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Injecting Dynamic JavaScript to Focus ASP.NET Controls

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Because Web applications are generally stateless, it takes some extra effort to offer some of the convenience of a Windows Forms application. Web controls don't support the focus behavior in the code behind but a client-side script does. In this article, Paul Kimmel, the author of Visual Basic .NET Power Coding (Addison-Wesley), demonstrates one technique for refocusing the previous control after a postback.
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It is well-documented that Windows Forms more easily can provide a better user experience and Web Forms offer greater flexibility in deployment. Often, developers are tasked with reconciling these two unique features into a single Web application.

Because Web applications are generally stateless, it takes some extra effort to offer some of the convenience of a Windows Forms application. Web controls do not support the focus behavior in the code behind but a client-side script does. This article demonstrates one technique for refocusing the previous control after a postback.

Writing Code to Statically Focus a Control

Simple Web applications with static content seldom need any cleverness (or much code, for that matter). However, when programmers write complex applications that have pages that contain a lot of controls, keeping track of user activity and facilitating data input can be highly important.

For example, if one is writing applications that are especially geared toward "heads-down data entry," reducing the number of extra steps can make a big difference in productivity.

Recently, while working on a large Web application, we devised several collapsible regions that mitigated the need for multiple trips to the Web server. Unfortunately, when the rendered page returned to the client, the expanded region and focused control were lost. To solve this problem, dynamic JavaScript was generated and emitted to the client, in the form of a startup block, to refocus the point of last interest.

Let's begin by looking at the static script necessary to focus a control and then we'll convert this script to a dynamically rendered version.

Listing 1 demonstrates a static script block that focuses the same control every time. Startup script is a block of script that is placed within the <form> tag and is run as the page is loaded.

Listing 1 Static Script to Refocus the TextBoxAddress Control When a Page is Rendered.

  <script language="javascript"> 
   var control = document.getElementById("TextBoxAddress");  
   if( control != null ) control.focus(); 
  </script>
  </form>
 </body>
</html>

The script requests the control by its object name. If the control was successfully found and returned, the control.focus() method is called.

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