Re: Possible support for buffer local idle timers?

[Eli Zaretskii <eliz@gnu.org>]

> Date: Tue, 21 Sep 2021 10:36:05 +1000
> From: Campbell Barton <ideasman42@gmail.com>
> 
> >    . does the timer start measuring idle time only when the buffer is
> >      the current buffer, or regardless of that?
> 
> would just go with default behavior of global idle timers since a user 
> switching buffers will typically reset idle timers.

What do you mean by "reset timers" here?

> >    . what to do when the timer expired but the buffer wasn't current,
> >      and then the buffer became current? does the callback gets called
> >      right away, or do we "miss" the timer in that case?
> 
> default behavior could be to run timers on the previously active buffer 
> (instead of missing them), although tracking this information could get 
> more involved.
> 
> It may be useful for callers to request not to run in the case the 
> buffer becomes inactive.

I don't understand what you mean by "acting on a buffer".  The timer
function can do anything it wants in any buffer.

> >    . how to handle repeated timers?
> 
> As far as I can see repeated timers would be the primary use-case, for 
> this feature.
> 
> I'm not sure what you mean by "how to handle", all callbacks registered 
> to run a repeated buffer local timer could share a timer, this would 
> store a list of callbacks which would run each time.

It's related to the second question: if timers don't fire when "their"
buffer is not the current buffer, then repeating timers will
accumulate.  Then it becomes an issue what to do when the buffer
eventually becomes the current one: do we run the timer function N
times or just once?

> It may be that all things considered - there are too many ambiguities 
> and corner cases for this to be implemented cleanly.

Indeed, I think the idea is not clear enough, and in general timers do
not interact cleanly with the notion of the current buffer, which in
Emacs is very ephemeral.  Emacs switches momentarily to other buffers,
including temporary buffers, a lot, so you could miss a timer tick
very easily due to that.

> Again, this is meant to be an alternative to packages registering their 
> global repeating idle timers that are never removed.

Never removed why? because of bugs?

> If there are better alternatives to this (such as starting/stopping 
> global-idle-timers on switching buffers, perhaps using: 
> window-state-change-hook, then that might be better, although even then, 
> it may be worth presenting this as a buffer-local-idle-timer API.

I find the notion of stopping a timer when Emacs switches buffers
strange.  A timer runs with the time, and time does not stop when you
switch buffers.  There's no such notion in Emacs as "buffer-local
time", and if there were, I wouldn't know how to explain its
semantics.  Why does it matter how long was a buffer the current
buffer?