Saturday, November 5, 2011

Introduction to AJAX

Asynchronous Javascript (AJAX) technology enables web applications to provide features like-
  • Partial Page Updates - Allows you to define an area of a webpage to postback and update independently from the rest of the page. Only the updated content is refreshed when the request completes
  • Indicate Progress of an operation - This allows you to track progress of server-side process and continuously update the user.
  • Desktop-like UI - With AJAX, you can provide users with controls such as modal dialog boxes, progress indicators, masked edit bixes, tooltips and more.
  • Cross Browser, cross platform support - Javascript, AJAX and JQuery are all supported on multiple browsers and multiple platforms.
Every ASP.NET page that uses AJAX must have one and only one ScriptManager class which enables-
  • Registration of custom scripts
  • Registration of web service method calls from client
Since you can use only one instance of ScriptManager class in an ASP.NET page there is a need to use ScriptManagerProxy class in the following scenarios-
  • When AJAX is used in a master page and also needed in content page
  • When AJAX is used in custom or user controls
An UpdatePanel control allows to define areas within a page that are partially updateable. Partial page updates are smaller asynchronous postbacks to the server.

UpdatePanel has some important properties like-
  1. Update Mode - The update mode can be set to one of the following modes
    • Always - If set to this mode the contents of the UpdatePanel is refreshed on every postback
    • Conditional - If set to conditional the refresh takes place based on ChildrenAsTriggers property (described below), or through an explicit call to the Update method or if Parent panel is refreshed.
  2. ChildrenAsTriggers - In case of nested UpdatePanels any update to a child UpdatePanel will not refresh contents of the Parent UpdatePanel unless ChildrenAsTriggers property is set to true.

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