When an ASP.NET page runs, the page goes through a life cycle in which it performs a series of processing steps. These include initialization, instantiating controls, restoring and maintaining state, running event handler code, and rendering. It is important for you to understand the page life cycle so that you can write code at the appropriate life-cycle stage for the effect you intend.
ASP.Net life cycle specifies the following characterizes as:
- ASP.Net processes pages to produce dynamic output
- The application and its pages are instantiated and processed
- ASP.Net compiles the pages dynamically
The ASP.Net life cycle could be divided into two groups:
- Application Life Cycle
- Page Life Cycle
ASP.Net Application Life Cycle:
The application life cycle has the following stages:
- User’s requests for accessing application resource, (pages). Which in turn Browser sends this request to the web server.
- A unified pipeline receives this user request and the following events will take place as:
· An object of the ApplicationManager class is created.
· An object of the HostingEnvironment class is created to provide
information regarding the resources.
· Top level items in the application are compiled.
- Response objects are created. And application objects: HttpContext, HttpRequest and HttpResponse are created and initialized.
- An instance of the HttpApplication object is created and assigned to the request. The request is processed by the HttpApplication class. And Different events are raised by this class for processing the request.
ASP.Net Page Life Cycle:
When a page is requested, it is loaded into the server memory, and processed. After processing is completed, it sent to the browser. Then it is unloaded from the memory. At each of this steps, methods and events are available, which could be overridden according to the need of the application. In other words, you can write your own code to override the default code.
This life cycle of the ASP.NET page starts with a call to the ProcessRequest() method. This method begins by initializing the page's control hierarchy. Next, the page and its server controls proceed lock-step through various phases that are essential to executing an ASP.NET Web page. These steps include managing view state, handling postback events, and rendering the page's HTML markup. The life cycle ends by handing off the Web page's HTML markup to the Web server, which sends it back to the client that requested the page.
The page life cycle phases are:
- Instantiation of the controls on the page
- Restoration and maintenance of the state
- Execution of the event handler codes
- Page rendering
Understanding the page cycle helps in writing codes for making some specific thing happen at any stage of the page life cycle. It also helps in writing custom controls and initializing them at right time, populate their properties with view-state data and run control behavior code.
Following are the different stages of an ASP.Net page:
Page request . when ASP.Net gets a page request, it decides whether to parse and compile the page or there would be a cached version of the page; accordingly the response is sent.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.
Starting of page life cycle . This is second stage, in this stage the Request and Response objects are set. If the request is an old request or post back, the IsPostBack property of the page is set to true. The UICulture property of the page is also set.
Page initialization . At this stage, the controls on the page are assigned unique ID by setting the UniqueID property and themes are applied. For a new request postback data is loaded and the control properties are restored to on the basis of view-state values.
Page load. At this stage, control properties are set using the view state and control state values.
Validation . Validate method of the validation control is called and if it runs successfully, the IsValid property of the page is set to true.
PostBack event Handling. At this stage if the request is a postback (old Request i.e. the Request is posted back second or successive time), the related event handler is called.If the request is a postback, and control event handlers are called. After that, the Validate method of all validator controls is called, which sets the values when an ASP.NET page runs, the page goes through a life cycle in which it performs a series of processing steps. These include initialization, instantiating controls, restoring and maintaining state, running event handler code, and rendering. It is important for you to understand the page life cycle so that you can write code at the appropriate life-cycle stage for the effect you intend. 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.)
Page Rendering. in this stage, view state for the page and all controls are saved. The page calls the Render method for each control and the output of rendering is written to the OutputStream class of the Page's Response property.
Unload. the rendered page is sent to the client and page properties, such as Response and Request are unloaded and all cleanup done.
ASP.Net Page Life Cycle Events:
At each stage of the page life cycle, the page raises some events, which could be coded. An event handler is basically a function or subroutine, bound to the event, using declarative attributes like Onclick or handle. Following are the page life cycle events:
1- PreInit . PreInit is the first event in page life cycle. It checks the IsPostBack property and determines whether the page is a postback. It sets the themes and master pages, creates dynamic controls and gets and sets profile property values. This event can be handled by overloading the OnPreInit method or creating a Page_PreInit handler.
2- Init . Init event initializes the control property and the control tree is built. This event can be handled by overloading the OnInit method or creating a Page_Init handler.
3- InitComplete . InitComplete event allows tracking of view state. All the controls turn on view-state tracking.
4- LoadViewState . LoadViewState event allows loading view state information into the controls.
5- LoadPostData . during this phase, the contents of all the input fields defined with the <form> tag are processed.
6- PreLoad . PreLoad occurs before the post back data is loaded in the controls. This event can be handled by overloading the OnPreLoad method or creating a Page_PreLoad handler.
7- Load . the Load event is raised for the page first and then recursively for all child controls. The controls in the control tree are created. This event can be handled by overloading the OnLoad method or creating a Page_Load handler.
8- LoadComplete . the loading process is completed, control event handlers are run and page validation takes place. This event can be handled by overloading the OnLoadComplete method or creating a Page_LoadComplete handler.
9- PreRender . the PreRender event occurs just before the output is rendered. By handling this event, pages and controls can perform any updates before the output is rendered.
10- PreRenderComplete . as the PreRender event is recursively fired for all child controls, this event ensures the completion of the pre-rendering phase.
11- SaveStateComplete . state of control on the page is saved. Personalization, control state and view state information is saved. The HTML markup is generated. This stage can be handled by overriding the Render method or creating a Page_Render handler.
12- UnLoad . the UnLoad phase is the last phase of the page life cycle. It raises the UnLoad event for all controls recursively and lastly for the page itself. Final cleanup is done and all resources and references, such as database connections, are freed. This event can be handled by modifying the OnUnLoad method or creating a Page_UnLoad handler.