title: react-navigation and component lifecycle
body: <p>I have a React Native application that uses react-navigation. In one of my screen, I'm using camera to read the QR code what works fine but I have to use setState({camera: false}) to prevent loading of QR code multiple times. Usually, it works. Even after re-entering page from main menu.</p>
  
  <p>Trouble is when user press '&lt;' button (iOS / in the header) and he should re-enter page with camera. I'm not able to find a function where I should do setState({camera: true}). Without react-navigation, there is standard lifecycle (componentWillMount, ...) but in this case I'm not able to find out where to put my code, so I can detect that page was re-entered.</p>
  
  <p>I'm aware of <a href="https://github.com/react-community/react-navigation/issues/51" rel="noreferrer">https://github.com/react-community/react-navigation/issues/51</a> but I still miss the solution. </p>