General RIA Programming Model
Although there are many different RIA solutions based on different underlying technology platforms, the general RIA programming model is actually converging into a single common model.

Declarative UI Development
The general RIA programming model is centered on using an XML-based UI markup language to create a rich user interface. The XML-based UI markup provides a much higher level of abstraction than HTML for building rich user interfaces. XML UI frees programmers to focus on the application's core logic and explicitly complements the roles of a typical enterprise development team (see reference "XML for Client-side Computing").

Below are examples from a scripting-based approach (Laszlo Systems) as well as an OOP-based approach (Nexaweb). Both are zero-install and can run inside any popular Web browser today without any software download.

On the client side, Laszlo requires a Flash engine (Flash 6 and above) while Nexaweb requires a JVM (JDK 1.1 and above).

Laszlo is a Flash-based RIA solution. It uses an XML UI markup language called "lzx" to describe the UI and uses ActionScript to code application logic. (See Figure 1) The Laszlo server will automatically compile lzx files into the Flash binary format (SWF), deliver the SWF files for rendering inside a Flash engine, and execute the application. Following is an example of the Laszlo code:

<canvas height="450">
   <window x="10" y="10" width="300" height="200" title="my window" resizable="true" closeable="true">
      <button x="10" y="100">Hello, World</button>
   </window>
</canvas>

By comparison, Nexaweb is a Java-based RIA product. Developers would use an XML-based UI markup to create a rich user interface and build client-side business logic by writing client-side Java objects called Managed Client Objects that are standard Java program objects. (See Figure 2) The Nexaweb client runtime dynamically renders the XML UI markup to present a rich user interface, and dynamically downloads client-side Java objects to the client side for execution in a "on-demand" fashion. Here is a simple Nexaweb UI that defines a tree and a button managed by a layout manager:

<xal xmlns="http://www.openxal.org/xal">
   <window title="New Window">
   <boxLayout orientation="vertical" pack="start" align="stretch"/>
     <tree>
       <column/>
         <row expanded="true">
           <cell text="Tree Item 1"/>
           <row>
             <cell text="Sub Tree Item 1"/>
           </row>
           <row>
             <cell text="Sub Tree Item 2"/>
           </row>
         </row>
         <row expanded="true">
           <cell text="Tree Item 2"/>
           <row>
             <cell text="Sub Tree Item 3"/>
         </row>
       </row>
     </tree>
       <button text="OK"/>
   </window>
</xal>

As shown in these two code examples, though Nexaweb uses Java and Laszlo uses Flash, RIA UI development is conceptually identical between the two different RIA solutions.

Application Logic Development
RIA solutions enable a wide range of options for application logic development. In typical HTML applications, most of the logic has to be on the server side. In typical desktop applications, most of the logic resides on the desktop. RIAs offer significant flexibility so developers put the logic either on the client side or server side. They can also adjust the location - ranging from very limited logic on the client side all the way to almost 100% logic on the client side, as shown in Figure 3.

The flexibility in partitioning application logic brings significant benefits. Some applications are best suited to having all their logic centralized on the server side while other applications require that the logic run on a local desktop. Traditionally, developers have to make tradeoffs depending on whether they choose to build the application as Web application or a desktop application, and bear with the problems associated with that particular choice. RIAs combine the best of both worlds, enabling developers to meet different application requirements without making costly tradeoffs.

In contrast to UI development, the choice of a particular RIA approach - Java, AJAX, .NET, or Flash - has a direct impact on and creates significant differences in application logic development. Choosing a scripting-based solution requires that the logic be written as scripts, which limits the amount and scope of logic that can be developed and maintained cost-effectively. An OOP-based RIA solution gives maximum flexibility to logic development and maintenance, but requires a higher-level skill set than scripting.

There's a code sample of logic development using Laszlo in Listing 1.

By contrast, OOP-based approaches use an object-oriented true programming language for the application logic development and typically enforce separation between the logic and the UI markup. For example, Nexaweb uses standard Java for the application logic called mco. Nexaweb also enforces a clear separation between the UI and logic, preventing the mixing of the UI with the logic in the same document for better application maintenance. (See Figure 4) Nexaweb separates the UI from the application logic:

<xal>
   <mco:declarations xmlns:mco="http://nexaweb.com/mco">
     <mco:mco id="myMco" src="com.nexaweb.test.MyTestMco"/>
</mco:declarations>

   <dialog title="Login Dialog">
     <boxLayout orientation="vertical" pack="start" align="start"/>
     <label text="Username:"/>
     <textField height="25" text="enter username here..." width="200"/>
     <label text="Password:" />
     <textField height="25" text="enter password here..." width="200"/>
     <panel height="25" width="100"/>
     <button height="25" text="Button" width="100" onCommand="mco://myMco.handleOnCommand()"/>
   </dialog>
</xal>