how to implement a custom validator for a custom TokenListControl

I am writing a custom checkbox widget based on prompt-toolkit. So far it works great. Now I want to add a validator so I can validate that for example at least one checkbox has been selected.
I understand that prompt-toolkit validators work on a Document which in turn is part of a Buffer. A TokenListControl has none of that. I could raise a ValidationError from my validator which runs in own scope - so it does not help much...
Any suggestions on how to best approach this? Are there any hooks I can use, do I use a bottom_toolbar (additional TokenListControl) to implement the validation message display myself?

[jonathanslenders]

Hi @markfink, thanks for the feedback.

So far, a TokenListControl does indeed not accept a validator. You can indeed use a custom bottom_toolbar that displays such a message when a certain key was pressed. (So you will need a custom key binding for the enter key, then check everything and give appropriate feedback in the bottom_toolbar widget.)

It's a bit awkward to get all of this working. For some time I've been planning to refactor a part of the internals of prompt_toolkit to make these kind of things easier. That's probably some job for 2017. ;)