How to efficiently include a repository relative path in the mode-line?

Question

Currently in the mode-line the filename %f, is absolute.

I would like to have the path relative to the version-control root, without having to find this every redraw.

How could this be cached, allowing for changes to the buffers filename to be refreshed?

---

Current code for reference.

(defvar-local buffer-file-name--vc-root-relative nil "
Use to cache the VC relative path.

- nil: Unset.
- t: No VC root, use regular filename.
- Otherwise this is the VC relative path.
")

(defun buffer-file-name-vc-root-relative-fn (filepath) "
Cache the vc-root relative name of the buffer,
return this, or a mode-line formatting character.
"
  (if filepath
    (progn
      ;; lazy initialize.
      (when (eq buffer-file-name--vc-root-relative nil)
        (let
          ((base-path
             (vc-call-backend
               (vc-responsible-backend filepath) 'root filepath)))
          (setq buffer-file-name--vc-root-relative
            (if base-path
              ;; Account for possibility of filepath containing a '%'.
              (replace-regexp-in-string "%" "%%"
                (file-relative-name filepath base-path)))
            t)))
      (if (eq buffer-file-name--vc-root-relative t)
        "%f"
        buffer-file-name--vc-root-relative))
    ;; No filepath, return buffer name.
    "%8b"))

Answer 1

after-set-visited-file-name-hook is a reliable way to detect changes to the filename.

This clears the cached value when a new filename if set.

;; Clear cached relative file name when saving under a new name,
(add-hook
  'after-set-visited-file-name-hook
  (lambda ()
    (setq buffer-file-name--vc-root-relative nil)))