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Controller
In the previous section we discussed a set of abstractions that are sub-classes of the component. A component class abstracts some behavior and to implement that, it might add a set of event listeners to its elements. Those event handlers are encapsulated in the component class and serves as its controllers. But in a bigger scheme of things, they are fully contained in the component class and can be considered a part of the View.

Besides component-specific listeners, there is a set of application-specific event listeners that respond to user inputs or timer events. These are called controllers. An event handler is a trigger point for some behavior implemented by the rest of the controller. This behavior can commonly be categorized as follows.

1. Controllers that simply change the state of the view such as color, enabled/disabled, hide/visible, and font based on some application logic.
2. Controllers that trigger re-rendering of the UI or its part due to some data change or navigation.
3. Controllers that execute some business logic or do computation. Here you need to worry about the security issues based on other parameters such as if the application is running over an intranet or Internet. Based on such parameters, the business logic can be executed in the browser space or delegated to the server. In almost all cases, if some other data look-up is needed, then it's recommended that the task be delegated to a back-end service through the controller servlet.
4. Controllers that fetch data from the back-end server, update the client model, and send a trigger to the View for re-rendering data.
5. Timer controllers that do something periodically. For example, a timer can periodically check for the availability of new updates on the server.

Controllers encapsulate application logic-specific details. Some of these details can be abstracted as base classes in the application code. We'll visit some of these controllers in the context of an application in the following sections.

MVC Interaction
We've described the Model, View, and Controller components of the MVC pattern. Now let's consider a typical sequence of operations on the client side to understand the interactions among them better.

• As a result of access to a URL by the user, the browser downloads a page and triggers the execution of certain controller code due to its "onload" event.
• In the first step, the controller retrieves the HTML template from the server using an AJAX call.
• Next the controller retrieves data from the server using an AJAX call and inserts it into the cache.
• Then the controller instantiates one or more components, creating an association between the components and HTML elements.
• To populate the data, the controller invokes a "setValue" call on one or more components.
• Lastly, the controller adds application-specific event handlers to the elements.

This content is reprinted from Real-World AJAX: Secrets of the Masters published by SYS-CON Books. To order the entire book now along with companion DVDs, click here to order.