Projectile is a staple of my emacs configuration. In my opinion, it is one of the all time best packages available, along with org-mode and magit.

One of my most commonly used commands with projectile is helm-projectile. This lets me locate any file in my project by pressing C-c p h. Using the narrowing abilities of helm, I can quickly filter the list of options to exactly the one I want.

Because I work on some projects that are extremely large in size, I have enabled projectiles caching mechanism. This tells projectile to maintain a local cache of all the files within a project for faster searching. You can enable this with (setq projectile-enable-caching t)

Projectile helps maintain the current state of this cache by adding a hook to find-file. Whenever a new file is found or created, projectile will automatically add that new file to the cache. This saves me from having to manually invalidate the cache with C-c p i every time a create a new file.

However, if I delete a file, projectile does not automatically remove it from the cache. The next time I search for a similarly named file, the deleted file will still show up in the helm list of candidates. While not a deal breaker, it is certainly a source of annoyance for me. Since emacs is all about tweaking your environment to eliminate those little headaches, I knew I had to fix it.

At first, I thought adding a hook to delete-file would be the easy, straightforward fix. I couldn’t find any documentation for a delete-file-hook. Having tried to check out the source code, I realized that while find-file is implemented in lisp, delete-file is actually implemented in C.

Having been unable to find a hook associated with this function, what was I do to?

This is where defadvice comes in. I won’t go into detail about how defadvice works, but the short version is that defadvice allows you to suggest behavior to emacs before, after, or around an invocation of a particular function. You can read more about defadvice in the manual.

The below snippet adds advice before the delete-file function is called. It checks to see if the file is part of a projectile project; if the file is cached by projectile, it removes it from the cache.

NOTE This defadvice function is no longer necessary. This functionality has now been incorporated into projectile.

(defadvice delete-file (before purge-from-projectile-cache (filename &optional trash))
  (if (and (projectile-project-p) projectile-enable-caching)
      (let* ((project-root (projectile-project-root))
             (true-filename (file-truename filename))
             (relative-filename (file-relative-name true-filename project-root)))
        (if (projectile-file-cached-p relative-filename project-root)
            (projectile-purge-file-from-cache relative-filename)))))

(ad-activate 'delete-file)

That’s it. With just a few short lines, projectile will now invalidate the cache as soon as I delete a file, just as I would like. Isn’t emacs great?