Emacs persistent folding mode

Question

There are plenty of ways to fold code in Emacs and I've settled in on using the outline minor mode... it works great!

However, I really want my folding to be persisted when I close and re-open files. It is quite frustrating to have folding set up in a file the way I like it, only to have that lost when I restart Emacs.

Has anyone found a way to keep the folding state of a file persistent?

Answer 1

Edit: Now that I understand the question...

How about something like the following nippet of code. It seems to work for me, though I haven't figured out how to avoid being prompted for the file local variable every time.

(defvar omm-state nil
  "file local variable storing outline overlays")
(defun omm-state-mode (&optional arg)
  "poor man's minor mode to re-apply the outline overlays "
  (interactive)
  (omm-re-enable-outline-state)
  (add-hook 'before-save-hook 'omm-state-save))
(defun omm-get-all-overlays ()
  "return a list of outline information for all the current buffer"
  (save-excursion
    (let ((all-overlays (overlays-in (point-min) (point-max))))
      (mapcar (lambda (o)
                (list (overlay-start o) (overlay-end o) (overlay-get o 'invisible)))
              (reverse all-overlays)))))
(defun omm-re-enable-outline-state (&optional arg)
  "turn on outline-minor-mode and re-apply the outline information"
  (outline-minor-mode 1)
  (when (listp omm-state)
    (mapcar (lambda (p)
              (apply 'outline-flag-region p))
            omm-state)))
(defun omm-state-save ()
  "save the outline state in a file local variable
Note: this just replaces the existing value, you need to start
it off by adding something like this to your file:

# Local Variables:
# omm-state:()
# mode:omm-state
# End:            
"
  (ignore-errors
    (save-excursion
      (goto-char (point-max))
      (when (search-backward "omm-state:" nil t)
        (goto-char (match-end 0))
        (kill-sexp)
        (princ (omm-get-all-overlays) (current-buffer)))))
  nil)

This solution requires you "seeding" your file with something like:

# Local Variables:
# omm-state:()
# mode:omm-state
# End: