title: React Navigation previous screen knows when it is activated
body: <p>I'm trying to figure out a way to call a function in my previous screen if it is revisited.</p>
  
  <ol>
  <li>Currently on <strong>Screen 1</strong></li>
  <li>Click a Button (Goes to <strong>Screen 2</strong>)</li>
  <li>Click Back (Goes Back to <strong>Screen 1</strong>)</li>
  </ol>
  
  <p><code>this.props.navigation.dispatch(NavigationActions.back())</code></p>
  
  <ol start="4">
  <li>Function on <strong>Screen 1</strong> is called. I don't need to pass any parameters or anything I just need to know when the screen has been called again. I'd rather not clear the entire state and reset it if I don't have t.</li>
  </ol>