The org.zkoss.zk.ui.util.DesktopCleanup interface
From Documentation
Listener:
org.zkoss.zk.ui.util.DesktopCleanup
A listener could implement DesktopCleanup to cleanup a desktop when it is being destroyed.
When ZK Loader is going to destroy a desktop, it invokes the cleanup method of this interface such that developers could plug the application-specific codes to cleanup a desktop.