Essential SharePoint

Understanding Event Order

From an event perspective, web part programming is very different from Windows Forms programming. In Windows Forms applications, code runs as long as the form is displayed and responds as events occur. Web part code runs only short bursts—beginning when the request is received by the server and ending shortly after the response is returned to the client browser.

The performance difference between the server and client Sum controls illustrates a basic principle of web part design: avoid unnecessary postbacks. The button web control and submit button trigger postback events by default, but you can also set the AutoPostback property on textbox, list, and checkbox controls to cause postbacks. When a postback event occurs, the browser sends the current state of the page back to the server, which then processes web part events and overridden methods in the order shown in Table 9-2.

Table 9-2. Order of major server events/methods in a web part

Event/method	Use to
OnInit event	Create the web part.
LoadViewState method	Override the web part's base class behavior when reading control properties from the web part's ViewState object.
CreateChildControls method	Add web controls to the Controls collection.
Onload event	Initialize resources used by the web part.
Cached child control events	Process cached events from child controls, such as TextChanged or SelectedIndexChanged.
Postback child control event	Process the event that caused the postback, for example ...