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CHAPTER 3 s WEB FORMS WITH ASP.NET
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establish the initial values of the controls on the page. For example, when you place a text box on a page, you set its initial text to a string (even if it s an empty string). Then, when you change its value from its default to another value in the Visual Studio editor, the new value is stored within the tag representing the text box. If the user edits the text box s value in their browser, ASP .NET detects the change in textual values but doesn t change the underlying HTML for the text box. Instead, when ASP.NET recognizes that the value has changed, it stores this change, as well as all other changes, into a name/value pair collection. This collection is then encrypted and serialized into a Base64 string, which will contain only letters and numbers (and perhaps the equal sign: =), and inserts this into a hidden field within the HTML page s form tag. As the page is transmitted between client and server, this information serves to maintain the state of each control on the page. So, for example, if your text box is originally empty, it is rendered on the client as .NET looks at the view state empty. If the user changes it and later triggers a postback, ASP when it receives the response and sees that the text box value has changed. ASP.NET then processes the postback and fires all applicable events. When ASP.NET finishes this processing, it generates new HTML and then takes the current value of the text, which may have changed yet again by some server-side process, encrypts and serializes it into the view state hidden field once again, and then sends it back to the client. The client sees the changes in the text box, and the process starts all over again. Thus, we get the best of both worlds. The server doesn t use precious resources keeping the state of the client, but yet the client state is still preserved. When the page is posted back to the server, the server can use the view state information to reinitialize the page controls to their correct state. There is a downside, however, which is that the size of the page is increased by the view state information. This has additional costs: it takes longer for the page to download and initialize, and the volume of information posted back to the server is larger because it contains all the view state information. When using complex controls such as Grids, the additional overhead can be significant. If an excessively large page is giving you a problem, you can override view state by setting the EnableViewState property to false. Thus, when designing your application, you can optimize it for page size by enabling view state only for the controls that you want to maintain state for.
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Processing Web Forms
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We ve seen that ASP .NET processes the page, generating HTML as it goes, and this HTML is returned to the client. In this section, we ll look at this process in a little more detail and see the major steps performed as part of this action. These include page initialization, code initialization, validation, event handling, data binding, and cleanup. This happens for each page request. Figure 3-4 shows the order of these steps.
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CHAPTER 3 s WEB FORMS WITH ASP.NET
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Figure 3-4. ASP .NET page life cycle
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The following sections will look at each step in this cycle.
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Page Framework Initialization
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This is the stage at which ASP .NET initializes the page in memory on the server. It first parses through the definition of the ASPX, initializing each of the controls on the page as it was originally defined. Should this not be the first time the page has been requested, ASP .NET will then look through the view state and update the status of each control according to the name/value pairs that it deserializes out of view state. At this point, it fires the Page.Init event. You can use this event to perform actions upon initializing the page, but UI actions are not permitted because the UI rendering has not yet occurred.
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