RE: How does `undo` know that `keyboard-quit`has been called?

[Drew Adams <drew.adams@oracle.com>]

> I would like to write a command that behaves like `undo`, that is: a
> command that reverses its behaviour when the user calls `keyboard-
> quit`.
>   I have read the Lisp code for both `undo` and `keyboard-quit` and
> I can't understand how `undo` achieves such behaviour. (I know that I
> could always advise `keyboard-quit`.)

If you just want to stop doing something when a user hits `C-g'
then wrap `condition-case' around the doing of the something:

(condition-case nil
     (do-something)
  (quit (handle-C-g-however-you-like)))

See the Elisp manual, node `Handling Errors`.