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        <p>
        Generally, the term component denotes an object that is reusable and can interact with other objects in the context of a framework. The term control, on the other hand, denotes an object that is sort of a specialized component. The main trait that differentiates components and controls is the user interface. Components are non-UI objects; controls are primarily UI-based objects.

        <p>The execution of each asp.net ajax page is controlled by an application object that is intstantiated in the body of the library. Whenever an asp.net ajax page is loaded in the browser, an instance of Sys._Application class is promptly created and assigned to the Sys.Application object: Sys.Application = new Sys._Application();
        </p>

        <h3>Components</h3>
        <ul>
        <li>Derive from the component base class</li>
        <li>Typically have no UI representation, such as timer component that raise events at intervals but is not visible on the page.</li>
        <li>Have no associated DOM elements</li>
        <li>Encapsulate client code that is intended to be resuable across applications</li>
        </ul>

        <ol>
        <li>Sys.Application.init(event handle need Sys.Application.add_init(MyInit);</li>
        <li>Sys.Application.load (event handler function pageLoad(sender, args){} </li>
        <li>Sys.Application.unload (event handler function pageUnLoad(sender, args){}</li>
        <li>Sys.Application.disposing( inherit from Component class)</li>
        <li>Sys.Application.propertyChanged( inherit from Component class)</li>
        </ol>