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bug#22404: 25.1.50; Forcing `window-scroll-functions` to run.
From: |
Keith David Bershatsky |
Subject: |
bug#22404: 25.1.50; Forcing `window-scroll-functions` to run. |
Date: |
Thu, 28 Jan 2016 19:08:22 -0800 |
Thanks, John, for taking another look at #22404.
Let's say for example that someone has a function named `foo` which uses
`window-start` and `window-end`, and the user wants that function to run every
command loop. Everyone will immediately think of the `post-command-hook`
because the buffer or position may have been modified by `this-command`
(whatever that may be). The way Emacs is presently designed, people are
needlessly calling `foo` multiple times when scrolling occurs because the
`post-command-hook` can't provide correct numbers. Most people probably don't
care because `foo` is trivial in terms of the time it takes to run -- so if it
runs 3 times per command loop, no problem. I personally think calling `foo` 3
times per command loop, when it only needed to run once, is a poor design.
There is no 100% guaranteed test from the `post-command-hook` to know whether
the `window-scroll-functions` hook will run, and if so, whether it will run 2
times (instead of just one).
Here is an excerpt from `window.h` that tells us what `optional_new_start` is
currently used for:
/* True means we have explicitly changed the value of start,
but that the next redisplay is not obliged to use the new value.
This is used in Fdelete_other_windows to force a call to
Vwindow_scroll_functions; also by Frecenter with argument. */
bool_bf optional_new_start : 1;
It is also used for `set-window-buffer`; so perhaps that should be added to the
doc-string.
By setting `optional_new_start` to `true`, we force the
`window_scroll_functions` hook to run every command loop (during redisplay).
If that happens, then the `post-command-hook` is no longer needed to run `foo`.
So we have just reduced `foo` being called 2 to 3 times, to 1 to 2 times
instead. I have been studying `xdisp.c` and have concluded that it is
extremely difficult to know whether the `window-scroll-functions` hook will run
1 or 2 times, because that hook can modify the buffer and there are some other
hooks that may modify the buffer too -- thus requiring a second call to the WSF.
So, the moral of the story is whether it is a good thing to call `foo` 2 to 3
times per command loop, instead of 1 to 2 times per command loop. And, it
boils down to whether `foo` is costly. It is costly for me because I use
`vertical-motion`. It may be costly to other users for a different reason.
Another idea would be to create a new animal that permits users to
THROW_ANY_SWITCH from `elisp`. It doesn't have to be `optional_new_start`
specifically. If users have an `elisp` mechanism to THROW_ANY_SWITCH, they can
throw it from anywhere -- `pre-command-hook`, `post-command-hook`, etc. [By
switch, I mean the bool_bf items in `window.h`.]
Keith
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At Thu, 28 Jan 2016 18:14:43 -0800,
John Wiegley wrote:
>
> >>>>> Keith David Bershatsky <esq@lawlist.com> writes:
>
> >> What do you need from window-scroll-functions? Only the correct values of
> >> window-start and window-end? Or something else?
>
> > Correct, I just need the 100% accurate `window-start` and
> > `window-end`. :) :)
>
> Keith, your use case here does strike me as a bit exotic. I'm not saying it's
> not a bug, but I'm wondering if your suggested solution is really the best
> path to the final result you're reaching for. The cost of `vertical-motion',
> for example, seems to be a driving force behind your request.
- bug#22404: 25.1.50; Forcing `window-scroll-functions` to run., (continued)
bug#22404: 25.1.50; Forcing `window-scroll-functions` to run., Keith David Bershatsky, 2016/01/21
bug#22404: 25.1.50; Forcing `window-scroll-functions` to run.,
Keith David Bershatsky <=
bug#22404: 25.1.50; Forcing `window-scroll-functions` to run., Keith David Bershatsky, 2016/01/29
bug#22404: 25.1.50; Forcing `window-scroll-functions` to run., Keith David Bershatsky, 2016/01/31