How to hook into buffer modified state?

Question

Such as in the built-in variable mode-line-modified which displays an asterisk in the mode line if the buffer is modified.

Specifically, I want to turn hl-line-mode off when the buffer is modified and on when it's not.

I've read the documentation about "Change Hooks", but I want hl-line-mode to toggle when I undo a change like the mode line asterisk.

Answer 1

Building off of the comments, here are two ways to achieve what you're trying to do. (Not extensively tested, so YMMV.)

Option 1: post-command-hook

Create a function that tests whether or not the buffer is modified, and then hook it into post-command-hook:

(defun hl-line-mode-toggle-maybe ()
  "Turn on `hl-line-mode' when buffer is unmodified, turn it off
when it is modified."
  (hl-line-mode (if (buffer-modified-p) -1 1)))

(add-hook 'post-command-hook #'hl-line-mode-toggle-maybe)

The advantage is that it's pretty simple. The disadvantage is that Emacs will run this test after every single command, which seems like overkill.

Option 2: first-change-hook, after-save-hook, and undo

Create a function to turn hl-line-mode off and hook it into first-change-hook. Create a function to turn hl-line-mode on and hook it into after-save-hook. Provide after advice on undo such that it turns hl-line-mode on when an undo had returned the buffer to an unmodified state:

(defun hl-line-mode-off ()
  "Turn off `hl-line-mode'."
  (hl-line-mode -1))

(defun hl-line-mode-on ()
  "Turn off `hl-line-mode'."
  (hl-line-mode 1))

(add-hook 'first-change-hook #'hl-line-mode-off)
(add-hook 'after-save-hook   #'hl-line-mode-on)

(defadvice undo (after hl-line-when-unmodified activate)
  (unless (buffer-modified-p)
    (hl-line-mode 1)))

The advantage is that it calls these functions infrequently rather than after every command. The disadvantage is that you've had to delve into advice, and there may be other, unanticipated ways to get a buffer to an unmodified state that don't trigger the "turn hl-line-mode back on" step.