Fires when the NavigationFrame is about to change its currently selected page and allows you to cancel this upcoming modification.
The SelectedPageChanging event's handler receives an argument of the DevExpress.XtraBars.Navigation.SelectedPageChangingEventArgs type.
Handle the SelectedPageChanging event and set the Cancel property to true to restrict the frame navigation to the specific NavigationPage. If not canceled, the NavigationFrame.SelectedPageChanged event occurs.
The current selection in the frame page is stored in the NavigationFrame.SelectedPage property.