When associating user data with a key on a node,
Node.setUserData() can also accept, in its third argument, a handler which will be called when the object is cloned, imported, deleted, renamed, or adopted. Per the specification, exceptions should not be thrown in a
UserDataHandler. In both
Node.cloneNode(), although user data is not copied over, the handler will be called.
handle ( operationInt, keyString, DOMUserData, srcNode, dstNode ) (no return)
NODE_RENAMED is currently not supported since
Node.renameNode() is not supported.