title: Flutter tabview refresh issue
body: <p>I have a TabBarView in my main.dart and every tab got a class to show the content(it's listview object), when i go between the tabs, the listview page refresh everytime, is it normal for tabbarview? I don't expect it will refresh everytime when i go between the tabs.</p>
  
  <p>is it the problem my class? how to fix this? the code is something like this.</p>
  
  <pre><code>    class ListWidget extends StatefulWidget {
    final catID;
  
    ListWidget(this.catID);
  
  
    _ListWidgetState createState() =&gt; new _ListWidgetState(catID);
  }
  
  class _ListWidgetState extends State&lt;ListWidget&gt; {
  
    var catID;
  
    void initState() {
      super.initState();
      _fetchListData();
    }
  
    @override
  
    Widget build(BuildContext context) {
      // TODO: implement build
  
      return new Scaffold(.......
  }
  </code></pre>