ASP.NET Page Life Circle Asp.net页生命周期

Let's now get into some meat. If we look at MSDN - ASP.NET Page Life Cycle Overview we learn that there are a number of events that occur each time a page is loaded/refreshed/post-back. To understand this, I wrote an application to trace what actually goes on. Here's the general run-down.

1. (Obviously) the user requests a page.
2. IIS determines what application the page maps to (assuming an ASPX page). If the application is currently not running, then:
   • Application is loaded
   • Application_Start of Global.asax is called (this is in response to an application start event).
3. If the request is not yet associated with a session, or associated with a stale session, and session state is required, then:
   • Session state is obtained
   • Session_Start of Global.asax is called.
4. A new object instance of the page class is created (so no, member fields do not retain their values).
5. Page controls are created (CreateControlCollection and AddedControl).
6. Post-back mode is determined (DeterminePostBackMode).
7. OnPreInit is called (firing PreInit event)
8. OnInit is called (firing Init event). This is a good time to add any page assertions / reject pages.
9. OnInitComplete is called (firing InitComplete event). Watch the next few steps. After OnInitComplete, page state is loaded. If you have dynamically created page content, then that exact same page content must be re-created at this point to ensure view state is correctly loaded for it. That is, the set of controls for the page must match the set of controls that view state was saved for, even if it's not the desired set of controls for this iteration of the page.
10. Page state is loaded (LoadPageStateFromPersistenceMedium is called)
11. Any additional child controls are created / initialized (EnsureChildControls is called) - EnsureChildControls should be manually called if required at any time before Page_Load(). Think about these two steps for a moment. Before OnPreLoad is called, all the controls of the page are populated. To achieve this, much activity can occur after OnInit including loading data from a database.
12. OnPreLoad is called (firing PreLoad event).
13. OnLoad is called (firing Load event, and thereby calling Page_Load). Note that if a page has a template, the Page_Load on the page is typically called before Page_Load on the template).
14. Page validation is called if requested by any controls. Manually call Validate if validation required before a control demands it (i.e. don't rely on this).
15. Page event handlers (e.g. Click event) are called at this time.
16. OnLoadComplete is called (firing LoadComplete event).
17. EnsureChildControls is called again to ensure any additional controls are created/initialized.
18. OnPreRender is called (firing PreRender event). This is the ideal time to perform irreversible control changes prior to rendering.
19. SaveViewState is called to determine any view state that needs to be saved to the page.
20. SaveStateComplete event is called at this time (even though the page has not yet been rendered).
21. CreateHtmlTextWriter is called to obtain the context required to render a page.
22. Render is called to perform actual page rendering.
23. OnUnload is called to indicate the Page object is no longer required. This does not necessarily indicate the page has been rendered, and definitely does not indicate that the client has received the page.

There are some surprises here. In particular, I've found that I generally use the "OnInit", "OnLoad", and "OnPreRender" page events.

• OnInit
  • Ensure all app specific page assertions are done by this point. For example, on one project I use this point in time to determine if the site is actually "online" or not, and if not, redirect to some special page.
  • Ensure controls match up with previous page state prior to a postback ready to receive view state. The override "EnsureChildControls" is actually the correct place to do this.
• OnLoad (more specifically "Page_Load")
  • Any code that must execute prior to control event handling.
• PreRender
  • Setting up the final page look, particularly for any generated controls. I used to do this in response to OnLoad and control event handling, to much pain since not all of the control events are handled at that time. PreRender is a nice choice as there will be no more events before the actual page rendering.

If an unhandled exception is thrown, the normal page flow above is aborted, and the following occurs:

1. OnError is called (firing Error event).
2. OnUnload is called. The page is effectively thrown away at this point.
3. Application_Error (in Global.asax) is called to handle the error and redirect to the error page.

Next week, I'll go into some more interesting events outside of the page, but still part of the page life-cycle, to allow for example, page redirection.

If anyone is interested, I'll post the code I used to analyze the page events.

Reference:http://it.toolbox.com/blogs/codesharp/console-to-web-3-page-life-cycle-20597

Basic OF .NET

Application: BeginRequest
Application: PreAuthenticateRequest
Application: AuthenticateRequest
Application: PostAuthenticateRequest
Application: PreAuthorizeRequest
Application: AuthorizeRequest
Application: PostAuthorizeRequest
Application: PreResolveRequestCache
Application: ResolveRequestCache
Application: PostResolveRequestCache
Application: PreMapRequestHandler
Page: Construct
Application: PostMapRequestHandler
Application: PreAcquireRequestState
Application: AcquireRequestState
Application: PostAcquireRequestState
Application: PreRequestHandlerExecute
Page: AddParsedSubObject
Page: CreateControlCollection
Page: AddedControl
Page: AddParsedSubObject
Page: AddedControl
Page: ResolveAdapter
Page: DeterminePostBackMode
Page: PreInit
Control: ResolveAdapter
Control: Init
Control: TrackViewState
Page: Init
Page: TrackViewState
Page: InitComplete
Page: LoadPageStateFromPersistenceMedium
Control: LoadViewState
Page: EnsureChildControls
Page: CreateChildControls
Page: PreLoad
Page: Load
Control: DataBind
Control: Load
Page: EnsureChildControls
Page: LoadComplete
Page: EnsureChildControls
Page: PreRender
Control: EnsureChildControls
Control: PreRender
Page: PreRenderComplete
Page: SaveViewState
Control: SaveViewState
Page: SaveViewState
Control: SaveViewState
Page: SavePageStateToPersistenceMedium
Page: SaveStateComplete
Page: CreateHtmlTextWriter
Page: RenderControl
Page: Render
Page: RenderChildren
Control: RenderControl
Page: VerifyRenderingInServerForm
Page: CreateHtmlTextWriter
Control: Unload
Control: Dispose
Page: Unload
Page: Dispose
Application: PostRequestHandlerExecute
Application: PreReleaseRequestState
Application: ReleaseRequestState
Application: PostReleaseRequestState
Application: PreUpdateRequestCache
Application: UpdateRequestCache
Application: PostUpdateRequestCache
Application: EndRequest
Application: PreSendRequestHeaders
Application: PreSendRequestContent