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I would like to make a snippet of code that only executes if the point is on a different line in a particular buffer since the last user command. The only way I could think to do it was using the post-command-hook:

(setq my-prev-line 0)

(defun my-install-move-hook (buf)
  (with-current-buffer buf
    (add-hook 'post-command-hook 'my-move-hook nil t)))

(defun my-move-hook ()
  (when (and my-prev-line (= my-prev-line (line-number-at-pos)))
    (message "The point moved from line number %d to %d" my-prev-line (line-number-at-pos))
    (setq my-prev-line (line-number-at-pos))))

That seems like overkill, since post-command-hook triggers after every command, not just user-invoked commands. Would it be better if the line number was stored during pre-command-hook?

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  • 2
    There is only rarely any distinction between "commands" and "user-invoked commands", so that comment sounds a little odd. Which are the 'not-user-invoked' commands you are wishing to avoid?
    – phils
    May 17 at 10:11
  • A user-invoked command is a command that is directly invoked by the user typing a keyboard shortcut for the command or running M-x <name of command>. For example, suppose that I have commands foo and baz, and baz calls foo 3 times. If the user invokes either foo or baz directly, there has been only one user-invoked command, but the post-command-hook would be called either once or four times (assuming that foo did something trivial).
    – shankar2k
    May 17 at 13:02
  • 1
    If bar calls (foo ...) that's not a separate command. A typical command might cause vast numbers of function calls, but it's still just one time around the command loop, and post-command-hook runs at the end of the command loop. Calling a function is not the same thing as invoking a command (even when the function being called could itself be invoked as a command).
    – phils
    May 17 at 14:01
  • 1
    Calculating line numbers is expensive. Why don't you place a marker at the beginning of the line in the pre-command hook and check if it's on the same line as the point in the post-command hook? May 17 at 14:59
  • 1
    @shankar2k, because finding the position of the start of the line is fast (at worst, Emacs steps one character at a time until the beginning of the line). Calculating the liner number requires Emacs to search through the entire text from the beginning to the point. May 18 at 12:49
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You say: "post-command-hook triggers after every command, not just user-invoked commands." And in comments you say that by "user-invoked" you really mean invoked interactively.

It's not true that post-command-hook triggers each time a function that is a command is invoked. It's triggered for such a function only when it is invoked interactively:

(defun foo () (interactive) (message "THIS: %s, FOO" this-command) (bar))
(defun bar () (interactive) (message "THIS: %s, BAR" this-command))
(defun toto () 
  (message "TOTO, THIS: %s" this-command)
  (when (equal this-command 'foo)
    (message "TOTO, FOO was invoked")))

(add-hook 'post-command-hook 'toto)

This is what *Messages* shows, when you evaluate (foo), that is, you do not invoke foo interactively (i.e., as a command):

THIS: eval-last-sexp, FOO
THIS: eval-last-sexp, BAR
"THIS: eval-last-sexp, BAR"
TOTO, THIS: eval-last-sexp

And this is what *Messages* shows when you do M-x foo, that is, you invoke foo interactively (i.e., as a command):

TOTO, THIS: execute-extended-command
TOTO, THIS: self-insert-command [3 times]
THIS: foo, FOO
THIS: foo, BAR
TOTO, THIS: foo
TOTO, FOO was invoked

You can put conditional code in your hook function, to do whatever you want when foo is invoked as a command. That's what the test (equal this-command 'foo) does above. You can see that toto prints TOTO, FOO was invoked only when foo is invoked as a command (e.g. M-x).


Alternatively, depending on what you really want, use function called-interactively, to test whether foo has been invoked as a command.

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