Building Web Sites with ASP.NET - Part 2
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Published: 14 Nov 2008
In this second part of the series, Brian demonstrates how to add AJAX features for an application that already has been built using the server-based approach of posting back and processing data on the server. This article shifts the application to doing more work on the client. After providing a detailed overview of the topic, he examines the steps required to implement AJAX features with the help of a sample application which renders news content using the ListView control with the help of relevant source code.
by Brian Mains
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ASP.NET AJAX has many helpful features that make developing ASP.NET applications asynchronous. Sometimes though, with the out-of-the-box controls in the ASP.NET framework, it is hard to tap into the existing control architecture to add custom JavaScript capabilities.  Oftentimes, creating your own custom control just to perform those actions on the client-side is what is needed. To understand how to add AJAX to an existing ASP.NET control requires a thorough understanding that the various paths server controls can take through the process. What I mean by that is some server controls render different HTML markups based upon the input settings. One instance is the Textbox control: in single line mode, it renders an <input type="text"> element, in multi line mode, it renders a <textarea> element, and in password mode it renders an <input type="password"> element.

In this article, I am focusing more on building additional features of an existing ASP.NET web site to incorporate some new AJAX features. The idea for these new features is to prevent as many postbacks as possible, and to create a richer user experience.

So why create a richer experience, and prevent posting back? Preventing post backs reduces the amount of binding that often takes place. A .NET application may often bind data to a GridView or ListView just to refresh the user interface, for the purposes of changing display modes, sorting, paging, or the like. All of the data gets rebound only to change modes or because of the possibility that some other user or process modified or added data to the system. This approach provides more of an instantaneous response than posting back to the server can do. In addition, it reduces the payload of pushing HTML to the client, which increases the payload being transferred across the wire.

The key to performing work on the client is to do two kinds of work: sending the data to the server, and manipulate the client. In a server-only approach without AJAX, the data that the user manipulates gets posted back along with the action that occurred, letting the underlying control know that an insert or update occurred and re-render its interface by binding again.

On the client, all of the work to create or refresh the user interface must occur on the client; simply binding again does not refresh the user interface. There are many other nuances to be aware of when shifting logic from the server to the client. So while the server approach is easy, and it is easy to use an UpdatePanel, it may often be better to use a client-side scripting approach to generating the user interface.

When shifting logic from the server to the client, be aware of some initial problems. First, any control marked as Visible="false" does not render in markup. The underlying HTML element rendered is not in the document object model, and thus cannot be manipulated and may cause client-side scripting errors. The better alternative would be to add null checks in JavaScript code, or set the style="display:none" property on the server control (which maps over to the client correctly for most controls).

Another complexity can come from understanding how the server control renders. I mentioned this before, but I will mention another example that can happen with the CheckBoxList control.  The list can render in vertical or horizontal modes, and can repeat depending on the settings of the RepeatLayout, RepeatDirection, and RepeatColumns properties. Depending on the RepeatLayout property, the underlying structure can render as a table or in flow mode. In flow mode, the actual text is not inside the checkbox tag, but rendered in a label element (not to be confused with the label server control).

Another issue to be aware of is dynamically generated ID's. Always provide an ID, because a null ID does not generate the id attribute on the client side, and this does not jive with Firefox.  Firefox needs the id to use document.getElementById, while Internet Explorer will use the name attribute as well.

With that said, let us begin to look at how we are going to improve upon the process.

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User Comments

Title: פיתוח אתרים   
Name: moshiko bracha
Date: 2011-01-16 12:08:23 PM
asp .net is the best way to build today... nice post tnx for the info
Title: Mr.   
Name: dengone
Date: 2009-12-19 8:35:10 PM
I think I have got what you write.
Thanks a lot.
Title: Executive Director (Commercial & Industrial Business System Development (Head Office)   
Name: Thomas Woon Wei Leou
Date: 2008-11-25 3:52:12 PM
I am presently undertaking my model (BSc (Honours in Computer Science & Management); Module of :- Intelligent Decision Making & Selection in tendon with an Advance Intelligent Making using Selective Decisional Process.

I think there would be a great & wider audience in this present field of study & interest. A simple application would be a Webbot (A Realistic Human Clone; with Full Emotions & Senstivity interecting with the Environment (Real World in Applcation Real Life Time Situation)

If there is some interest, I am very much happy to contribute Ideals for Projects of Future System, Science & Technology an Beyond the Real Matter World, i.e. Visions & Dreams & Our Final Destiny.) Regards.

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