Watchman exists to watch files and record when they change. It can also trigger actions (such as rebuilding assets) when matching files change.

Concepts
- Watchman can recursively watch one or more directory trees (we call them roots).
- Watchman does not follow symlinks. It knows they exist, but they show up the same as any other file in its reporting.
- Watchman waits for a root to settle down before it will start to trigger notifications or command execution.
- Watchman is conservative, preferring to err on the side of caution; it considers files to be freshly changed when you start to watch them or when it is unsure.
- You can query a root for file changes since you last checked, or the current state of the tree
- You can subscribe to file changes that occur in a root
