Re: What's the best (i.e. least bad) way to re-redisplay?

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 23 Aug 2021 18:41:59 +0000
> From: Alan Mackenzie <acm@muc.de>
> 
> I envisage a situation where an identifier is identified as a type during
> jit-lock fontification, and thus font-lock-type-face needs to be applied
> to all occurrences of this identifier.
> 
> The problem is, there will be occurences in the foreground window before
> the one which triggered the fontification.  Redisplay will already have
> passed this earlier buffer position.  So I need to trigger another
> redisplay immediately after the current one.
> 
> What is the least bad way of doing this?

You mean, the same identifier is somewhere in the same window-ful, but
before the position where you suddenly decided your previous
fontification was incorrect?  (Why is this a reasonable scenario,
btw?)

> Currently, I'm thinking of something like the CC Mode fontification
> functions setting a flag, and a repeating timer function testing this
> flag every 0.025s, say, and triggering a redisplay if it's set.

Why not simply call a one-time timer function that runs
jit-lock-force-redisplay, when you decide that you need to refontify?
jit-lock.el already does that in some cases.  I see no reason to have
a periodic timer and a flag, because the same code that sets the flag
could instead arm a one-time timer.

> One problem is knowing whether or not redisplay is currently active.  Is
> there a simple way to do this in Lisp?  (I know there is in C.)

I don't understand your problem here.  Timers cannot run while
redisplay works, because timers are only run by the main loop.  So
when a timer runs, redisplay by definition isn't.