[vtkusers] How to update coordinates of one vtkPlane if coordinates of another vtkPlane change?

[Sophonet]

Hi list,

in my VTK app, I need two vtkPlanes (A and B) for different purposes. 
However, their information depends on each other. If coordinates of 
plane A change, I would like to automatically B recalculate its 
parameters, based on indirectly derived information from A.

Currently, I am manually triggering the update of B when I change the 
information of A, but for that "hack", I have to provide an interface 
through a few software layers, which I would like to avoid.

I have not found a way to listen to "modify" - events of A, since it 
seems that there is no "modified()" - command.

Any suggestions?

Thanks,

      Sophonet