title: React Native Navigation: how to disable backbutton on modals?
body: <p>I've a login modal opened using <code>showModal()</code>.</p>
  
  <p>It has no navbar buttons, so ios users cannot close this modal. </p>
  
  <p><strong>Problem</strong>: Actually Android users can use hardware back button to close the login modal.</p>
  
  <p>In my login modal I tried to add </p>
  
  <p><code>
  BackHandler.addEventListener('hardwareBackPress', function() {
    return true;
  }
  </code></p>
  
  <p>to disallow backbutton on Android, but it simply doesn't works.</p>
  
  <p>I did this because I read what follows on <a href="https://facebook.github.io/react-native/docs/backhandler.html" rel="nofollow noreferrer">official RN guide</a>:</p>
  
  <blockquote>
    <p>Android: Detect hardware back button presses, and programmatically invoke the default back button functionality to exit the app if there are no listeners or if none of the listeners return true.</p>
  </blockquote>
  
  <p>Adding a console.log into this function I see the event fired on 'normal' screens but NOT when I've a modal showed !</p>
  
  <p>What am i doing wrong?</p>