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Web forms are event-driven. An event represents the idea that something happened (see 5 for a full discussion of events). An event is raised when the user clicks a button, or selects from a listbox, or otherwise interacts with the UI. Of course, in web applications these user interactions happen on the client s machine in the web browser, but ASP.NET events are handled on the server. For this to work, user interactions require a round trip the browser needs to send a
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message to the server, and the server needs to respond to handle the event completely. This can take a while, so our hands are somewhat tied compared to classic Windows application event handling it s just not practical for ASP.NET to offer server-side event handlers for things like mouse move events. So ASP.NET offers only a limited set of events, such as button clicks and text changes. These are events that the user might expect to cause a significant change, and for which it s reasonable to perform a round trip to the server.
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Postback versus nonpostback events
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Postback events are those that cause the form to be posted back to the server immediately. These include click-type events, such as the button Click event. In contrast, many events are considered nonpostback, meaning that the form isn t posted back to the server immediately.
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You can force controls with nonpostback events to behave in a postback manner by setting their AutoPostBack property to true.
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Nonpostback events are raised at the point at which ASP.NET discovers it needs to raise them, which may be some considerable time after the user performed the actions to which the events relate. For example, the TextBox web control has a TextChanged event. You wouldn t expect a web page to submit a form automatically the moment you typed into a text box, and so this is a nonpostback event. If the user fills in several text fields in a form, the server knows nothing about that this change in state happens on the client side, and it s only when the user clicks a button to submit the form that ASP.NET discovers the changes. So this is when it will raise TextChanged events for all the text boxes that changed. Consequently, you can expect to see multiple events during the handling of a single submission.
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View state
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Users tend to expect controls in user interfaces to remember their state it s disconcerting when text boxes lose their content, or listboxes forget which item was selected. Sadly, the Web is inherently a stateless environment.* This means that every post to the server loses the state from previous posts, unless the developer takes great pains to preserve this session knowledge. The Web is rife with sites where you fill in a form, only for it to lose all of your data if anything goes wrong. Developers have to do a lot of extra work to prevent this. ASP.NET, however, provides support for maintaining some of the state automatically.
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* There are good architectural reasons for this, but it s bad for usability.
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Whenever a web form is posted to the server, the server re-creates it from scratch before it is returned to the browser. ASP.NET provides a mechanism that automatically maintains state for server controls (ViewState). Thus, if you provide a list and the user has made a selection, that selection is preserved after the page is posted back to the server and redrawn on the client.
Web Forms Life Cycle
Every request for a page made to a web server causes a chain of events at the server. These events, from beginning to end, constitute the life cycle of the page and all its components. The life cycle begins with a request for the page, which causes the server to load it. When the request is complete, the page is unloaded. From one end of the life cycle to the other, the goal is to render appropriate HTML output back to the requesting browser.
Since ASP.NET is a server-side technology, its view of the lifetime of the page is quite different from the user s view. By the time the user sees the page, the server has already finished with it. Once the HTML has reached the browser, you can switch off and unplug the web server and the user will be none the wiser for as long as she s looking at that page.
The life cycle of a page is marked by the following events. ASP.NET performs specific processing at each stage, but you can attach handlers to any of these events to perform additional work: Initialize Initialize is the first phase in the life cycle for any page or control. It is here that any settings needed for the duration of the incoming request are initialized. Load ViewState The ViewState property is populated. The ViewState lives in a hidden input tag in the HTML when ASP.NET first renders a page, it generates this field, and uses it to persist the state across round trips to the server. The input string from this hidden variable is parsed by the page framework, and the ViewState property is set. This allows ASP.NET to manage the state of your control across page loads so that each control isn t reset to its default state each time the page is posted. Process Postback Data During this phase, the data sent to the server in the posting is processed. Load CreateChildControls() is called, which creates and initializes server controls in the control tree. State is restored, and the form controls contain client-side data. Send Postback Change Modifications If there are any state changes between the current state and the previous state, change events are raised via the RaisePostDataChangedEvent() method.
Handle Postback Events The client-side event that caused the postback is handled. PreRender This is your last chance to modify control properties prior to rendering. (In web forms, rendering means generating the HTML that will eventually be sent to the browser.) Save State Near the beginning of the life cycle, the persisted view state was loaded from the hidden variable. Now it is saved back to the hidden variable, persisting as a string object that will complete the round trip to the client. Render This is where the output to be sent back to the client browser is generated. Dispose This is the last phase of the life cycle. It gives you an opportunity to do any final cleanup and release references to any expensive resources, such as database connections.
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