[this is a copy of my question on SO https://stackoverflow.com/questions/2200215/user-interface-functionality-modelling-languages but it fits better here]
I am looking for a UI functionality modelling language (UML-alike "thing", but for user interfaces) that is already accepted and maybe has its design patterns and handles the problem better than state or activity diagram.
(If there is no such thing I'm planning to develop one)
Clarification: It's functionality modelling (possible user interaction, events etc.), so it's not about where I put a button. It's about objects that have some events binded, and the interface behaviour logic.
I've been thinking of using BPMN but It was not created for this purpose.