The UpdatePanel is the workhorse of the ASP.NET Ajax library. It is responsible for defining regions of a web page that trigger partial page postbacks (as opposed to full page postbacks). Such partial page postbacks transfer less information between the client and server and have their user interfaces updated seamlessly, thereby leading to a more interactive user experience. (For more information on UpdatePanels, refer to Using the UpdatePanel.) One side-effect of a partial page postback is that the HTML elements within the UpdatePanel are replaced with the markup returned on postback. This behavior is not noticeable and is not an issue unless you have client-side event handlers wired up to the elements within the UpdatePanel. Such client-side event handlers are lost after a partial page postback.
Consider a very simple UpdatePanel that contains just a TextBox and a Button. Furthermore, assume we have JavaScript on the page that creates an event handler for the TextBox’s focus and blur events, which “highlights” the TextBox when the user focuses it and unhighlights it when losing focus. Initially, this script works as expected–clicking on the TextBox will “highlight” it. However, things break down once the Button is clicked. When the Button is clicked the UpdatePanel triggers a partial page postback and submits an asynchronous HTTP request back to the server. The requested ASP.NET page then goes through its life-cycle again, but this time only the markup in the UpdatePanel (and the hidden form fields on the page) are returned to the browser. The UpdatePanel then overwrites its existing markup with the markup just returned from the server. Unfortunately, this overwriting obliterates the focus and blur client-side event handlers, meaning that selecting the TextBox no longer highlights it.
In short, if there are client-side event handlers attached to HTML elements within an UpdatePanel it is imperative that they be rebound after a partial page postback. This article looks at three different ways to accomplish this. To read the entire article, Building Interactive User Interfaces with Microsoft ASP.NET AJAX: Rebinding Client-Side Events After a Partial Page Postback, click here.