What's a good way to have a hook that runs for every file in a given directory?

Question

What is a good way to assign a function to run when I open any file in a specified directory? Just like a hook but based on the location of a file rather than a major mode.

It would be nice if I could enable this by having a special file in the directory, but adding the hook in my central config file would be good too.

Answer 1

find-file-hook, and in your hook function, check whether the candidate file is in the given directory.

Answer 2

This sounds like what directory local variables are for. Just create a .dir-locals.el file in the directory with the settings you want, and every file will inherit those settings.

Answer 3

;;; .dir-locals.el

((nil
  (eval add-hook 'find-file-hook (lambda ()
                                   (message "my find-file hook called...")) nil t)))

The nil means this applies to all files in the directories bellow .dir-locals.el.

The t passed to add-hook makes find-file-hook buffer-local. Therefore the hook runs only for this file.

dir-locals runs before find-file-hook, so the whole thing works.