Encapsulation ... Again
There is one shortcoming in our CHECKBOX. The way we built it, the HTML injected during the constructor() contributes to the DOM of the page that hosts the HTC. Also global JavaScript variables like_value fall into the global scope of the document they're included in. This is dangerous since we run into the possibility of name clashes: the most obvious case is more than one instance of the same control. In addition, it presents a possibility that our control may accidentally reference other objects with the same names and vice versa.

To put it simply, a special mechanism is required to enable a truly modular approach for object authoring. Fortunately, HTC technology supports an intelligent answer - viewLink.

The easiest way to declare a control as encapsulated is to put one extra declaration between opening and closing PUBLIC:COMPONENT tags:

<PUBLIC:DEFAULTS viewLinkContent/>

Instantly, the control becomes encapsulated; it has its own HTML document tree, atomic to the master document. Every instance of the object has its own set of instantiated values and only public methods and properties can be accessed by the outside code. In other words, the viewLink mechanism fully enables the design and implementation of sophisticated Web applications using a true OO component-based approach.

In particular, we can simplify our code by removing uniqueID suffixes from the definition of internal checkboxes and HTML labels, since we are not afraid of name clashes anymore. Accordingly, we may replace the line:

eval( 'cb_'+uniqueID).checked = ( _value == onValue );

with the

cb.checked = ( _value == onValue );

as well as change addCheckbox() and addLabel() accordingly.

Conclusion
Since the AJAX race has just started, there are no AJAX standards and no commonly accepted RAD tools you can rely on to build your applications. While software vendors have a long way to go to create the robust development platforms, AJAX enthusiasts can prepare by encapsulating reusable blocks of code as business components with a well-defined API.

Navigating in this direction, this article outlined the OO "powers" of the AJAX language - JavaScript. It also illustrated one of the available component-authoring strategies - client-side custom tags technology. While we presented only IE-specific custom tags, we also provide a downloadable example of extensible bindings example for the Mozilla browser. All article-related examples can be downloaded from www.ajaxmaker.com/JDJ/AJAX/partII.html.