When an instance of
this event is passed to SystemEventListener#processEvent or
ComponentSystemEventListener#processEvent, the listener
implementation may assume that the source of this event
instance is the UIComponent instance that is about to be
validated.
Components with children
must publish this event before processing their child nodes in
UIComponent#processValidators. This is especially important for
iterating components such as
UIData, and form components, such as UIForm.