I wrote this little Obj-C class to watch for and report changes on a list of files. The MAGIC part is that, like iTunes, it tracks the file even when its path changes (eg if you move it in the finder).
It's part of a larger project but I thought this part was particularly useful.
https://github.com/ptrsghr/FileWatcher
Clickable link: https://github.com/ptrsghr/FileWatcher