Beginning Web Development, Silverlight, and ASP.NET AJAX

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58 CHAPTER 3 ■ WEB FORMS WITH ASP.NET handle the HTTP-POST from the form. Other initialization, such as configuring controls, retrieving database information, and so on, is usually done at this stage so that when the page is rendered, it is provided with all the information the user needs. It’s important to note that Page_Load always fires, whether this is the first time the page has loaded or it is a postback. With the latter, you might expect it not to load, because the page is already loaded, but it does. Due to the stateless nature of web applications, each postback is treated as a new request to the page, and view state is used to maintain state between calls, as discussed earlier. As a developer, you may need to discern whether you are in a postback situation or an initial load situation, and ASP.NET provides this information for you via the IsPostBack property on the Page object. Thus, Page.IsPostBack will be true upon a postback and false on the first load. If there are actions that you only want to perform on either case, you can gate them inside an if(Page.IsPostBack) clause. Performing Validation An important aspect of any web application is verifying that the data the user has input is valid. This ensures that your application is secure (e.g., so that no one is able to attempt Structured Query Language [SQL] injection attacks) and the user experience is optimized; only appropriate data is sent to the server and returned from the server. ASP.NET introduces a suite of self-sufficient validation controls that can be used to ensure that the content of your page meets validation rules. These are very useful controls, relieving you from writing reams of validation code to ensure that the correct data has been input. Performing Event Handling Earlier we saw how events are handled on a page. Some events provide an automatic postback that triggers an event handler. Others—such as a list box selection—aren’t handled immediately, unless they have their AutoPostBack property set to true. At this stage of the life cycle, ASP.NET invokes the event handlers for each of the events that were tripped. So, if your page has a number of events before the postback occurs—for example, if a number of lists are used with no AutoPostBack, and the user selects items on them before pressing a button on the page—then each of these events will have their handler called at this stage. In addition, a number of page-level events need to be performed. In the scenario in which two lists do not have AutoPostBack set to true, and there’s no button that causes the postback, events will be fired in the following order: 1. Page_Init 2. Page_Load
CHAPTER 3 ■ WEB FORMS WITH ASP.NET 59 3. ListBox1_SelectIndexChanged 4. ListBox2_SelectIndexChanged 5. Button_Click 6. Page_PreRender 7. Page_Unload Note that the order in which the SelectionIndexChanged events occur might vary, depending on the order in which the user clicked them before pressing the button. They don’t necessarily fire in chronological order, as this list indicates. Performing Data Binding Data binding is a vital part of making web applications easy to develop. In short, it enables us to automatically set properties on a control from the contents of a database or other data store. It involves little to no coding on the developer’s part. It is a tool provided by the ASP.NET Framework. We’ll be going into this very important functionality in more detail in Chapter 4. One issue to note is the timing at which the data-binding functionality occurs. Any data binding that updates the data store via an insert, a delete, or an update will occur after all the control events are handled, but before the Page_PreRender occurs. Then, after the PreRender event, the functionality that queries the data store will fire. This ensures that data isn’t stale, because any functionality on your page that updates the data store will happen before anything is read from the data store. Therefore, the data on your page is always up-to-date. Server Tidies Up Objects To maintain application scalability, the server sends the results of the page rendering to the client, and in its memory, cleans up everything for a particular session. The server now “forgets” about the page, and control is returned to the browser. Information that is necessary for the server to “remember” the page again at the next session is stored in view state, and the page is initialized with this information on the next postback. Pages and Controls Because ASP.NET is a true object-oriented development API, the page itself is an object that is accessible to the developer, as are the controls on the page (which are members of
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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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PART 1 Building Web Applications
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4 CHAPTER 1 ■ INTRODUCTION TO WEB
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6 CHAPTER 1 ■ INTRODUCTION TO WEB
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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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CHAPTER 9 .NET 3.0: Windows Workflo
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CHAPTER 9 ■ .NET 3.0: WINDOWS WOR
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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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418 ■INDEX copying web sites, 25
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424 ■INDEX Sequential Workflow Co
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Beginning Web Development, Silverlight, and ASP.NET AJAX

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