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Understanding AJAX and ASP.NET

This chapter from Microsoft ASP.NET 4 Step by Step covers AJAX, helping you understand the problems AJAX solves, how it is supported in ASP.NET, and how to write AJAX-enhanced websites to improve the user's experience.

After completing this chapter, you will be able to

  • Understand the problem AJAX solves.

  • Understand ASP.NET support for AJAX.

  • Write AJAX-enabled Web sites.

  • Take advantage of AJAX as necessary to improve the user’s experience.

This chapter covers AJAX, possibly the most interesting feature added to ASP.NET recently. AJAX stands for Asynchronous JavaScript and XML, and it promises to produce an entirely new look and feel for Web sites throughout the world.

Rich Internet Applications

Software evolution always seems to happen in this typical fashion: Once a technology is grounded firmly (meaning the connections between the parts work and the architecture is fundamentally sound), upgrading the end user’s experience becomes a much higher priority. Application technology is in this stage, and the general term for this kind of application is a Rich Internet Application (RIA). AJAX is one means of producing Rich Internet Applications. (Microsoft Silverlight is another popular means of creating RIAs.)

The primary reason for the existence of AJAX is to improve the standard HTTP GET/POST idiom with which Web users are so familiar. That is, the standard Web protocol in which entire forms and pages are sent between the client and the server is getting a whole new addition.

Although standard HTTP is functional and well understood by Web developers, it does have certain drawbacks—the primary one is that the user is forced to wait for relatively long periods while pages refresh. This has been a common problem in all event-driven interfaces. (The Windows operating system is one of the best examples.) AJAX introduces technology that shields end users from having to wait for a whole page to post.

Think back to the way HTTP typically works. When you make a request (using GET or POST, for example), the Web browser sends the request to the server, but you can do nothing until the request finishes. That is, you make the request and wait—watching the little progress indicator in the browser. When the request returns to the browser, you can begin using the application again. The application is basically useless until the request returns. In some cases, the browser’s window even goes completely blank. Web browsers have to wait for Web sites to finish an HTTP request in much the same way that Windows-based programs have to wait for message handlers to complete their processing. (Actually, if the client browser uses a multithreaded user interface such as Windows Internet Explorer, users can usually cancel the request—but that’s all they can really do.) You can easily demonstrate this problem by introducing a call to System.Threading.Thread.Sleep inside the Page_Load method of an ASPX page. By putting the thread to sleep, you force the end user to wait for the request to finish.

The AJAX solution to this problem is to introduce some way to handle the request asynchronously. What if there were a way to introduce asynchronous background processing into a Web site so that the browser would appear much more responsive to the user? What if (for certain applications) making an HTTP request didn’t stall the entire browser for the duration of the request, but instead seemed to run the request in the background, leaving the foreground unhindered and changing only the necessary portion of the rendered page? The site would present a much more continuous and smooth look and feel to the user. As another example, what if ASP.NET included some controls that injected script into the rendered pages that modified the HTML Document Object Model, providing more interaction from the client’s point of view? Well, that’s exactly what ASP.NET AJAX support is designed to do.