Creating Applications with Mozilla-Chapter 7. Extending the UI with XBL- P1

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Nội dung Text: Creating Applications with Mozilla-Chapter 7. Extending the UI with XBL- P1

Chapter 7. Extending the UI with XBL- P1 You now know that XUL is the basic tool set for creating your application interface, but even it has limitations. It is just a finite set of widgets that your programming needs may transcend. If you find that you reimplement many of the same groups of widgets in different applications, or if you want to extend your application's interface in some other way, you will find the eXtensible Binding Language (XBL) an invaluable tool. This chapter describes what XBL is and how it is used. Basically, XBL provides a way to attach new content or behavior to your application by using XBL bindings. XBL can extend, add to, and reorganize user interfaces. XBL can also help you organize scattered XUL code into a set of self- contained widgets that make building and maintaining your Mozilla application much easier. Appendix C provides a reference for the XBL element set, with which new elements can be created. 7.1. What Is XBL? XBL is an XML markup language invented specifically for creating widgets. XBL looks similar to XUL, and may even contain XUL or HTML and other markup (see the Section 7.1.3 section later in this chapter for more information about how other markup is used in XBL bindings), but its purpose is different. Flexibility and interoperability are the point of XBL. If the XUL textbox is inadequate, for example, you can use XBL to create and attach a new widget called , possibly based on textbox, that provides special attributes and functionality for validating input data against a database.
A binding is a single XBL language entity that can contain content as other markup (such as XUL) behavior that is represented as methods and properties, and event-handling capabilities. Bindings can be anything from small widget objects to large, complex blocks of code with extensive functionality. Figure 7-1 shows the different components that make up a binding: fields, properties, functions, event handlers, and content. The section Section 7.2, later in this chapter, provides more detail about a binding's structure. Figure 7-1. Mozilla XBL binding structure Bindings differ from XUL overlays because they are fully self-contained, reusable, and generally have no dependencies outside of the binding itself. Although XUL is used most often as content in an XBL binding, XBL can also bind to and from HTML and XML. If you have worked with Java or C#, you may recognize some parallels between XBL bindings and Java objects. 7.1.1. XBL Terminology The following terms are used to describe XBL and its use in the XPFE: XBL
An acronym for the eXtensible Binding Language. In some contexts, the term XBL refers to actual code (e.g., "the XBL in this example . . . "). XBL is an XML syntax. Binding A single unit of the XBL language, one or more of which is contained in a binding document. Most bindings are made up of content and implementation, although each are mutually exclusive; if you add event handlers to that list, each one can appear on its own in a binding. Binding document An XBL file with an .xml extension that contains one or more bindings. Bound document A XUL (or HTML) document that has one or more bindings attached to it as content. Bound element A bound element is a widget or element that uses a particular binding. It can be an existing element in the XUL or HTML set or a newly invented one. Anonymous content Content (e.g., XUL elements) contained in a binding that is hidden from the document object (DOM). Refer to the section Section 7.4, later in this chapter, for a more detailed discussion of its
characteristics and how to programmatically gain access to the content. Attachment and detachment Attachment is the process through which a binding is associated with a bound element. It is essentially a way of telling the element which binding to use. Detachment is the process of removing that link and with it, the binding display. Insertion point The point in anonymous content at which children of the bound element are inserted. The section Section 7.4.4, later in this chapter, details the insertion process. Inheritance During inheritance, characteristics of one object are passed on to another object. In XBL, this process is multifaceted. Bindings can inherit from other bindings, anonymous content can inherit attributes from the bound element, and a binding implementation can inherit the behavior of another widget. All concepts are explained in the section Section 7.5, later in this chapter. 7.1.2. An XBL Document XBL documents are files saved with an .xml filename extension. Most bindings implement XUL content and behavior with script, so XBL files reside in your XUL application's chrome content area and have full access to XPConnect-wrapped XPCOM objects.
Several bindings often reside inside the same XBL file. Performance benefits from this arrangement, if you have multiple related bindings, because only one XBL document needs to be loaded, rather than multiple documents. Organization is another factor. Mozilla has dozens of bindings that are interrelated by either inheritance or filename identifiers. Individual pieces to a menu widget reside in a file called menu.xml, button bindings are in button.xml, and so forth. Keeping these bindings together is wise. The XBL document's root container is the tag. Inside this element is one or more individual child bindings, defined by the tag. A simple XBL document is as follows: An XBL document is a valid XML document. The XML preamble you are used to seeing in XUL files is present. It also contains the single root element (in this case, ) and the child nodes that define the bindings (empty). Bindings are the atomic units of an XBL document. An XBL document may define any number of individual bindings, each of which is bound (i.e., associated with other XML/XUL elements by way of CSS class definitions) somewhere in the interface. In other words, an XBL document may be a set of unrelated or barely related bindings that are picked up by the XUL interface.
7.1.3. Namespaces and XBL Because XBL is a binding language for other markup, remember to distinguish between the XBL markup (such as and ) and markup from another language (such as XUL). Namespaces are a feature of the XML language that was invented to handle this separation of intermingled markup, and XBL uses namespaces. For more information on namespaces, refer to the W3C at http://www.w3.org/TR/REC-xml-names/. Namespaces are declared in the root element of an XML document. The most common implementation is the declaration of two namespaces: a default namespace for XBL and a namespace for the other markup. This code shows the root of a bindings document in which the XUL namespace declaration (xmlns:xul) and the XBL default namespace are declared: An NCName is the part of a namespace declaration that qualifies the markup type for that particular namespace. It is placed after a colon, and in many XPFE documents, is the markup language name (xul, xbl, or rdf). The XBL namespace is the default in this instance because it does not declare a namespace prefix (NCName).
You can choose to namespace your document in a different way. For example, if you have a large mass of XUL code in your binding and do not wish to use the xul: prefix repeatedly, you can declare the XBL namespace as xmlns:xbl; you won't need to use prefixes on the XUL content since it is set as the default. Another option is to namespace a parent element: This code enables all children inside the to be in the scope of the XUL namespace; therefore the explicit xul: tag prefix declaration is not necessary. 7.1.4. XBL and HTML Although XUL usually makes up the content of an XBL binding in Mozilla, HTML is another valid and popular binding format. Using the XBL with HTML combination can be advantageous. With it, web pages (rendered in Mozilla) can be more feature-rich and move beyond the limitations of the HTML specification's finite element set. It means a possible mingling of one or many markup languages, including HTML, XUL, and RDF. The following snippet, in which a simple binding defines the name of the browser in an HTML div, gives you a feel for its potential: Mozilla 1.0
The bound element in HTML is called browser_name and is attached to the anonymous content in the HTML document's inline style. Browser Information browser_name { -moz-binding: url("brand.xml#browser"); } Guide ... Although the element is not a valid HTML element, one of XBL's great capabilities is that Mozilla finds the binding, reads the content there, and makes the substitution. The browser name can be included in several places in the HTML document. Like a poor man's DTD, the binding lets you change the definition of browser_name in one place and propagate that change to every instance of its use. This feature is useful because it requires the touching of fewer files during code maintenance. 7.2. Anatomy of a Binding
The best way to understand a binding is to watch one evolve from start to finish. This section examines a binding from its inception, into the construction of the binding, and through its use in a XUL document. At the end of the section, you should be able to take the pieces and construct your own binding, so try to follow it like a step-by-step guide. Design is important in XBL. The implementation can sometimes be tricky; for example, when you intend to reuse the binding elsewhere or when others use it in a way you don't foresee. Ideally, a binding should be as small as possible to facilitate reuse. And it's a good idea to separate your binding into smaller pieces -- perhaps smaller "subbindings" -- so you can recombine when necessary. You could design the widget mentioned in the introduction -- for example, as a combination of the XUL widget and your own new binding, , which you could then use elsewhere. The widget constructed in this section is a good example of a small, reusable binding. It is a special text input widget called inputfield -- a self- contained extension to a XUL textbox that can be used on its own or as part of another binding. The binding combines a and a , allows child elements, and offers functions that work with the data and style of the . 7.2.1. CSS Attachment Attachment is the process through which the binding is connected to the bound document that uses it. This process is most commonly achieved through CSS, but can also be done by using the DOM. The section Section 7.4, later in this chapter, details the interaction between XBL and the
document object model. The CSS connection begins where the bound element is placed in the XUL file: Remember that XML ignores elements it doesn't recognize, so this new element won't be rendered until you add information to the stylesheet; a binding is attached with the special -moz-binding attribute. The style selector must be associated with the bound element in the XUL file. In the following example, the binding is attached to every tag because the element name itself is used as the style selector. However, - moz-binding can also be inside a class selector, an ID selector, or any other CSS selector you wish to use: inputfield { -moz-binding: url("inputfield.xml#inputfield"); } It also can be from an inline style: The constituent parts of this style rule are the -moz-binding property, the url binding locator that takes the bindings file (and possibly the path to it) as a parameter, and the id of the binding denoted with the # notation. For the binding to take, the XBL file must contain a binding with the same id.
The ID of inputfield matches the value specified in the URL after the # symbol. When the UI is drawn in Mozilla, the binding content, behavior, and handlers are applied to the bound document at the point where the element appears in the document. Figure 7-2 shows a visual representation of the constituent parts of a binding attachment occurring via CSS. Figure 7-2. CSS binding attachment components
In this example, we use our own new element name called , but you can also extend existing XUL widgets by including: Because they are bound through CSS, bindings cannot be guaranteed to be loaded until the whole document is loaded, which means that any inline scripts accessing bindings should be considered incorrect because you cannot guarantee that the binding is loaded. XBL content is considered "invisible" in the context of the document object because it is not contained directly in the XUL document. Refer to the later section Section 7.4 for more information on this concept. Because a document binding can have multiple instances, something must happen to make the content unique in each one. When a binding is attached to a document, its content is automatically cloned in memory. Every instance of a binding shares the same fields, properties, methods, and event handlers because separate copies of those are simply not necessary. These elements cannot be changed dynamically, but the content document model can. 7.2.2. The XBL Content Element The element requires an id attribute to make the binding unique within the entire document. In the general XML specification, there can only be one element in a document that has a certain ID string. As in XUL, if you use an ID twice, the last one parsed is the only one seen. This
situation can lead to unexpected behavior. Figure 7-3 shows the appearance of an inputfield binding. Figure 7-3. The inputfield alone in the XUL document An has a attached to it, shown here as "Input Field." It also has a regular . The "Eric's" label is not part of the binding, but is still displayed inside of it as a child. Child content is discussed later in the section "Extra Binding Content and Insertion Points." The binding content is defined as: , the first element in the binding, lets any elements that existed in the original XUL document pass through the binding's display if it exists inside the tag.
In this case, the XUL label is inserted into the anonymous content at the point of the element when the binding is rendered. This ability is useful for changing the ordering of content and adding extra content within a binding. You can limit which tags are displayed as child content by using something like: These filtering capabilities open the possibility of multiple in your binding. The next content element is . Notice how the XML namespace of xul is used in the content. Using this notation is the most common way to apply namespace XUL elements in bindings. The label element has an XBL-namespaced inherits attribute. This code translates an attribute used on the original bounded tag into something usable by a content element: The final element in the content is a typical XUL textbox that has a namespace like the label. The anonid attribute on the textbox is fabricated and used here to avoid bugs and scope issues with the id attribute in content. The id attribute should be used only on the and tags, but anonid works well as direct DOM access to this element and is shown in the next section. 7.2.3. The Implementation Element
The next part of the binding, and also the most complex, is the behavior. The element contains the , , , , and -- all of which handle the binding's implementation features. All elements can contain JavaScript, which changes the binding into a dynamic widget that does more than display content on the screen. The binding implementation accepts user input, dynamically changes the UI, interacts with remote sites through web protocols, and surfaces Mozilla library functions in widgets. 7.2.3.1. Constructor In the example binding, some variables and style rules are set up for access by the rest of the binding to make the code cleaner. These rules are set up by using the constructor:
this.input.inputField.setAttribute("onchange",""); ]]> The first JavaScript command accesses the with the anonid label and puts it into the this.input variable. getAnonymousElementByAttribute is a custom DOM method used to access anonymous content. The section Section 7.4.1, later in this chapter, talks more about the XBL DOM methods. The use of the this keyword and the "dot notation" comes from Java. If you have programmed in Java, these bindings can be considered similar to Java classes. They are self-contained and can be extended. Using this is not necessary but it's common practice and clarifies that the variable or property is a member of the binding, especially if you define elements in the binding's constructor. In the next two commands, an object called inputField contains the style object property. You may be familiar with this structure in HTML elements, and in fact, this inputField is a version of the HTML textbox. The in XUL derives from that HTML element. The color and backgroundColor are set here manually to return something other than the initial value of a blank string when they are accessed. The last line in the sets up the onchange event handler for the textbox. This event handler is also used in a property for this binding. 7.2.3.2. Destructor
This section of a binding executes anything that needs to be done immediately before a binding is unloaded. Here is an example: this.input=null;