Emacs Lisp, how to monitor changes of a file/directory

Question

I'm looking for a way to check periodically if a files under a certain directory were changed from the last check (a functionality symilar to FAM daemon or to gio.monitor_directory). In emacs lisp.

• Are there any library/snippet that provide this functionality?
• If not, how can I implement such a function?

Answer 1

(defun install-monitor (file secs)
  (run-with-timer
   0 secs
   (lambda (f p)
     (unless (< p (second (time-since (elt (file-attributes f) 5))))
       (message "File %s changed!" f)))
   file secs))

(defvar monitor-timer (install-monitor "/tmp" 5)
  "Check if /tmp is changed every 5s.")

To cancel,

(cancel-timer monitor-timer)

Edit:

As mentioned by mankoff, the above code snippet monitors file modification in the last 5 seconds, instead of since last check. To achieve the latter, we will need to save the attributes each time we do the checking. Hope this works:

(defvar monitor-attributes nil
  "Cached file attributes to be monitored.")

(defun install-monitor (file secs)
  (run-with-timer
   0 secs
   (lambda (f p)
     (let ((att (file-attributes f)))
       (unless (or (null monitor-attributes) (equalp monitor-attributes att))
         (message "File %s changed!" f))
       (setq monitor-attributes att)))
   file secs))

(defvar monitor-timer (install-monitor "/tmp" 5)
  "Check if /tmp is changed every 5s.")