GNU bug report logs - #19684
25.0.50; `this-command` is set to `nil` before `window-scroll-functions` hook runs.

Previous Next

To add a comment to this bug, you must first unarchive it, by sending
a message to control AT debbugs.gnu.org, with unarchive 19684 in the body.
You can then email your comments to 19684 AT debbugs.gnu.org in the normal way.

Toggle the display of automated, internal messages from the tracker.

Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Keith David Bershatsky <esq <at> lawlist.com>
To: bug-gnu-emacs <at> gnu.org
Subject: 25.0.50;
 `this-command` is set to `nil` before `window-scroll-functions` hook
 runs.
Date: Sat, 24 Jan 2015 23:19:48 -0800

To the extent that a user wishes to use the value of `this-command` within a function attached to the `window-scroll-functions` hook, a workaround is needed because the default value is `nil`.  In other words, something at the C source code level is setting the value to `nil` before the `window-scroll-functions` hook runs.

There are two (2) workarounds that I am aware of.

1.  Expressly call `redisplay` at the tail end of the series of functions attached to the `post-command-hook`.  Calling `redisplay` somehow prevents `this-command` from being reset to `nil` and the `window-scroll-functions` hook can then take advantage of the existing value of `this-command`.

2.  Create a separate variable -- e.g., `my-this-command` and set the value within a function attached to the `post-command-hook`.  Like so:  (setq my-this-command this-command).  And, then use the value of `my-this-command` within the function attached to the `window-scroll-functions` hook.

The most reliable test I have discovered to pinpoint when the `window-scroll-functions` hook will activate is by having the following code within a function attached to the `post-command-hook`:  (pos-visible-in-window-p (point) (get-buffer-window (current-buffer) (selected-frame))).  If the result is `t`, then the `window-scroll-functions` hook will be activating.

I do not see a visual difference between either of the two workarounds mentioned above, even though I would expect to see a flicker on the screen with the first workaround.  However, there is no flicker.  The second workaround is probably a safer bet.

Although this bug report has been submitted with Emacs Trunk from October 1, 2014; the behavior is the same with Emacs Trunk built on January 1, 2015.

Thanks,

Keith

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

In GNU Emacs 25.0.50.1 (x86_64-apple-darwin10.8.0, NS appkit-1038.36 Version 10.6.8 (Build 10K549))
 of 2014-10-01 on MP.local
Repository revision: 117996 dmantipov <at> yandex.ru-20141001132108-zdsxru2390mqyjlu
Windowing system distributor `Apple', version 10.3.1038
Configured using:
 `configure --with-ns'

Configured features:
ACL LIBXML2 ZLIB

Important settings:
  locale-coding-system: utf-8-unix

Major mode: Fundamental

Minor modes in effect:
  sd-mode: t
  sb-mode: t
  tb-mode: t
  shell-dirtrack-mode: t
  cm-mode: t
  frame-bufs-mode: t
  bc-mode: t
  ds-mode: t
  ml-mode: t

Recent input:
<escape> x r e p o r t - e m a c s - b u g <return
>

Recent messages:
Wrote /Users/HOME/.0.data/.0.emacs/.lock

Load-path shadows:
/Users/HOME/.0.data/.0.emacs/.0.wl/md4 hides /Users/HOME/.0.data/.0.emacs/Emacs_10_01_2014.app/Contents/Resources/lisp/md4
/Users/HOME/.0.data/.0.emacs/.0.wl/hex-util hides /Users/HOME/.0.data/.0.emacs/Emacs_10_01_2014.app/Contents/Resources/lisp/hex-util
/Users/HOME/.0.data/.0.emacs/.0.wl/sasl hides /Users/HOME/.0.data/.0.emacs/Emacs_10_01_2014.app/Contents/Resources/lisp/net/sasl
/Users/HOME/.0.data/.0.emacs/.0.wl/sasl-ntlm hides /Users/HOME/.0.data/.0.emacs/Emacs_10_01_2014.app/Contents/Resources/lisp/net/sasl-ntlm
/Users/HOME/.0.data/.0.emacs/.0.wl/sasl-digest hides /Users/HOME/.0.data/.0.emacs/Emacs_10_01_2014.app/Contents/Resources/lisp/net/sasl-digest
/Users/HOME/.0.data/.0.emacs/.0.wl/sasl-cram hides /Users/HOME/.0.data/.0.emacs/Emacs_10_01_2014.app/Contents/Resources/lisp/net/sasl-cram
/Users/HOME/.0.data/.0.emacs/.0.wl/ntlm hides /Users/HOME/.0.data/.0.emacs/Emacs_10_01_2014.app/Contents/Resources/lisp/net/ntlm
/Users/HOME/.0.data/.0.emacs/.0.wl/hmac-md5 hides /Users/HOME/.0.data/.0.emacs/Emacs_10_01_2014.app/Contents/Resources/lisp/net/hmac-md5
/Users/HOME/.0.data/.0.emacs/.0.wl/hmac-def hides /Users/HOME/.0.data/.0.emacs/Emacs_10_01_2014.app/Contents/Resources/lisp/net/hmac-def
/Users/HOME/.0.data/.0.emacs/.0.wl/rfc2368 hides /Users/HOME/.0.data/.0.emacs/Emacs_10_01_2014.app/Contents/Resources/lisp/mail/rfc2368
/Users/HOME/.0.data/.0.emacs/.0.wl/utf7 hides /Users/HOME/.0.data/.0.emacs/Emacs_10_01_2014.app/Contents/Resources/lisp/gnus/utf7
/Users/HOME/.0.data/.0.emacs/.0.wl/smime hides /Users/HOME/.0.data/.0.emacs/Emacs_10_01_2014.app/Contents/Resources/lisp/gnus/smime
/Users/HOME/.0.data/.0.emacs/.0.wl/pgg hides /Users/HOME/.0.data/.0.emacs/Emacs_10_01_2014.app/Contents/Resources/lisp/obsolete/pgg
/Users/HOME/.0.data/.0.emacs/.0.wl/pgg-pgp5 hides /Users/HOME/.0.data/.0.emacs/Emacs_10_01_2014.app/Contents/Resources/lisp/obsolete/pgg-pgp5
/Users/HOME/.0.data/.0.emacs/.0.wl/pgg-pgp hides /Users/HOME/.0.data/.0.emacs/Emacs_10_01_2014.app/Contents/Resources/lisp/obsolete/pgg-pgp
/Users/HOME/.0.data/.0.emacs/.0.wl/pgg-parse hides /Users/HOME/.0.data/.0.emacs/Emacs_10_01_2014.app/Contents/Resources/lisp/obsolete/pgg-parse
/Users/HOME/.0.data/.0.emacs/.0.wl/pgg-gpg hides /Users/HOME/.0.data/.0.emacs/Emacs_10_01_2014.app/Contents/Resources/lisp/obsolete/pgg-gpg
/Users/HOME/.0.data/.0.emacs/.0.wl/pgg-def hides /Users/HOME/.0.data/.0.emacs/Emacs_10_01_2014.app/Contents/Resources/lisp/obsolete/pgg-def

Features:
(shadow disp-table mime-setup mail-mime-setup semi-setup mime-image
modb-standard elmo-imap4 emacsbug lawlist-desktop frameset lawlist-bk
lawlist-arc lawlist-dv lawlist-mc rect lawlist-ztree lawlist-wl
w3m-load mime-w3m w3m browse-url doc-view jka-compr image-mode
w3m-hist w3m-fb bookmark-w3m w3m-ems mule-util w3m-ccl w3m-favicon
w3m-image w3m-proc w3m-util wl-demo wl-spam elmo-spam elsp-header
elsp-generic wl-fldmgr wl-score smtp sasl sasl-anonymous sasl-login
sasl-plain wl-news wl-address pldap wl-thread wl-action wl-e21
wl-draft wl-template wl-summary ps-print ps-def lpr wl-refile
wl-message wl-mime pgg mime-pgp mime-play filename mime-edit
eword-encode pgg-parse pccl pccl-20 ccl pgg-def signature sendmail
elmo-mime mmelmo-buffer mmelmo-imap mime-view mime-conf calist
semi-def mmimap mime-parse mmbuffer wl-highlight elmo-filter
elmo-multi elmo-nntp wl-folder wl wl-util crm pp elmo-pop3 elmo-net
elmo-cache elmo-map elmo-dop elmo-flag elmo-localdir wl-vars elmo
elmo-signal elmo-msgdb modb modb-generic modb-entity mime mmgeneric
elmo-util emu invisible inv-23 poem poem-e20 poem-e20_3 utf7
eword-decode mel mime-def alist std11 mcharset mcs-20 mcs-e20 pces
pces-e20 pces-20 broken pcustom elmo-date elmo-vars path-util poe pym
static product wl-version elmo-version apel-ver luna lawlist-vr-hr
lawlist-test lawlist-sb lawlist-vl lawlist-ln lawlist-hl
lawlist-parens lawlist-dired tramp-sh lawlist-ws lawlist-calc
lawlist-flyspell lawlist-yas lawlist-elisp lawlist-tex-mode skeleton
compare-w lawlist-txt-mode lawlist-tabbar lawlist-github ido view
vc-git vc vc-dispatcher tramp tramp-compat tramp-loaddefs trampver
shell thingatpt time-stamp log-view log-edit message mml mml-sec
mm-decode mm-bodies mm-encode gmm-utils mailheader pcvs-util add-log
json help-mode grep compile find-lisp epa epg epg-config ediff-merg
ediff-wind ediff-diff ediff-mult ediff-help ediff-init ediff-util
ediff ert ewoc debug eieio-base diff-mode conf-mode autorevert
filenotify lorg-calendar org-element lorg-search org-agenda
context-menu dired-read-filename dired count-business-days
lawlist-toodledo url-http tls url url-proxy url-privacy url-expand
url-methods url-history mailcap url-auth url-cookie url-domsuf
url-util url-parse auth-source eieio eieio-core password-cache url-gw
url-vars lawlist-org lawlist-calendar org byte-opt bytecomp
byte-compile cconv org-macro org-footnote org-pcomplete pcomplete
org-list org-faces org-entities noutline outline org-version
ob-emacs-lisp ob ob-tangle org-src ob-ref ob-lob ob-table ob-keys
ob-exp ob-comint ob-core ob-eval org-compat org-macs org-loaddefs
find-func cal-menu easymenu calendar cal-loaddefs format-spec
lawlist-frame lawlist-cm lawlist-bbdb gnus gnus-ems nnheader gnus-util
mail-utils wid-edit mail-parse rfc2231 rfc2047 rfc2045 ietf-drums
mm-util mail-prsvr mailabbrev mail-extr rfc822 cl timezone
lawlist-grep lawlist-compile rx comint ansi-color ring
lawlist-frame-bufs advice derived lawlist-window lawlist-print
lawlist-undo pcase lawlist-tm lawlist-bc lawlist-as cl-macs gv
lawlist-ds lawlist-ml easy-mmode lawlist-misc lawlist-keymap edmacro
kmacro eldoc cl-loaddefs cl-lib help-fns savehist saveplace server
time-date tooltip electric uniquify ediff-hook vc-hooks
lisp-float-type mwheel ns-win tool-bar dnd fontset image regexp-opt
fringe tabulated-list newcomment elisp-mode lisp-mode prog-mode
register page menu-bar rfn-eshadow timer select scroll-bar mouse
jit-lock font-lock syntax facemenu font-core frame cham georgian
utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao korean
japanese hebrew greek romanian slovak czech european ethiopic indian
cyrillic chinese case-table epa-hook jka-cmpr-hook help simple abbrev
minibuffer nadvice loaddefs button faces cus-face macroexp files
text-properties overlay sha1 md5 base64 format env code-pages mule
custom widget hashtable-print-readable backquote make-network-process
cocoa ns multi-tty emacs)

Memory information:
((conses 16 1229677 95833)
 (symbols 48 61646 0)
 (miscs 40 105 150)
 (strings 32 132512 13095)
 (string-bytes 1 4616193)
 (vectors 16 33496)
 (vector-slots 8 697599 22896)
 (floats 8 942 252)
 (intervals 56 4202 0)
 (buffers 976 11))

Message #8 received at 19684 <at> debbugs.gnu.org (full text, mbox):

From: martin rudalics <rudalics <at> gmx.at>
To: Keith David Bershatsky <esq <at> lawlist.com>, 19684 <at> debbugs.gnu.org
Subject: Re: bug#19684: 25.0.50;
 `this-command` is set to `nil` before `window-scroll-functions`
 hook runs.
Date: Sun, 25 Jan 2015 11:38:51 +0100

> To the extent that a user wishes to use the value of `this-command`
> within a function attached to the `window-scroll-functions` hook, a
> workaround is needed because the default value is `nil`.  In other
> words, something at the C source code level is setting the value to
> `nil` before the `window-scroll-functions` hook runs.

Couldn't you use `last-command' here?

martin

Message #11 received at 19684 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Keith David Bershatsky <esq <at> lawlist.com>
Cc: 19684 <at> debbugs.gnu.org
Subject: Re: bug#19684: 25.0.50;
 `this-command` is set to `nil` before `window-scroll-functions`
 hook	runs.
Date: Sun, 25 Jan 2015 17:33:20 +0200

> Date: Sat, 24 Jan 2015 23:19:48 -0800
> From: Keith David Bershatsky <esq <at> lawlist.com>
> 
> To the extent that a user wishes to use the value of `this-command` within a 
> function attached to the `window-scroll-functions` hook, a workaround is needed 
> because the default value is `nil`.  In other words, something at the C source 
> code level is setting the value to `nil` before the `window-scroll-functions` 
> hook runs.

That "something" is the command loop itself: it resets 'this-command'
when the command finishes and returns to the loop.

Recall that in Emacs, execution of commands running the Lisp
interpreter is separated from redisplay: the latter runs when Emacs is
idle and is waiting for input.  Since most of the calls to
window-scroll-functions are made by the display engine, the command
already returned to the command loop by that time.

> Calling `redisplay` somehow prevents `this-command` from being reset
> to `nil`

I hope it is now clear why this happens: in this scenario redisplay is
entered before the command finishes, so 'this-command' was still not
reset.

> The most reliable test I have discovered to pinpoint when the 
> `window-scroll-functions` hook will activate is by having the following code 
> within a function attached to the `post-command-hook`:  
> (pos-visible-in-window-p (point) (get-buffer-window (current-buffer) 
> (selected-frame))).  If the result is `t`, then the `window-scroll-functions` 
> hook will be activating.

You are well-advised not to depend on such implementation details.

> Although this bug report has been submitted with Emacs Trunk from October 1, 
> 2014; the behavior is the same with Emacs Trunk built on January 1, 2015.

Emacs always behaved like that.  And it's not a bug.

Message #14 received at 19684 <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> IRO.UMontreal.CA>
To: Keith David Bershatsky <esq <at> lawlist.com>
Cc: 19684 <at> debbugs.gnu.org
Subject: Re: bug#19684: 25.0.50;
 `this-command` is set to `nil` before `window-scroll-functions` hook
 runs.
Date: Sun, 25 Jan 2015 11:14:22 -0500

tags 19684 notabug
thanks

> 1.  Expressly call `redisplay` at the tail end of the series of
>     functions attached to the `post-command-hook`.
>     Calling `redisplay` somehow prevents `this-command` from being
>     reset to `nil` and the `window-scroll-functions` hook can then
>     take advantage of the existing value of `this-command`.

It's not that calling `redisplay' prevents it.  It's that the normal
redisplay is run later (potentially much later if there are other things
pending), at a point which is not directly connected with the command,
hence this-command is not set any more because we're really not in the
process of running that command any more.


        Stefan

Message #19 received at 19684 <at> debbugs.gnu.org (full text, mbox):

From: Keith David Bershatsky <esq <at> lawlist.com>
To: Martin Rudalics <rudalics <at> gmx.at>, Eli Zaretskii <eliz <at> gnu.org>,
 Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: 19684 <at> debbugs.gnu.org
Subject: Re: bug#19684: 25.0.50;
 `this-command` is set to `nil` before `window-scroll-functions` hook
 runs.
Date: Sun, 25 Jan 2015 09:42:44 -0800

Thank you (Eli, Stefan and Martin) for helping me to better understand how the redisplay process works.

I was surprised to learn that I could not depend upon either `this-command` or `last-command` when using the `window-scroll-functions` hook because a forced redisplay prior thereto -- e.g., `(redisplay)` -- makes a difference in the values.

To obtain the name of the interactive function that ran before the `window-scroll-functions` hook was triggered:

  `this-command` may be used in the `window-scroll-functions` when there IS a forced redisplay prior thereto.

  `last command` may be used in the `window-scroll-functions` when there is NOT a forced redisplay prior thereto.

I am unaware of any documentation that could have helped me to discover the behavior described above.  Perhaps the Emacs team might consider adding something to the doc-string and/or the manual so that a user is advised as to the behavior, and to suggest an alternative approach:

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

(defvar my-command nil)

(defun set-my-command ()
  (setq my-command this-command))

(add-hook 'post-command-hook 'set-my-command)

(defun wsf-fn (win _start)
"NOTE:  Since the `window-scroll-functions` hook may be called more than
once, the user may not wish to set `my-command` to `nil` at this time."
  (message "my-command:  %s | this-command: %s | last-command: %s"
    my-command this-command last-command)
  (setq my-command nil))

(add-hook 'window-scroll-functions-hook 'wsf-fn)

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

Thanks,

Keith

Message #24 received at 19684-done <at> debbugs.gnu.org (full text, mbox):

From: Stefan Kangas <stefan <at> marxist.se>
To: Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: Keith David Bershatsky <esq <at> lawlist.com>, 19684-done <at> debbugs.gnu.org
Subject: Re: bug#19684: 25.0.50; `this-command` is set to `nil` before
 `window-scroll-functions` hook runs.
Date: Tue, 1 Oct 2019 22:16:57 +0200

Stefan Monnier <monnier <at> IRO.UMontreal.CA> writes:

> tags 19684 notabug
> thanks
>
>> 1.  Expressly call `redisplay` at the tail end of the series of
>>     functions attached to the `post-command-hook`.
>>     Calling `redisplay` somehow prevents `this-command` from being
>>     reset to `nil` and the `window-scroll-functions` hook can then
>>     take advantage of the existing value of `this-command`.
>
> It's not that calling `redisplay' prevents it.  It's that the normal
> redisplay is run later (potentially much later if there are other things
> pending), at a point which is not directly connected with the command,
> hence this-command is not set any more because we're really not in the
> process of running that command any more.

This was already tagged notabug; I'm now also closing it.

Best regards,
Stefan Kangas

Previous Next

GNU bug tracking system
Copyright (C) 1999 Darren O. Benham, 1997,2003 nCipher Corporation Ltd, 1994-97 Ian Jackson.