Watchman is a popular file watcher made by google and usually first choice for react-native developers. Watchman can also trigger actions (such as rebuilding assets) when matching files change. If you are a linux user it might be a little bit tricky to install it. Here is how I did it latest Mint, but I suppose it will be the same for Ubuntu.

1. git clone https://github.com/facebook/watchman.git
2. cd watchman
3. sudo apt-get install -y autoconf automake build-essential python-dev
4. ./autogen.sh
5. ./configure
6. make
7. sudo make install

Finally if all went well you can verify that it works by simply running
watchman –version