The PageRequestManager: the unsung hero in VS .NET

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The PageRequestManager: the unsung hero
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So far, the UpdatePanel has received much of the credit for partial-page rendering. With little effort, ASP.NET developers can leverage the control to declare regions of a page for partial updates. As a result, a normal postback is replaced with an asynchronous one that can update fragments of a page without causing the browser to refresh. You know that some of this behavior can t be done with a few server controls; essential to the solution is client-side scripting to at least update the UI. It should come as no surprise that the UpdatePanel server control looks to another resource on the client side to manage the updates and requests to the server. The PageRequestManager is the client-side counterpart to the UpdatePanel control. When partial rendering is enabled, this JavaScript object manages the asynchronous postbacks and updates that take place in the browser. To help you
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The PageRequestManager: the unsung hero
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comprehend how the UpdatePanel works, we ll shed light on where it all begins: with the PageRequestManager and its client-side event model.
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The client-side event model
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The PageRequestManager is a JavaScript object that becomes available when partial rendering is enabled on a page. Its primary responsibilities include managing the UpdatePanel controls on the page, performing asynchronous postbacks to the server, and processing the results to dynamically update the contents of the page. During this process, the PageRequestManager goes through a series of events, much like the ASP.NET page lifecycle, that presents an opportunity for you to take more control of what happens during an asynchronous postback. Figure 7.1 illustrates the events that occur when an asynchronous postback is triggered from within an UpdatePanel.
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beginRequest initializeRequest
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[CurrentTime] Update
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pageLoading
pageLoaded
load
endRequest
Figure 7.1 The PageRequestManager fires off a series of events before, during, and after an asynchronous postback. This allows the page and control developer to have more influence over how content is rendered during a postback.
Under the hood of the UpdatePanel
Perhaps the best way to ease into explaining how you can use these events is to first outline the order in which they occur. A high-level understanding of the intentions of each event will give you more insight into the client-side event model. Once we ve established this foundation, we can then take a more intrusive look into how the events work and how to leverage them. What follows is a brief explanation of each event in the model. The initializeRequest event When a trigger such as a button click or column sort on a GridView occurs, the asynchronous postback process is initiated. In response to this action, the PageRequestManager fires a client-side event called initializeRequest. As its name suggests, the early stages of a request to the server begin to take shape here. Along with information about which DOM element caused this to occur, the event establishes an opportunity for you to cancel or give precedence to a particular asynchronous postback. The beginRequest event If the asynchronous postback hasn t been canceled or aborted in the previous event, the next step in the timeline is the beginRequest event. Raised just before the asynchronous postback is sent to the server, this occasion is typically used to relay to the user a visual cue that an asynchronous process is about to begin. When a process can end up being lengthy, it s important to keep the user in tune with the application by providing instant feedback.
The UpdateProgress control (discussed in chapters 1 and 4) leverages the beginRequest event to display its contents as a visual cue to the user during an asynchronous postback. It then uses the endRequest event to hide the visual cue signifying an end to the request.
In addition, you can invoke custom scripts in response to the event. After this occurs, the asynchronous postback is sent to the server. The pageLoading event After the postback is processed on the server, its response is sent back down to the client, and the pageLoading event is raised. During this event, the updated HTML for declared regions of the page is sent down to the client. Additional scripts are also delivered to assist in managing the state of the UpdatePanel controls and subsequent postbacks. Because this event occurs before any updates are made, it presents you with an opportunity to inspect the data from the server and apply customizations, cleanup, or additional handling.
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