The Unison has file system change monitoring (in the Unison config, repeat = watch) through a process
that runs and monitors file system activity. The protocal is defined in https://github.com/bcpierce00/unison/blob/master/src/fswatch.ml#L19. The Unison distribution includes file system monitors for Window and Linux.
There is a Python implementation of the file system monitor, https://github.com/hnsl/unox which I have used for quite some time now, but it stopped working on my MacBook Pro running High Sierra on an APFS file system.
I thought it would be fun to implement the watcher in Go. The only dependency is on https://github.com/fsnotify/fsevents but the version in the vendor directory has some bug fixes applied that have not yet made it into master yet (fsnotify/fsevents#38 and fsnotify/fsevents#39).