Beginning Web Development, Silverlight, and ASP.NET AJAX

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52 CHAPTER 3 ■ WEB FORMS WITH ASP.NET WSG7LH03G2iHU=" /> First Number Second Number Add Subtract The Result is: 13 Using ASP.NET Events and Automatic Postbacks Having access to the control view state means that your server-side code isn’t limited to getting and setting properties—it can also respond to events within the controls. Remember, as the code is running on the client, and since the client is constrained to the
CHAPTER 3 ■ WEB FORMS WITH ASP.NET 53 workflow that HTML forms offer, something is necessary to bubble the events up to the server so that the server knows what event to respond to. The flow of processing follows this model: • When the page first runs, ASP.NET parses through the declarations and generates HTML to be sent to the client. All objects that it uses are destroyed from server memory once the HTML has been generated and issued to the client. • The user triggers a postback to the server, either by clicking an HTML submit button or performing an action that the server will interpret as an event, such as checking a CheckBox control. • When the page is returned to the server, ASP.NET is able to interpret the view state information to restore the state of each of the controls. • The view state information also contains the operation that triggered the postback. ASP.NET obtains this information and raises the appropriate event, and if you’ve written an event handler for this event, then your event handler’s code will execute. Other events that didn’t cause the postback will also fire, such as a list selection, and if you’ve provided an event handler for those events, you can process them as well. • The modified page is rendered back into HTML and returned to the client, and all objects—including those invoked by the event handlers—are destroyed. Not all client browser events fire a postback right away, and ASP.NET is designed to process each event at the most appropriate time. For example, if you come from a Windows development background, you may know that the Windows Forms TextBox fires a TextChanged event whenever the user types something into the text box. However, in a web context, you wouldn’t want to do this, because you’d trigger a postback every time you press a key when the cursor is in a text box, severely hampering your application’s performance and scalability. Instead, the web-based text box triggers a postback when you press Enter or Tab if so configured. However, when you start getting into complex controls, you may not want to use a postback for every user interaction. Consider a scenario where you have a UI containing a list control. In some cases, you might want to have a postback when the user selects an item on the list (so that you can respond to it right away), and in other cases, you might not want one. You can set up the former situation using an automatic postback (using the control’s AutoPostback property), where the control triggers the postback instead of the user. For the latter, just query the status of the list when the user selects a submit button or something similar to generate a postback for the page. Listing 3-10 shows a simple web UI containing a three-item list. This list has an event handler defined for it that is designated to fire whenever the list selection changes.
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The EXPERT’s VOIce ® in .NET Beg
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Beginning Web Development, Silverli
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Contents About the Author . . . . .
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■CONTENTS ix ■CHAPTER 5 ASP.NET
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■CONTENTS xi ■CHAPTER 11 Ajax A
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■CONTENTS xiii ■CHAPTER 15 Enha
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About the Author ■LAURENCE MORONE
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Introduction This book is aimed at
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CHAPTER 4 ■ DATA BINDING WITH ASP
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CHAPTER 6 Deploying Your Web Site I
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CHAPTER 6 ■ DEPLOYING YOUR WEB SI
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CHAPTER 7 .NET 3.0: Windows Communi
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CHAPTER 7 ■ .NET 3.0: WINDOWS COM
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CHAPTER 8 .NET 3.0: Windows Present
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CHAPTER 8 ■ .NET 3.0: WINDOWS PRE
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234 CHAPTER 10 ■ .NET 3.0: PROGRA
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CHAPTER 11 Ajax Applications and Em
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CHAPTER 11 ■ AJAX APPLICATIONS AN
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CHAPTER 12 AJAX Extensions for ASP.
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CHAPTER 12 ■ AJAX EXTENSIONS FOR
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CHAPTER 13 Ajax Scripts and Service
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CHAPTER 13 ■ AJAX SCRIPTS AND SER
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332 CHAPTER 14 ■ JAVASCRIPT PROGR
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CHAPTER 15 Enhancing the Web Experi
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CHAPTER 15 ■ ENHANCING THE WEB EX
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416 ■INDEX applications. See also
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Beginning Web Development, Silverlight, and ASP.NET AJAX

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