Perfect - that’s what I was looking for.
So when using something like this:
foo:foo()
{
foo->AddObserver(vtkCommand::<event-name>, this, &foo::bar);
}
The callback is able to run the member function, which enables me to use member variables aswell, without setting clientData of the AddObserver() call from my initial post.
Thank you @dgobbi
Best Regards
Phlipp