The PageRequestManager: the unsung hero in VS .NET

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The PageRequestManager: the unsung hero
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The pageLoaded event The pageLoaded event signifies the completion of the partial updates to the page. During an asynchronous postback, this event is fired immediately after the pageLoading event. However, this event is also fired by the PageRequestManager when it s initially loaded on a page. What is important to remember right now is that the pageLoaded event (as its name states) is fired each time a page is loaded, regardless of what caused the page to load (or reload). Later, we ll demonstrate how to distinguish between normal and asynchronous requests during this event. The load event The next event fired is not a PageRequestManager event, but rather one that belongs to the Application object (introduced in chapter 2). To recap, the load event signifies that all scripts have been loaded and that all client-side objects in the application have been created and initialized. Because this event is significant to the event lifecycle that exists on the client, the PageRequestManager raises it during asynchronous postbacks, on behalf of the Application object.
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The PageRequestManager has a function called pageLoaded. When it s called, it s passed in a parameter that indicates whether this is the initial load for the page. If this is the initial load, the Application object naturally raises the load event during the client-side page lifecycle. If this isn t the initial load, a call is made to the raiseLoad function in the Application object, which, as its name suggests, raises the load event.
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The endRequest event Finally, if everything goes smoothly, the endRequest event is raised by the PageRequestManager after the load event. This event indicates that the processing for the request has completed. We chose our words carefully when we said if everything goes smoothly ; this event is also raised when an error is thrown during the postback processing. If an error occurs, details about the error are passed along with the event arguments, allowing you to handle the error yourself or let it be dealt with elsewhere in the page. We ll spend more time on error handling later in the chapter. You can rely on the fact that the endRequest event will always be raised at the end of a partial postback. You should now have a high-level understanding of the client-side event model offered by the PageRequestManager. If the previous overview wasn t thorough enough, have no fear we ve only just begun to explore how the model behaves. Soon, we ll provide more insight into what is happening behind the scenes and how to program against it effectively.
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Under the hood of the UpdatePanel
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The next step is to understand how an asynchronous postback works. We ll walk through the process of how it originates from the client, how it s processed, and how it s sent back to the browser for partial updates to the UI.
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The anatomy of an asynchronous postback
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In this section, we ll walk through an asynchronous postback and uncover how it works behind the scenes. We ll examine how the pieces are initially put into place and how a request is formulated, sent to the server, and eventually parsed and updated in the UI. In the end, you ll have a deep appreciation of the partial-page rendering behavior that the ASP.NET AJAX framework provides. Laying the foundation While a page is loaded, a number of things are put into place that lay the groundwork for partial postbacks. The first significant event that occurs is the OnInit event for the ScriptManager. The ScriptManager calls the PageRequestManager server object to determine whether the page is in the process of handling an asynchronous postback. It then exposes this value through the read-only property IsInAsyncPostBack.
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