Re: Use cases for post-redisplay hooks

[Keith David Bershatsky]

Yes, a `needs-recalculation` flag would be very useful -- i.e., when the values 
for `window-start` and `window-end` are correct and not going to change, then 
run the Lisp function (attached to the hook) that depends upon those values 
being correct.  A counter, however, may not be a reliable indicator of the 
above test.

[My assumption is that overlays could be placed before the screen visually 
refreshes, so that the overlays are visually present when redisplay finishes.]

Keith

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At Thu, 7 Jul 2016 12:08:09 -0400,
Clément Pit--Claudel wrote:
> 
> On 2016-07-04 18:50, Keith David Bershatsky wrote:
> > The reason I mentioned the `window-scroll-functions` hook was an
> > assumption (on my part) that Clément had initially categorized my
> > potential usage of the proposed `post-redisplay-hook` as only
> > relating to scrolling.  I just wanted to clarify / differentiate my
> > proposed use case as needing `window-start` and `window-end` each
> > command loop.
> 
> IIUC then the post-redisplay-hook thing wouldn't work very nicely: in 
> general, it's going to run more often than once per command loop cycle.
> 
> > With respect to the hook running more than once each command loop, it
> > is only a problem for me because my calculations are costly time-wise
> > (e.g., `vertical-motion').  So, I only want to do it one time each
> > command loop.
> 
> Would registering a post-command hook and incrementing a counter in there 
> help? You would then predicate your costly calculations on the value of that 
> counter having changed since the last time the calculations ran.
> 
> Even simpler: in a pre/post-command hook switch a `needs-recalculation' flag 
> to t, and after completing the calculations (from whichever other hook you're 
> using for them) switch `needs-recalculation' to nil. If the function doing 
> the calculations is called when `needs-recalculation' is nil, then can exit 
> immediately.
> 
> Or is there something more complex that I'm missing?
> 
> Clément.
> 
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