Typical web pages built with ASP.NET web server controls perform postbacks initiated by a user action on the page, such as clicking a button. In the response, the server renders a new page. Frequently this re-renders controls and text that did not change between postbacks.
With partial-page rendering, you can refresh individual regions of the page asynchronously and make the page more responsive to the user. You can implement partial-page rendering using ASP.NET AJAX web server controls, and optionally write client script that uses the APIs in the Microsoft AJAX Library.
Server controls for partial-page updates
To add AJAX functionality to ASP.NET web pages, you identify individual sections of the page that you want to update. You then put the content of these sections into UpdatePanel controls. The contents of an UpdatePanel control can be HTML or other ASP.NET controls. You can add an UpdatePanel control to the page as you would add any other control. For example, in Microsoft Expression Web you can drag it from the Toolbox task pane to the web page, or you can add it by adding the code for the control in the Code view of the page.
By default, postbacks that originate from controls inside the update panel (child controls) automatically initiate asynchronous postbacks and cause a partial-page update. You can also specify that controls outside the update panel cause an asynchronous postback and that they refresh the UpdatePanel control's content. A control that causes an asynchronous postback is referred to as a trigger.
An asynchronous postback behaves much like a synchronous postback. All the server page life-cycle events occur, and view state and form data are preserved. However, in the rendering phase, only the contents of the UpdatePanel control are sent to the browser. The rest of the page remains unchanged.
To support partial-page rendering, you must put a ScriptManager control on the page. The ScriptManager control keeps track of all the update panels on the page and of their triggers. It coordinates partial-page rendering behavior on the server, and determines which sections of the page to render as a result of an asynchronous postback.
For more information about partial-page rendering, see Partial-Page Rendering Overview in the MSDN Library.