Buck has a daemon that does it automatically. Often when you go to build it takes 0 seconds because the daemon has already done it before you get to it:

https://buckbuild.com/command/buckd.html

Note the daemon is automatically started when you `buck build`.

(disclosure: my team created Buck when I was at FB)

Although the daemon doesn't automatically build things. Buck does use watchman (https://github.com/facebook/watchman) to watch files though, and it has a way to run commands when files change: https://facebook.github.io/watchman/docs/watchman-make.html