8.5. Lifecycle


A user sits at her browser and types in a URL. A web page appears, with text, images, buttons, and so forth. She fills in a text box and clicks on a button. What is going on behind the scenes?

Every request made of the web server initiates a sequence of steps. These steps, from beginning to end, constitute the lifecycle of the page.

When a page is requested, it is loaded, processed, sent to the user, and unloaded. From one end of the lifecycle to the other, the goal of the page is to render appropriate HTML and other output back to the requesting browser. At each step, there are methods and events available to let you override the default behavior or add your own programmatic enhancements.

To fully understand the lifecycle of the page and its controls , it is necessary to recognize that the Page class creates a hierarchical tree of all the controls on the page. All the components on the page, except for any Page directives (described shortly), are part of this control tree. You can see the control tree for any page by adding TRace="True" to the Page directive.

The page itself is at the root of the tree. All the named controls are included in the tree, referenced by control ID. Static text, including whitespace, NewLines, and HTML tags, are represented in the tree as Literal controls. The order of controls in the tree is strictly hierarchical. Within a given hierarchy level, the controls are ordered in the tree using the same sequence in which they appear in the page file.

Web components, including the Page, go through the entire lifecycle every time the page is loaded. Events fire first on the Page, then recursively on every object in the control tree.

The following is a detailed description of each of the phases of the component lifecycle in a web form. There are two slightly different sequences of events in the lifecycle: on the first loading of the page and on subsequent postbacks. This lifecycle is shown schematically in Figure 8-33.

During the first page load, the lifecycle is composed of the following steps:


Initialization

The initialization phase is the first phase in the lifecycle for any page or control. The control tree is built during the initialization phase . In this phase, you can initialize any values needed for the duration of the request.

The initialize phase is modified by handling the Init event with the OnInit method.


Load

User code runs and the form controls show client-side data.

The load phase can be modified by handling the Load event with the OnLoad method.


PreRender

This is the phase just before the output is rendered. CreateChildControls is called, if necessary, to create and initialize server controls in the control tree. Modifications are made via the PreRender event, using the OnPreRender method.


Save ViewState

The view state is saved to a hidden variable on the page, persisting as a string object that will complete the roundtrip to the client. This can be overridden using the SaveViewState method.


Render

The page and its controls are rendered as HTML. You can override using the Render method. Within Render, CreateChildControls is called, if necessary, to create and initialize server controls in the control tree.


Dispose

This is the last phase of the lifecycle. It gives you an opportunity to do any final cleanup and release references to any expensive resources, such as database connections. This is important for scalability. It can be modified using the Dispose method.

Figure 8-33. The lifecycle of a web page


During postback , the lifecycle is:


Initialization

Same as on first load.


Load ViewState

The ViewState property of the control is loaded from a hidden variable on the page, as described in "View State" earlier in this chapter. You can modify this behavior by overriding the LoadViewState method.


Postback Data is loaded

During this phase, the data sent to the server via the POST method is processed. Any updates to the view state necessitated by the postback are performed via the LoadPostData method.


Load

Same as on first load.


Change events are raised

If there are any state changes between the current state and the previous state, change events are raised via the RaisePostDataChangedEvent method. Again, the events are raised for the controls in the order in which the controls appear in the control tree.


Handle postback events

Exactly one user action caused the postback. That user action is handled now, after all the change events have been handled. The original client-side event that instigated the postback is handled in the RaisePostBackEvent method.



Programming Visual Basic 2005
Programming Visual Basic 2005
ISBN: 0596009496
EAN: 2147483647
Year: 2006
Pages: 162
Authors: Jesse Liberty

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