title: Leave screen callback React Navigation Tab Navigator
body: <p>I have a React Navigation Tab Component like this:</p>
  <pre><code>const RootNavigator=TabNavigator({
  Home:{
      screen: Home,
      navigationOptions:{
          tabBarIcon: ({focused}) =&gt; (
              &lt;Icon
                  name={focused? 'ios-home':'ios-home-outline'}
                  style={{color: '#464646'}}
                  size={16}
              /&gt;
          )
      }
  },
  Notifications:{
      screen: Notifications,
      navigationOptions:{
          tabBarIcon: ({focused}) =&gt; (
              &lt;TabNotifications focused={focused} /&gt;
          )
      }
  }, {});
  </code></pre>
  <p>Is there a way to make a callback when leaving a screen?</p>
  <p>In this case, I would like to perform a function when I leave the Notifications tab. Such as mark the notifications as seen and remove the badge indicator.</p>
  <p>As of now, I am pulling the Notification icon from another component in order to show the number badge.</p>
  <p>Thanks in advance.</p>