title: How to implement ViewPage onTouch?
body: <p>I've tried:</p>
  
  <ol>
  <li><code>onTouch</code> for a page widget, it works, but only after a page
  transformation finishes</li>
  <li>Add a widget above <code>ViewPage</code> in the <code>Stack</code>, but how in this case handle <code>onTouch</code>
  and pass gestures to the child?</li>
  </ol>