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

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

[Keith David Bershatsky]

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

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

[martin rudalics]

 > 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

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

[Eli Zaretskii]

> Date: Sat, 24 Jan 2015 23:19:48 -0800
> From: Keith David Bershatsky <esq@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.

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

[Stefan Monnier]

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

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

[Keith David Bershatsky]

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)

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Thanks,

Keith

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

[Stefan Kangas]

Stefan Monnier <monnier@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