Re: track-changes and undo

[Joost Kremers <joostkremers@fastmail.fm>]

On Mon, Apr 22 2024, Stefan Monnier wrote:
> The gain from `track-changes` is just to provide you with the "before"
> string for deletions so it takes care of reading it in
> `before-change-functions` and then providing it to you in the
> `after-change-functions` (with the advantage that it detects/handles the
> various corner cases where that pairing fails).

I think I'll try and see if I can make it work with the `:immediate` option.
It would of course mean that the buffer is modified inside
`after-change-functions`, which you warn against, but it looks like that's the
only way.

> One other thing that you might have trouble to reproduce with
> `track-changes` is the following test:
>
>     (and (= beg (point-min)) (= end (point-max)))
>
> that you have in `cm-before-change`.  I'm not completely sure what this
> is for, tho.  Is it for `revert-buffer`?

I honestly don't remember... Based on the comment, it looks like
`switch-to-buffer` triggers `before-change-functions`, but a) that doesn't make
much sense; and b) the code seems to work just fine without that line. (I even
fired up a Vagrant box with an old Ubuntu release with Emacs 24, which would be
the most recent version when I wrote that code).

> [ I tend to do "destructive reads", so the patch below is the result of
>   reading that part of your code.  ]

Thanks.

> +(define-minor-mode cm-follow-changes  ;FIXME: Shouldn't it end in `mode'?
> +  "Minor mode to follow changes."
> +  :global nil
> +  (if cm-follow-changes
> +      (progn
> +        (add-hook 'before-change-functions #'cm-before-change t t)
> +        (add-hook 'after-change-functions #'cm-after-change nil t))
> +    (remove-hook 'before-change-functions #'cm-before-change t)
> +    (remove-hook after-change-functions #'cm-after-change t)))

Making it a minor mode makes sense, of course. And don't ask me why I used
`add-to-list` instead of `add-hook`. 🤔
  

-- 
Joost Kremers
Life has its moments