Re: Is there something like `on-display-functions'?

[Eli Zaretskii <eliz@gnu.org>]

> Date: Thu, 28 Jan 2010 10:38:56 +0000
> Cc: Stefan Monnier <monnier@iro.umontreal.ca>, emacs-devel@gnu.org
> From: Alan Mackenzie <acm@muc.de>
> 
> As a zero order approximation, something like
> `before-display-functions', where an element would look like this:
> 
>    (defun foo-before-display (buffer beg end) .....)
> 
> , BEG and END defining the piece of BUFFER about to be displayed.  The
> hook would be called before fontification.

What do you mean by ``before fontification'', and why does it have to
be before that?

> (Is it possible in general to determine BEG and END before
> fontification?).

Only approximately.  E.g., imagine a `display' property whose value is
a long string -- without computing how it will be displayed, there's
no way of knowing how much of the screen estate it will take.  As
another, less exotic example, imagine font-lock definitions that
enlarge the font significantly: until you fontify that part, you
cannot know how much screen space it will take.

BEG and END are well known _after_ redisplay prepares what will be
shown on the glass, though.  Therefore, please tell why do you ask to
call this hook _before_ fontifications.