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Asp.net Page Life-cycle

2013-05-26 10:40 716 查看
General Page Life-cycle Stages

Stage
Description
Page request
The page request occurs before the page life cycle begins. When the page is requested by a user, ASP.NET determines whether the page needs to be parsed and compiled (therefore beginning the life of a page), or whether a cached version of the page can be sent
in response without running the page.
Start
In the start stage, page properties such as Request and Response are
set. At this stage, the page also determines whether the request is a postback or a new request and sets the IsPostBack property.
The page also sets the UICulture property.
Initialization
During page initialization, controls on the page are available and
each control's UniqueID property is set. A master page and themes are also applied to the page if applicable.
If the current request is a postback, the postback data has not yet been loaded and control property values have not been restored to the values from view state.
Load
During load, if the current request is a postback,
control properties are loaded with information recovered from view state and control state.
Postback event handling
If the request is a postback, control event handlers are called. After that,
the Validate method of all validator controls is called, which sets the IsValid property
of individual validator controls and of the page. (There is an exception to this sequence: the handler for the event that caused validation is called after validation.)
Rendering
Before rendering, view state is saved for the page and all controls. During the rendering stage, the page calls the Render method
for each control, providing a text writer that writes its output to the OutputStream object of the
page's Response property.
Unload
The Unload event is raised after the page has been fully rendered, sent to the client, and is ready to be discarded.
At this point, page properties such as Response and Request are
unloaded and cleanup is performed.
Life-cycle Events

Page Event
Typical Use
PreInit
Raised after the start stage is complete and before the initialization stage begins.

Use this event for the following:

Check the IsPostBack property to determine whether this is the first time the page is being processed. The IsCallback andIsCrossPagePostBack properties
have also been set at this time.

Create or re-create dynamic controls.

Set a master page dynamically.

Set the Theme property dynamically.

Read or set profile property values.


Note
If the request is a postback, the values of the controls have not yet been restored from view state. If you set a control property at
this stage, its value might be overwritten in the next event.

Init
Raised after all controls have been initialized and any skin settings have been applied. The Init event of individual
controls occurs before the Init event of the page.

Use this event to read or initialize control properties.
InitComplete
Raised at the end of the page's initialization stage. Only one operation takes place between the Init and InitComplete events:
tracking of view state changes is turned on. View state tracking enables controls to persist any values that are programmatically added to the ViewState collection.
Until view state tracking is turned on, any values added to view state are lost across postbacks. Controls typically turn on view state tracking immediately after they raise their Init event.

Use this event to make changes to view state that you want to make sure are persisted after the next postback.
PreLoad
Raised after the page loads view state for itself and all controls, and after it processes postback data that is included with theRequest instance.
Load
The Page object calls the OnLoad method
on the Page object, and then recursively does the same for each child control until the page and all controls are loaded.
The Load event of individual controls occurs after the Load event
of the page.

Use the OnLoad event method to set properties in controls and to establish database connections.
Control events
Use these events to handle specific control events, such as a Button control's Click event
or a TextBox control's TextChangedevent.


Note
In a postback request, if the page contains validator controls, check the IsValid property
of the Page and of individual validation controls before performing any processing.

LoadComplete
Raised at the end of the event-handling stage.

Use this event for tasks that require that all other controls on the page be loaded.
PreRender
Raised after the Page object has
created all controls that are required in order to render the page, including child controls of composite controls. (To do this, the Page object
calls EnsureChildControls for each control and for the page.)

The Page object raises the PreRender event
on the Page object, and then recursively does the same for each child control. ThePreRender event
of individual controls occurs after the PreRender event of the page.

Use the event to make final changes to the contents of the page or its controls before the rendering stage begins.
PreRenderComplete
Raised after each data bound control whose DataSourceID property is set calls its DataBind method.
For more information, seeData Binding Events for Data-Bound Controls later in this topic.
SaveStateComplete
Raised after view state and control state have been saved for the page and for all controls. Any changes to the page or controls at this point affect rendering, but the changes will not be retrieved on the
next postback.
Render
This is not an event; instead, at this stage of processing, the Page object calls this method on each control. All ASP.NET
Web server controls have a Render method that writes out the control's markup to send to the browser.

If you create a custom control, you typically override this method to output the control's markup. However, if your custom control incorporates only standard ASP.NET Web server controls and no custom markup, you do not need to override theRender method.
For more information, see Developing Custom ASP.NET Server Controls.

A user control (an .ascx file) automatically incorporates rendering, so you do not need to explicitly render the control in code.
Unload
Raised for each control and then for the page.

In controls, use this event to do final cleanup for specific controls, such as closing control-specific database connections.

For the page itself, use this event to do final cleanup work, such as closing open files and database connections, or finishing up logging or other request-specific tasks.


Note
During the unload stage, the page and its controls have been rendered, so you cannot make further changes to the response stream. If
you attempt to call a method such as the Response.Write method, the page will throw an exception.

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