Beginning ASP.NET 4.5 in CSharp and VB Opsylum

Using ASP.NET AJAX in Your Projects x 351
Even though this model has been used for years to serve web pages, it has a few big drawbacks. First,
because the entire page is loaded after a postback, the HTML sent to the browser is much larger than
it needs to be. Think back to the contact form you created in the previous chapter. Right after the user
has submitted the contact form, the server shows a Label control with the text Message Sent. It does
that by fully loading a new page that hides the form controls and shows the message. Even though the
rest of the page hasn’t changed (the menu, the sidebar, the footer, and so on), they are still sent from
the server to the client. Ideally, you would only want to send back the HTML that has changed. In the
case of the contact form, that could be as little as the text Message Sent. The right-hand side of
Figure 10-1 shows how this works. Rather than sending the entire page as a response, the server
sends a partial response (containing little more than the text Message Sent), which is then used by the
browser to update just the part of the page that has changed, leaving the rest of the page as it was.
The second drawback of a full page reload has to do with the way the browser renders the page.
Because the entire page is replaced, the browser has to dismiss the old one and then draw the new
one. This causes the page to “flicker,” which results in an unattractive user experience. You can
deploy Ajax techniques to overcome these two problems, as you see in the remainder of this chapter.
The concepts behind Ajax have been around for many years. Browsers since Internet Explorer 5
have shipped with the XMLHttpRequest object that enabled you to make calls to the server from
JavaScript to send and receive data. However, people also used other techniques to emulate the
behavior of what is now called Ajax, including Macromedia Flash, iframe elements, or hidden
frames.
However, when the term Ajax was introduced, things really took off. In an attempt to stay ahead of
the curve, Microsoft started building ASP.NET AJAX, the Ajax framework that is now fully integrated
in ASP.NET and Visual Studio 2012. This framework offers a number of benefits that you as
a web developer can take advantage of to create responsive applications.
In particular, ASP.NET AJAX enables you to:
‰ Create flicker-free pages that enable you to refresh portions of the page without a full reload
and without affecting other parts of the page
‰ Provide feedback to your users during these page refreshes
‰ Update sections of a page and call server-side code on a scheduled basis using a timer
‰ Access server-side WCF services and page methods and work with the data they return
The nice thing about ASP.NET AJAX is that it is very easy to get started with. Creating a flickerfree
page is a matter of dragging and dropping a few controls from the Toolbox onto your page.
When you understand the basics of the Ajax framework, you can extend your knowledge by looking
at more advanced topics such as calling WCF services.
USING ASP.NET AJAX IN YOUR PROJECTS
ASP.NET AJAX is fully integrated in ASP.NET and VS, which means you can start using it right
away. Each new ASP.NET 4.5 web project you create in VS is already Ajax-enabled. In addition, the
Toolbox contains an AJAX Extensions category with a number of Ajax-related controls that you can