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Re: Use cases for post-redisplay hooks

From: Clément Pit--Claudel
Subject: Re: Use cases for post-redisplay hooks
Date: Mon, 11 Jul 2016 00:17:11 +0200
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On 2016-07-04 12:39, Clément Pit--Claudel wrote:
> Hi emacs-devel,
> As a prelude to implementing the actual functionality, I'd like to get a 
> clearer picture of what use cases exist for post-redisplay hooks.

Summarizing the current discussion:

(1) Is still valid, though Richard did suggest that we could have Emacs take 
the screenshots itself. It would indeed be a useful feature to have, though it 
is partly orthogonal, and seems non-trivial to implement. Richard also 
suggested special casing this feature and implementing only that instead of a 

(2) Does not sound like it would benefit significantly from such a hook (but I 
may have missed something).

(3) Does not resolve to something concrete.


> Here are the ones that I'm currently aware of; can you think of other ones?
> 1. (my own) Generating emacs screenshots. You can see a demo at 
> http://web.mit.edu/cpitcla/www/emacs-screencast-annot.gif (let me know what 
> you think!). Other hooks do not work because:
>   - pre-redisplay-functions are called too early
>   - pre- and post-command-hook do not run often enough (in the screencast 
> above, see the butterfly animation, for example)
>   - window-related hooks do not run often enough either
>   - redisplay-end-trigger-functions are tricky to use, and deprecated
> 2. (Keith David Bershatsky + Michael Heerdegen @ 
> https://lists.gnu.org/archive/html/bug-gnu-emacs/2016-01/msg01013.html) 
> Updating overlays after scrolling.  The bug thread is pretty long and subtle, 
> but this issue has also been discussed on emacs.stackexchanged IIRC.
> 3. (Stefan? @ 
> https://lists.gnu.org/archive/html/emacs-devel/2009-02/msg00785.html) Running 
> code every time "something visible" happens (e.g. due to process filters)
> For number 1, and potentially number 3, a hook that run after redisplay 
> completes entirely would work. For number 2, things are less clear. Michael, 
> Keith, and Eli, do you have ideas of when in the display cycle such a hook 
> should run?
> Thanks!
> Clément.

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