title: Pull to refresh gesture in flutter material
body: <p>I want to detect when user want to refresh page in a flutter app .<br>
  for that i want to know when user swipe up on top of page by force then call a function to refresh data like most of the material app UX of pull to refresh gesture.<br>
    anyone know how to implement it in a scaffold widget like below:</p>
  
  <pre><code>return new Scaffold(
           appBar: new AppBar(
             title: new Text('title'),
           ),
           body: new ListView(
              //mylist
           ),
           onPullToRefresh:this.myEventHandlerFunction
        )
  </code></pre>