hook for buffers becoming writable/read-only

hook for buffers becoming writable/read-only

[Steve Yegge]

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I have a minor mode that wishes to disable itself when the bufferbecomes
writable, but I do not see a way to do it.
The closest existing scenario I can see is view-mode, but view-mode
is special-cased in toggle-read-only, rather than listening on a hook.
This does not seem clean.

Moreover, most modes do not use toggle-read-only; they set the
buffer-read-only variable directly.  This has the (in my mind) unwanted
side-effect of leaving a writable buffer in view-mode if it is made
writable by setting the variable directly.

The semantics are admittedly a bit vague with respect to temporary
changes from let-binding buffer-read-only or inhibit-read-only, but
I would be satisfied if the hook were run on any persistent change
to the buffer-read-only variable.

Perhaps rather than creating a new hook, we could simply call the
before-change-functions and after-change-functions when the buffer
read-only status changes.

-steve

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Re: hook for buffers becoming writable/read-only

[Lennart Borgman]

On Wed, Jul 22, 2009 at 5:39 AM, Steve Yegge<stevey@google.com> wrote:
> I have a minor mode that wishes to disable itself when the buffer
> becomes writable, but I do not see a way to do it.

Don't you have to define "when" a bit closer here?


> The closest existing scenario I can see is view-mode, but view-mode
> is special-cased in toggle-read-only, rather than listening on a hook.
> This does not seem clean.
> Moreover, most modes do not use toggle-read-only; they set the
> buffer-read-only variable directly.  This has the (in my mind) unwanted
> side-effect of leaving a writable buffer in view-mode if it is made
> writable by setting the variable directly.
> The semantics are admittedly a bit vague with respect to temporary
> changes from let-binding buffer-read-only or inhibit-read-only, but
> I would be satisfied if the hook were run on any persistent change
> to the buffer-read-only variable.
> Perhaps rather than creating a new hook, we could simply call the
> before-change-functions and after-change-functions when the buffer
> read-only status changes.
> -steve

Re: hook for buffers becoming writable/read-only

[Stefan Monnier]

> I have a minor mode that wishes to disable itself when the buffer becomes
> writable, but I do not see a way to do it.

Indeed, there is no such hook.  I can think of two ways to get something
similar:
- use the usual post-command-hook to check the state of the
  variable and react accordingly.
- if your minor mode is mostly a bunch of key-bindings, you can do what
  I did in diff-mode and use buffer-read-only as the minor-mode
  variable:

  [...]
  ;; Neat trick from Dave Love to add more bindings in read-only mode:
  (lexical-let ((ro-bind (cons 'buffer-read-only diff-mode-shared-map)))
    (add-to-list 'minor-mode-overriding-map-alist ro-bind)
    ;; Turn off this little trick in case the buffer is put in view-mode.
    (add-hook 'view-mode-hook
	      (lambda ()
		(setq minor-mode-overriding-map-alist
		      (delq ro-bind minor-mode-overriding-map-alist)))
	      nil t))
  [...]


-- Stefan

Re: hook for buffers becoming writable/read-only

[Steve Yegge]

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The post-command-hook approach worked like a charm.  Thanks!
-steve

On Wed, Jul 22, 2009 at 11:23 AM, Stefan Monnier
<monnier@iro.umontreal.ca>wrote:

> > I have a minor mode that wishes to disable itself when the buffer becomes
> > writable, but I do not see a way to do it.
>
> Indeed, there is no such hook.  I can think of two ways to get something
> similar:
> - use the usual post-command-hook to check the state of the
>  variable and react accordingly.
> - if your minor mode is mostly a bunch of key-bindings, you can do what
>  I did in diff-mode and use buffer-read-only as the minor-mode
>  variable:
>
>  [...]
>  ;; Neat trick from Dave Love to add more bindings in read-only mode:
>  (lexical-let ((ro-bind (cons 'buffer-read-only diff-mode-shared-map)))
>    (add-to-list 'minor-mode-overriding-map-alist ro-bind)
>    ;; Turn off this little trick in case the buffer is put in view-mode.
>    (add-hook 'view-mode-hook
>              (lambda ()
>                (setq minor-mode-overriding-map-alist
>                      (delq ro-bind minor-mode-overriding-map-alist)))
>              nil t))
>  [...]
>
>
> -- Stefan
>

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