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

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

[Raffaele Ricciardi]

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`.)

Thank you.

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

[Stefan Monnier]

> 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`.)

undo reverses its behavior after any other command than undo.
I.e. keyboard-quit is not special in this respect.  The only part that's
special about keyboard-quit is that if you use it like that, it
typically doesn't do anything else (well, it beeps).


        Stefan

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

[Drew Adams]

> 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`.