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Re: Use cases for post-redisplay hooks

From: Clément Pit--Claudel
Subject: Re: Use cases for post-redisplay hooks
Date: Thu, 7 Jul 2016 15:44:13 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.8.0

But I meant ­— can't you update this flag yourself, and do at most one 
recalculation per command loop?

On 2016-07-07 13:53, Keith David Bershatsky wrote:
> 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
> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
> 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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