Tk 21 proxy callback fn

I do think we'll need a little more detail on the callback-fn.  Specifically, since these are Java types, we probably need to mention the actual class names so that people can look at the docs for them if they need to.

The docstring might end up being pretty verbose in that case, so perhaps the docstring could simply reference a section of the README?  And the readme could have more detail and potentially even links to the javadocs for the two classes that are in play here?  Feel free to create a new subsection in the readme if it seems warranted.

Thanks!