Used by the pageLoading event of the Sys.WebForms.PageRequestManager class to send event data that represents the UpdatePanel controls being updated and deleted as a result of the most recent postback.
var args = new Sys.WebForms.PageLoadingEventArgs();
This class contains private members that support the client-script infrastructure and are not intended to be used directly from your code. Names of private members begin with an underscore ( _ ).
The UpdatePanel control renders <div> elements. After a postback, the pageLoading event contains information about which <div> elements on the page were updated and created. The panelsDeleting property is an array of <div> elements that are being deleted. The panelsUpdating property is an array of <div> elements that are being updated.