title: How to know if a react native app goes to background?
body: <p>Is it possible to know if a RN app has gone to background? Any callback or trigger?</p>
  
  <p>If I listen to <code>componentDidUnmount</code> or <code>componentWillUnmount</code> of the screen I'm on, it will only be fired if I go back/forth to another screen</p>