Event Description
initializeRequest It's raised when an asynchronous postback is first initialized. This event can be used to cancel requests in cases where another request is already in process.
beginRequest It's raised when an asynchronous postback begins. This event can be used to animate an UpdatePanel container within a page to provide users with a visual cue that an asynchronous postback request is starting.
pageLoading It's raised after data is received from an asynchronous postback request but before the data is updated in the page. This event can be used in cases where you'd like to change data returned from the server using JavaScript or provide an effect on the UpdatePanel to signify that content is about to be updated.
pageLoaded It's raised when data in a page is updated after a synchronous or asynchronous request.
endRequest It's raised after an asynchronous postback request is completed. Any errors that occurred during the request can be processed here.

Table 1 The PageRequestManager Class Events
Here are the PageRequestManager class events.