With great power comes great responsibility in .NET framework

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With great power comes great responsibility
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The online poll control in a state before any user interaction
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and a new page is served to the browser. Figure 6.2 shows the online poll after the user has completed it. As the site s traffic increases and weekly poll feature grows in popularity (man, you re good!), the site begins to slow down considerably. After some research, it becomes apparent that limited bandwidth and the stress of loading the entire page after each online poll is submitted are the main causes of the site s inadequate performance. This time, the company has approached you in search of suggestions for alleviating the stress recently placed on the site. They have made it clear that they would like to change the site as little as possible and are in search of a solution that is simple and easy to manage. Your first thought is to use an IFRAME element. IFRAME stands for inline frame; this element is commonly used to include external objects such as HTML documents in a page. If you move the online poll feature to its own page and then host it in an IFRAME, only the contents in the frame will be posted back to the server. This solution lessens the strain put on the server, but it still causes that portion of the page
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Partial-page rendering with UpdatePanels
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Feedback from the online poll after it has been completed
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to reload and flicker in the process. For aesthetic reasons, this solution isn t acceptable to the customer. With the IFRAME not returning pleasing results and the use of pop-up windows being out of the question, you decide to explore an Ajax solution by making a request to the server using the XMLHttpRequest object and then updating portions of the page dynamically from the client (with JavaScript). This approach requires you to abandon the user control and move most of the logic from the server to the client. It also introduces the effort of managing browsers that don t support the XMLHttp protocol and writing the online poll logic in two areas to compensate (once in JavaScript, again on the server). So far, this seems like the most reliable solution, but it leaves you wondering if there is a better option. At last, you decide to give ASP.NET AJAX a try. You start by purchasing a copy of ASP.NET AJAX in Action (a wise choice). You realize that the solution you re looking
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With great power comes great responsibility
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for is possible with the UpdatePanel control. Listing 6.1 shows a portion of the markup before adding the UpdatePanel to the page.
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Listing 6.1 Markup for the home page before adding the UpdatePanel
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<%@ Page Language="C#" AutoEventWireup="true" CodeFile="SimpleExample.aspx.cs" Inherits="SimpleExample" %> <%@ Register Src="~/UserPoll.ascx" TagPrefix="demo" TagName="UserPoll" %> ... <div id="masthead"> <a href="SimpleExample.aspx" > <img src="images/header.png" alt="Emily's Flowers" style="border: 0px;" /> </a> </div> ... <div id="container"> <div id="page_content"> ... </div> <div id="right_col"> <demo:UserPoll ID="FlowerPoll" runat="server" /> </div> </div>
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The online poll control
Listing 6.2 shows the same code with the addition of the ScriptManager and UpdatePanel controls. With these quick and minor updates, the next time the user fills out the online poll, the portion of the page encapsulated by the UpdatePanel is updated dynamically instead of a page refresh occurring.
Listing 6.2 Adding the ScriptManager and UpdatePanel to replace the page refresh with a partial-page update
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="SimpleExample.aspx.cs" Inherits="SimpleExample" %> <%@ Register Src="~/UserPoll.ascx" TagPrefix="demo" TagName="UserPoll" %> <asp:ScriptManager ID="ScriptManager1" runat="server" />
Required ScriptManager
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... <div id="masthead"> <a href="SimpleExample.aspx" > <img src="images/header.png" alt="Emily's Flowers" style="border: 0px;" /> </a> </div> ... <div id="container"> <div id="page_content"> ... </div> <div id="right_col"> <asp:UpdatePanel ID="UpdatePanel1" runat="server"> <ContentTemplate> <demo:UserPoll ID="FlowerPoll" runat="server" /> </ContentTemplate> </asp:UpdatePanel> </div> </div>
UpdatePanel enables partially updated region
Amazingly, that is all you needed to do to keep the page refresh from taking place. A postback still happens, but it takes place asynchronously (for more about asynchronous operations, see chapter 1). Best of all, the hard work needed to accomplish this was done for you.
A common misconception about the UpdatePanel is that it enables a section of a page to be partially rendered without a postback. The truth is, a postback still occurs, and the price of going through the entire page lifecycle comes with it. Without a postback, none of this would be possible, and the application logic would have to change. Instead, an asynchronous postback occurs, and the only action that is replaced from the user s perspective is the full-page refresh. Throughout this book, we ll make a conscious effort to distinguish between a page refresh and a postback. Before the UpdatePanel, the two were incorrectly considered to be the same thing.
This simple example should give you a glimpse into what the UpdatePanel can accomplish. Now, we ll take a closer look at the control its properties, methods, and limitations.
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