The org.zkoss.zk.ui.util.UiLifeCycle interface
From Documentation
Listener:
org.zkoss.zk.ui.util.UiLifeCycle
A listener could implement UiLifeCycle to handle something depending on the life cycle of UI, such as attaching a component to a page, moving a component and so on. Once registered, an instance of the specified class is created and shared within the whole application.