Microsoft SharePoint. Building Office 2007 Solutions in VB 2005 ...

374CHAPTER 10 ■ SHAREPOINT WEB PARTSAfter the web part is initialized, the ViewState of the web part is populated. ViewState isa property inherited from System.Web.UI.Control. The ViewState is filled from the state informationthat was previously serialized. Once the ViewState property is populated, the controlreturns to the same state it was in when it was last processed on the server. The ViewState ispopulated through a call to the LoadViewState method.Although it is normally not necessary to override the LoadViewState method, it is usefulif you would like to persist your own data within a web part. Overriding the LoadViewStatemethod along with the corresponding SaveViewState method lets you store custom data withinthe web part’s view state. In Listing 10-5, I use this technique to show how these methods fitinto the overall web part life cycle.When the viewstate becomes available, the web part’s user interface may be created. AsI showed earlier, this happens with a call to the CreateChildControls method. In this method,all of the constituent controls are created and added to the Controls collection. It is importantto note that the CreateChildControls method does not always get called at the same point inthe life cycle. When web parts are first rendered on the page, the method generally occurs afterthe OnLoad method. After a postback, however, the method is called before the OnLoad method.You can even force it to be called using the EnsureChildControls method. All this means thatyou must not make any hard assumptions about when this method will execute.Once the web part interface has been created and the properties set from the viewstate,the server can make changes to the properties of the web part based on values that are postedby the client browser. Any new values that are posted during the request—such as text fieldvalues—are applied to the corresponding property of the web part. At this point, the web parthas reached the state it was in just before the postback occurred.After all of the new property values are applied to the web part, the page may begin usingthe information to process the end user request. This begins through a call to the OnLoad methodof the WebPart class. The OnLoad method fires for every web part regardless of how many propertieshave changed. Web part developers use the OnLoad method as the basis for the functionalityembodied in the web part. During this event, web parts may obtain a connection to a databaseor other system to retrieve information for display. The key thing to remember about this methodis that it always fires after the posted data has been applied to the web part.Once the OnLoad method completes, any events triggered by the client interaction with theweb part are fired. This includes all user-generated events such as the Click event associatedwith a button. It is critical for the web part developer to understand that the user-generatedevents happen after the OnLoad event. This means that you must be careful not to rely on theresults of user-generated events when you write code for the OnLoad event.Once the web part has finished handling the user-generated events, it is ready to createthe output of the control. The web part begins creating this output with a call to theOnPreRender method of the WebPart class. The OnPreRender method gives the web part developerthe opportunity to change any of the web part properties before the control output isdrawn. This is the perfect place to run a database query that relies on several user-suppliedvalues, because all of the values will be available at this point in the life cycle.After the OnPreRender event is complete, the web part output may be drawn. Drawingbegins through a call to the RenderContents method, which I discussed in the “Rendering WebParts” section. In this method, the web part must programmatically generate its HTML output.This output will be rendered in the appropriate zone on the page in the portal.The next step is to save the state of any controls on the page in the ViewState. TheViewState of the web part is serialized and saved to a hidden field in the web page.