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AJAX Components

Declarative Components

Although defining components in an imperative manner can be useful, it is also increasingly common to see components defined using a declarative approach. You probably already know at least one declarative language; HTML and XSLT are two common examples of declarative languages. When using declarative components, the developer doesn't need to worry about how things are achieved behind the scenes but instead only needs to worry about declarative structure; for example, in HTML, the web browser parses and interprets the tags based on some predefined set of rules. The result of this is that when the HTML parser finds text surrounded in <em> tags, it presents that text with emphasis. Exactly how the text is emphasized by default is left up to the web browser, although that can, of course, be overridden by the developer using CSS. Because the markup or declaration specifies what a component does rather than how it works is the biggest advantage to declarative programming.

When discussing imperative coding, you learned that making a ham-and-cheese sandwich ended up being a bit of a pain to achieve the right outcome. On the other hand, a ham-and-cheese sandwich created using a declarative approach would go something more like this:

Ham and cheese sandwich please. Enjoy!

Rather than having to specify each of the steps involved in making the sandwich, it is more like going to your local café and ordering the sandwich from the waiter. It is certainly fewer steps and probably a lot more convenient to make the sandwich declaratively rather than imperatively; however, there are some drawbacks. The most apparent drawback here is that if you aren't careful, the waiter might bring you a ham-and-cheese sandwich without any mustard!

You might be familiar with declarative programming from any one of the gamut of server-side web application frameworks employing a declarative approach, such as JavaServer Faces, JSP, and ASP.NET. In these languages, you can define a part of the page using a declarative syntax that is then processed on the server and produce standard HTML that is delivered to the client like any other web page.

Server-Side Declarative Programming

In ASP.NET, you can define a web page with a declarative DataGrid control like this:

What happens to this declaration is that the .NET framework loads the ASPX web page containing the declaration, and the declaration is processed and replaced with regular HTML by the server, which then gets streamed up to the client as though it were plain HTML page. Of course you can see that there is certainly more to the story than just that simple declaration because there is no mention of what data is to be rendered in the DataGrid. Although these various server-side technologies do provide a nice declarative interface, they still require a little bit of code to hook everything together. Behind the scenes of the ASPX HTML page is a code page that might have some C# code such as this to connect the DataGrid to a database:

By combing the declarative and imperative approaches, developers get the best of both worlds, enabling them to develop a simple application rather quickly, still having the control to tweak all aspects of the application components.

There are many advantages to taking a declarative approach to building applications. The most obvious advantage of markup is that it is more "designable" than code in that it enables far better tool support. ASP.NET or JavaServer Face components in Visual Studio or JavaStudio Creator are good examples of this where you can drag components into a web page during application design and visually configure without writing code.

The fact that a declaration is just a snippet of XML means that XML Schema can be used to ensure that a declaration adheres to an expected XML structure. Validating against a rigid XML schema makes declarative components much less error prone than the pure JavaScript counterparts. Writing declarations in a web editor such as Eclipse or Visual Studio can also be made easier by using autocomplete features (for example IntelliSense for Visual Studio) that ensure the declaration adheres to the XML schema as the declaration is being written. In fact, at some point, things can become even more simplified because a DataGrid in one declarative framework, like Adobe's MXML language, is little more than an XSLT transformation away from a DataGrid in some other language like XForms—thus, achieving the same functionality across platforms without changing and recompiling a single line of code. Of course, with some effort, this can be said of almost any programming language; however, declarative programming does have the advantage that the order in which statements are declared has no impact on the operation of the component, and declarations are XML-based and, therefore, readily machine readable.

Although a declaration can get a developer most of the way to building a great application, there is always that last little bit that requires more fine control to customize a component in specific ways. In these instances, you can still fall back on the programming language that the declarative framework is build on, be it Java, C#, or JavaScript.

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