Mozilla functions referred to in this book, you can look up toOpenWindowByType by using the LXR web-based source code viewer, described in Appendix A and available at http://lxr.mozilla.org/.

2.2.5 使用XUL窗口

XPFE应用程序的基础就是一个窗口。每一个XUL文件必须至少有一个元素，而且必须是这个文件的根元素，包含了除xml序言的所有其他元素。一个最基本的空窗口基本上是这个样子的：



xmlns:html="http://www.w3.org/1999/xhtml"
xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">


通常一个应用程序有不只一个窗口，加上一些对话框和二级窗口。每一个窗口都是是由元素定义的（尽管最近的XUL规范增加了dialog和page元素，也可以用作xul文件的根元素，但是他们也都是由window元素继承而来的）。

当应用程序变得越来越复杂的时候，就需要一种方式来跟踪这些窗口并且确保它们互相之间可以正常的通信了。在Mozilla中可以使用type属性来达到这样的目的，在Mozilla中可以使用toOpenWindowByType()这样的函数来管理特定的窗口类型。

提示：
所有在这本书里面涉及到的Mozilla内置函数，都可以在基于web的代码浏览器LXR来查看，具体参见附录A，LXR则位于http://lxr.mozilla.org/。