When
an ASP.NET application is executed, the web page goes through a life
cycle in which it performs a series of processing steps. When a web
page is requested, it is loaded into the Web Server memory, processed
and sent to the web browser. Then it (the object) is unloaded from
the memory. At each of these steps, methods and events are available,
ready to be overridden according to the need of the
application.
The
Page class creates a control tree of all the controls on the page.
All the components on the page, except the directives are part of
this control tree.
ASP.NET
Page Life Cycle Phases
- Initialization
- Instantiation
of controls on the web page
- Restoration
and Maintenance of state
- Execution
of the Event Handler code
- Page
Rendering
ASP.NET
Page Life Cycle Stages
- Page
Request – The ASP.NET page
life cycle (PLC) begins after the page is requested by a
user. ASP.NET then determines whether the web page needs to be
parsed and compiled, or a cached version of the page can be sent in
response without running the page.
- Start
– The page properties such as Request and Response are set
at the Start stage. It is determined whether the request is a
postback (old request) or a new request and IsPostBack
property of the Page is set accordingly. The UICulture
property of the Page is also set at this stage.
Events
a)
PreInit – Raised after the
Start stage is complete and before the Initialization stage begins
- Initialization
– The controls on the web page are available during page
initialization, and UniqueID property of each control is set.
If applicable, master page and themes are also applied to the page
during initialization. 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.
Events
a)
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.
b)
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.
- Load
– During load, if the current request is a postback, control
properties are loaded with information recovered from view state and
control state.
Events
a)
PreLoad – Raised after the
page loads view state for itself and all controls, and after it
processes postback data that is included with the Request instance.
b)
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.
- Postback
event handling – If
the request is a postback, the event handlers of the controls 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.)
Events
a)
Control Events – These
events should be used to handle specific control events, such as a
Button control's Click event or a TextBox control's TextChanged
event.
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.
b)
Load Complete – Raised at
the end of the event-handling stage.
c)
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. The
PreRender event of individual controls occurs after the PreRender
event of the page.
d)
PreRender Complete – Raised
after each data bound control whose DataSourceID property is set
calls its DataBind method.
e)
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.
- Rendering
– The view state is saved for the page and all controls
before rendering. 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.
Events
a)
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. A user control (an .ascx file) automatically incorporates
rendering, so you do not need to explicitly render the control in
code.
- 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.
Events
a)
Unload - Raised for each
control and then for the page. 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
