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Write AJAX-style Applications Using the ASP.NET 2.0 Client Callback Manager

With applications like Google Maps for inspiration, it seems everybody's been talking about what you can do with AJAX. But many don't know that writing AJAX-style applications that can do partial screen refreshes without a trip to the server are easy to write in ASP.NET using the built-in Client Callback Manager.

t seems everybody is talking about AJAX these days. The technology behind AJAX is hardly new, yet some really 'killer' new applications of XMLHttpRequest object have emerged recently, and these have breathed new life and excitement into the concept of client-side image refresh.

The most notable of these new applications is Google's Google Maps. Using the Google Maps (see Figure 1), you can search for a location in the U.S. and then zoom in into the map and drag and roll the map without needing to refresh the page.

AJAX is a collection of technologies that has been around since the early days of the Web. Here is what AJAX incorporates (according to Jesse James Garrett, who first coined the term 'AJAX' for Asynchronous JavaScript + XML):

  • standards-based presentation using XHTML and CSS;
  • dynamic display and interaction using the Document Object Model;
  • data interchange and manipulation using XML and XSLT;
  • asynchronous data retrieval using XMLHttpRequest;
  • and JavaScript binding everything together.

Figure 1. Google Maps: Go ahead and zoom right down to the street level. You'll never have to do a server refresh.
In a nutshell, AJAX allows you to write responsive Web applications that do not need to constantly refresh the page in order to update the information on the page.

Unknown to some, ASP.NET 2.0 includes the Client Callback Manager to allow developers to write AJAX-style Web applications. The Client Callback Manager uses XMLHTTP behind the scenes to encapsulate the complexities in sending data to and from the servers and clients (hence for it to work, you need a Web browser that supports XMLHTTP; at the moment the Client Callback Manager only works with Microsoft Internet Explorer.

In this article, I will show you how to use the Client Callback Manager to write AJAX-style applications.

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