The PSN_SETACTIVE notification message notifies a page that it is about to be activated. This notification message is sent in the form of a WM_NOTIFY message.
PSN_SETACTIVE lpnmhdr = (NMHDR FAR *) lParam;
Returns zero to accept the activation or -1 to activate the next or previous page (depending on whether the user chose the Next or Back button). To set the activation to a particular page, return the resource identifier of the page.
The PSN_SETACTIVE notification message is sent before the page is visible. An application can use this notification to initialize data in the page.
To set the return value, the dialog box procedure for the page must use the SetWindowLong function with the DWL_MSGRESULT value, and the dialog box procedure must return TRUE.