bug#17453: Framework extending window functions for Follow Mode (etc.).

[Alan Mackenzie <acm@muc.de>]

Hello, Juri

It's been nearly a week since your last email.  In that time, I tried
one of the approaches (see below) that worked just fine.... except it
wouldn't work with replace.el.  I got discouraged by this, and moved
onto other things for a while.  Now I'm back.

On Thu, Nov 19, 2015 at 02:45:43AM +0200, Juri Linkov wrote:

> >> Another thing that disturbs me is moving lazy-highlighting checks
> >> to a new function isearch-lazy-highlight-maybe-new-loop.

> > What disturbs you about this change in particular?

> The same consequences you mentioned earlier: various time periods
> of lazy-highlighting staying active.

OK.

> >> What do you think about moving only window-checking into new function
> >> i.e. only checks for window-start/window-end/etc that need (sit-for 0)
> >> and leaving other checks in isearch-lazy-highlight-new-loop?

> > Actually, I think that "(sit-for 0)" isn't needed - I left it in to deal
> > with isearch-lazy-highlight-initial-delay being set to 0, thinking that
> > Follow Mode's post-command-hook might "not have had time" to run.  But
> > surely post-command-hook will be called before checking timers.  I'll
> > need to look at the source code (probably keyboard.c) to be absolutely
> > sure of this.

The "(sit-for 0)" is absolutely not needed: post-command-hook is indeed
always invoked before Emacs checks for timer expiry; I tried this out.

> > I don't think splitting the checks between
> > isearch-lazy-highlight-new-loop (in the command loop) and a function
> > triggered by a timer is a good idea.  I tried it out, and it kind of
> > jars when the lazy highlighting sometimes stays 0.25s, sometimes goes
> > instantly.  How about maybe ...

> > How about maybe putting the test for new highlighting in Isearch's
> > post-command-hook (and using the APPEND argument of `add-hook' to make
> > sure Isearch's post-hook-function comes after Follow Modes's)?  That
> > way, all the lazy h. would be removed immediately after the command, as
> > at present.

This is precisely what I tried.  It works very well in Isearch.
However, the problem is in replace.el (the replace functionality, not
`occur'): `query-replace', instead of using the Command Loop like
Isearch does, implements its own command loop.  This seems suboptimal:
it doesn't invoke post-command-hook (or pre-command-hook) until the
entire replace session is over.

This means the use of `query-replace' whilst Follow Mode is enabled is
not going to work properly, without some radical change in replace.el.

Probably the smallest change would be to invoke new hooks
`pre-replace-command-hook' and `post-replace-command-hook' from
`query-replace''s command loop.

A more satisfying change would be to get rid of `perform-replace' and
use Emacs's command loop the way Isearch does.  This would probably not
be all that difficult.  Do you know if there's any special reason
`query-replace' implements its own command loop?

What do you think?

[ .... ]

> We need to test the same in all modes that use lazy-highlighting too.

Yes.  That's a problem, right now.

-- 
Alan Mackenzie (Nuremberg, Germany).