public interface SessionActivationListenerUsed to notify an object stored in a session, when the session is going to be deactivated or has been activated.
When a session is going to be deactivate, it checks every variable to see whether this interface is implemented. If implemented,
willPassivate(org.zkoss.zk.ui.Session)will be called. Similarly,
didActivate(org.zkoss.zk.ui.Session)is called if the session has been activated.