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life cycle of a asp page

Posted By: Nabin     Posted Date: October 08, 2009    Points:2   Category :ASP.Net
what is the life cycle of an asp page?
which event is fired 1st when an asp page is loaded ans also what is the sequence of events?

Responses
Author: Venkat             
Posted Date: October 08, 2009     Points: 5   

HI,

The sequence of various stages are

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 step, 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. Additionally, during the start step, the page's UICulture property is set.

Page initialization:

During page initialization, controls on the page are available and each control's UniqueID property is set. Any themes are also applied to the page. 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.

Validation:

During validation, the Validate method of all validator controls is called, which sets the IsValid property of individual validator controls and of the page.

Postback event handling:

If the request is a postback, any event handlers are called.

Rendering:

Before rendering, view state is saved for the page and all controls. During the rendering phase, the page calls the Render method for each control, providing a text writer that writes its output to the OutputStream of the page's Response property.

Unload:

Unload is called 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 any cleanup is performed.


The various Event and its sequence are:
PreInit:

Use this event for the following:

Check the IsPostBack property to determine whether this is the first time the page is being processed.

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. Use this event to read or initialize control properties.

InitComplete:

Raised by the Page object. Use this event for processing tasks that require all initialization be complete.

PreLoad:

Use this event if you need to perform processing on your page or control before the Load event.

Before the Page instance raises this event, it loads view state for itself and all controls, and then processes any postback data included with the Request instance.

Load :

The Page calls the OnLoad event method on the Page, then recursively does the same for each child control, which does the same for each of its child controls until the page and all controls are loaded.

Use the OnLoad event method to set properties in controls and 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 TextChanged event.

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
Use this event for tasks that require that all other controls on the page be loaded.

PreRender:

Before this event occurs:

The Page object calls EnsureChildControls for each control and for the page.

Each data bound control whose DataSourceID property is set calls its DataBind method. For more information, see Data Binding Events for Data-Bound Controls later in this topic.

The PreRender event occurs for each control on the page. Use the event to make final changes to the contents of the page or its controls.

SaveStateComplete:

Before this event occurs, ViewState has been saved for the page and for all controls. Any changes to the page or controls at this point will be ignored.

Use this event perform tasks that require view state to be saved, but that do not make any changes to controls.

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 that is sent 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 the Render 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 :

This event occurs 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.


More details can be found here
http://msdn.microsoft.com/en-us/library/ms178472.aspx

Mark As Answer if this solve your problem
Warm Regards,
Venkat
Author: Santosh             
Posted Date: October 08, 2009     Points: 5   

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 a button. New data appears in response. How does this work?

When an ASP.NET page is requested from the server, the server loads the page into server memory, processes the page, sends the page to the user, and then unloads it from memory.
From the beginning of the life cycle to the end, the goal is to render appropriate HTML to the requesting browser. At each step, methods and events are available that allow you to override the default behavior or add your own programmatic enhancements.

There are two slightly different sequences in the life cycle: one for the first time a page is loaded, and a second when the page reloads itself in a postback.

During the first page load, the life cycle consists of the following steps:

1. A request for the page is made from a browser to the web server. The ASP.NET Framework first determines whether the page already exists in a cache (a section of memory specifically reserved for recently used items). If so, the page is retrieved and returned to the browser and we are done. If not, then the actual page life cycle starts at this point.

2. During the Start phase, the postback mode is determined. If the page was requested by a different page, then it was not a postback. If the page was returned to the server for processing and redisplay, then it is a postback.
The IsPostBack and PreviousPage properties are set accordingly. The Request and
Response properties of the page along with a number of other properties are also
set.

3. The Page Initialization phase contains two events often handled by your code:
PreInit and Init.
If you do not handle these explicitly yourself, ASP.NET will perform the default behavior on your behalf.

During the PreInit event, the target device is determined before the page is initialized, the master page is set, the control tree is built, and the controls are assigned unique IDs, which are made available to your code. Personalization and themes are loaded and applied to the page in this step.

PreInit is the first event in the life cycle that can be trapped and handled. That is, this is the first event that you can write your own code for, to change the default behavior of initializing the page.

4. During the Init event, control properties are read or initialized. If this is a postback, it is important to realize that the controls won't reflect any changes to the page made before the postback-that happens in the PreRender phase. They will contain values specified in the markup file.

5. During the Load event, all the control properties are set. View state information is available, and controls in the page's control hierarchy can be accessed. The load phase is routinely modified in a Page_Load method.

6. During the Validation phase, the Validate method is called on all the validation controls on the page. The IsValid property is set for all those controls and for the page as a whole.

7. During the Rendering phase, personalization, control, and view state is saved. Each control on the page is called in succession to render itself to the browser, that is, to compose itself into HTML that is included in the page's Response property.

It is very common to handle the PreRender event with a Page_PreRender method, typically when you must take some action based on the final value of some other control on the page. During the Render event, the HTML is actuallygenerated and sent to the requesting page, although this event is rarely handled unless you are developing custom controls.

8. Unload is the last event of the life cycle. It gives you an opportunity to do any final cleanup (i.e., closing open files and releasing references to expensive resources, such as database connections).

During postback, the life cycle is the same as during the first load, except for the following:

1. During the Load phase, after initialization is complete, the view state and the control state are loaded and applied as necessary.

2. After the Validation phase completes, postback data is processed. Control event handlers are now executed.

This is important: control event handlers, such as a Button Click, are not called until after the Page Initialization and Load events are handled.

This is important because it is often critical in which order code in the various event handlers is executed.



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