What is “watchman” ?

Watch files and take action when they change. Watchman  watches files and record when they actually change. It can also trigger actions (such as rebuilding assets) when matching files change.
For more details: https://github.com/facebook/watchman

Installation

• Open Terminal by pressing command+space then type terminal and hit Enter key.
• Install homebrew first.

  ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)" < /dev/null 2> /dev/null

• Install watchman.

  brew install watchman

Now, use watchman!