Creating Applications with Mozilla-Chapter 7. Extending the UI with XBL- P2

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Nội dung Text: Creating Applications with Mozilla-Chapter 7. Extending the UI with XBL- P2

Chapter 7. Extending the UI with XBL- P2 The code you see here is not necessary in this binding, but it shows you the format for executing code in the destructor. Destructors are great to use when you have loaded a component in the binding and want to set the variable representing the component to null to avoid memory leaks. If you have a lot of bindings that load and then unload components while a Mozilla session is still running, a significant amount of memory can be eaten up unless you take this kind of step. 7.2.3.3. Properties Like Java properties, an XBL has getters and setters, so they can behave differently when you read or write data. Here is a subset of properties used in this binding: return this.input.value;
return this.input.inputField.style.backgroundColor; this.input.inputField.style.backgroundColor=val; return val; At this point, the characteristics of properties to watch out for include the readonly attribute, the getter and setter elements, and the existence of a val keyword that is used internally for accessing a property's current value. For your reference, this binding's property extracts are used for getting the value of the input field, returning an uppercase version of the inputted text, and getting or setting the input field's background color. 7.2.3.4. Methods Methods in XBL are self-contained functions represented by the tag and encapsulated within the element. They usually provide a binding object with a specific function like copying and saving some data or showing and hiding widget controls. Like properties, they can be called from within the binding, from another binding that subclasses that binding, and directly from the bound element.
this.input.value=''; this.input.value=newValue; The method code is contained in a tag, and each method can have 0 or more parameters, which gather the values passed into the method when called. 7.2.4. Handlers Handlers in XBL mimic regular document events like onclick and onmousedown, and provide a means for trapping them within your binding and carrying out tasks associated with them. this.input.focus( );
Each handler is contained in a tag and the event name is placed in the event attribute -- minus the "on" prefix. The handler in the code shown above places the focus in the inputfield when the mouse goes over it. See the section "Event Handling," later in this chapter, for more details. 7.2.5. Style The sample binding's last piece of the puzzle is style. Any XUL elements used in the content inherit the default styles for those widgets, but if you want to add more, you can include your own stylesheet like this: Notice the container element, which is a prerequisite for this feature. The use of stylesheets in bindings is covered more thoroughly at the end of the chapter in the section Section 7.7. At this point, you should be familiar with the pieces that make up a binding, which, at the top level, are the content, implementation, event handlers, and extra resources. The binding that you have constructed is small, yet it shows all the main concepts involved in structuring a binding. With some personalizing, it could be included in potentially any application. 7.3. Adding Behavior to Bindings Like XUL widgets, XBL uses JavaScript to provide functionality to bindings by accessing XPCOM methods via XPConnect. Like binding content, behavior is optional in a binding. Each can exist without the other. At times,
you might want only implementations, such as a base binding that contains certain properties that are inherited by other bindings. The element is the container for all other elements that make up a binding's behavioral portion. Example 7-1 shows an empty implementation shell that highlights the element's contained hierarchy. Example 7-1. XBL implementation element The code in Example 7-1 shows the element having a constructor, destructor, method, property, and field as possible children. Each component can exist in quantities of zero or more, with the exception of the constructor and destructor, of which there can be only zero or one.
The rest of this section describes each binding implementation component in more detail. 7.3.1. Binding Methods Bindings can exist solely as content generators, acting passively when they are drawn. But you can also create bindings that provide new capabilities and more interactive functions or that execute routines in your application. In the spirit of self-containment, functions can be added to a binding that carry out functionality related to that binding. These functions are the behavior of a binding. The ideal way to add behavior is to add methods to your binding with the element. Each parameter for a method defined in the element is contained within its own tag.
To use the method to print text to the command shell, call the name that is specified in the name attribute. All methods created in a binding are added to the binding element object and called as members of that object, as shown here: Using the
Two special methods exist in XBL that allow you to manipulate what happens when a binding is added or removed from a document. These methods use the native and tags. A constructor is valuable, for example, when you need to initialize a binding object or prefill the UI based on stored values. Destructors can clean up when a binding is discarded. When bindings are attached, their content is inserted into the document at the point you specify. After insertion -- or, more specifically, after the attachment occurs -- the code in the executes. Similarly, you can use the element to execute functions when a binding is destroyed. This example prints some text to output, but you can include code that carries out variable initialization or anything else you want to occur at these stages of the binding's life. Bound elements constructed before the document
load event execute this script before the document load event fires. In the case of extended bindings, base handlers are fired first, followed by derived handlers. 7.3.2. Binding Properties Properties on a binding are included by using the element. Fundamentally, properties are used to hold and manipulate values that are used elsewhere in the binding, such as when a text widget has its displayed value changed periodically by the application. Currently, there are two classifications for properties, one of which gets a raw value directly on the element. false; This example always sets the attribute named someAttribute to false. Though still supported, this use of the element was replaced by a new element called . Although it is not in the XBL 1.0 specification, the element is implemented and used in the Mozilla code base. It has the same syntax but a different name. We recommend using the element. false;
The second property usage defines functions that are carried out when getting and setting a property's value. Take this anonymous content with a label child: This content simply shows some text on the screen. The bound element that uses this content looks like this: The XUL label used in the binding inherits the title attribute value that is set on the bound element. Example 7-2 shows how to set this title and its retrieval. Both access the bound element, and any changes filter down to the label. Example 7-2. An XBL property setting and getting a value
The script keyword val is used internally to represent the latest property value. The request to change the property or retrieve its value can come from another property in the same binding (this.), from a binding method, or from a method in the bound document that accesses the binding object directly. This JavaScript sets the value of a property named title on the binding object: var titleElement = document.getElementById("my- binding"); titleElement.title = "The Adventures of an XBL hacker"; You can use the onget and onset attribute as an alternative to and elements. Properties are initialized after the content is generated but before the binding attached event is set off. This ensures that all properties are available once that event occurs.