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?

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.

;; Cache the vc-root relative name of the buffer.
(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)
  ;; Lazy initialize.
  (when (eq buffer-file-name--vc-root-relative nil)
    (setq buffer-file-name--vc-root-relative
      (let
        (
          (base-path
            (let ((base-vc-backend (ignore-errors (vc-responsible-backend filepath))))
              (when base-vc-backend
                (vc-call-backend base-vc-backend 'root filepath)))))
        (if base-path
          ;; Account for possibility of filepath containing a '%'.
          (concat
            "./"
            (replace-regexp-in-string "%" "%%" (file-relative-name filepath base-path)))
          ;; No base-path, set to 't', initialized with no base (return nil).
          t))))
  (unless (eq buffer-file-name--vc-root-relative t)
    buffer-file-name--vc-root-relative))

(add-hook 'after-set-visited-file-name-hook
  (lambda () (setq buffer-file-name--vc-root-relative nil)))

Then in the mode-line, this can be used to show the filename.

(or (buffer-file-name-vc-root-relative-fn filepath) "%f")