The PageRequestManager: the unsung hero in .NET

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The PageRequestManager: the unsung hero
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Figure 7.2 A capture of an asynchronous HTTP request confirms that the X-MicrosoftAjax fields is being added to the request.
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HTML for the entire page to render in the browser. Also, the Content-Type is set
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to text/html. For partial postbacks, this type is set to text/plain; and instead of the entire HTML being sent from the browser, the payload instead includes the following:
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Only the HTML rendered for regions of the UpdatePanel controls on the page The updated ViewState for the page A set of script blocks, hidden fields, and data items for managing and applying updates to the page
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Figure 7.3 gives you a glimpse into the payload sent from the server in response to an asynchronous request.
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Under the hood of the UpdatePanel
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Figure 7.3 The response from the server during an asynchronous postback includes uniquely formatted text that is parsed by the PageRequestManager in the browser to apply the partial-page updates.
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If you take a close look at the Response Content tab displayed in the bottom panel of figure 7.3, you ll see some strangely formatted content that is part HTML and part plain text. Prefixed with an integer that signifies the size of the payload, the character | is used as a delimiter for certain keys in the text (a format only a mother could love). It s important to note that this text shouldn t be tampered with and is parsed by the PageRequestManager to apply the updates to the page. Considering the warning, it doesn t mean you can t evaluate the content and add additional logic to your script based on the data. This brings you to the last and final step of the process: evaluating the data from the server and updating the interface.
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A client-side event viewer
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After the asynchronous postback Going back to when the request was made, you included a callback function called _onFormSubmitCompleted. This method is now invoked in the browser to signify that the server has completed its portion of the processing. If any errors occurred, they re also caught here in the browser and the endRequest event is raised prematurely in the client-side event model. Included in the endRequest event arguments is information about the error. If there are no errors, the pageLoading event is raised, and the PageRequestManager begins to parse the data from the response and apply the updates to the DOM. Additional scripts are also loaded at this time, and the scroll position, which was recorded before the request was sent, is restored at the end. Once complete, the pageLoaded, load, and endRequest events are raised in their respective order. At last, the partial-page rendering pattern comes to an end. It s time to put this valuable knowledge to work and get back to coding. The best way to come full circle with all the information we ve introduced is to apply it.
A client-side event viewer
Often, when .NET developers are learning about the page lifecycle, they throw together an application that displays the raised events on a page. This widespread technique helps them understand the order in which the events occur, the arguments that are passed along, and ultimately what can and can t be accomplished during each event. To reinforce your understanding of the client-side event model, you ll build a similar application that will let you observe what happens during partial-page updates. Figure 7.4 shows the application you ll build in this section: a client-side event viewer that hooks into the events of the PageRequestManager and Application objects. Serving as a platform for comprehending the client-side events, this learning tool also lets you experiment with different scenarios that often occur during development. The application has a little style applied to it as well (this is all about the user experience, isn t it ). Rather than take up space displaying the stylesheet, we ll leave that up to you to download from the book s website if you want to produce the same look.
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