title: Detect vertical swipe direction in Flutter
body: <p>How to detect when the user swipe vertically either upward on downward?</p>
  
  <p>I have been using <a href="https://pub.dartlang.org/packages/swipedetector" rel="nofollow noreferrer">swipedetector</a> package, but now, it gives me exceptions like </p>
  
  <pre><code>The getter 'globalPosition' was called on null.
  </code></pre>