title: Flutter - New StatefullWidget, why related createState() method not called?
body: <p>in the 'TestPageState', I put a button to call 'setState' method. new TestChildWidget() will create a new  TestChildWidget,default construtor TestChildWidget() is called, but why 'createState()' method in TestChildWidget not called? </p>
  
  <pre><code>void main() =&gt; runApp(MyApp());
  
  class MyApp extends StatelessWidget {
    // This widget is the root of your application.
    @override
    Widget build(BuildContext context) {
      return MaterialApp(
          title: 'Flutter Demo',
          theme: ThemeData(
            primarySwatch: Colors.blue,
          ),
          home: new TestPage()
      );
    }
  }
  
  class TestPage extends StatefulWidget{
    TestPageState createState(){
      print('TestPageState createState');
  
      return new TestPageState();
    }
  }
  class TestPageState extends State&lt;TestPage&gt;{
  
    @override
    Widget build(BuildContext context) {
      print('TestPageState Build');
  
        // TODO: implement build
        return Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: &lt;Widget&gt;[
            new TestChildWidget(),
            RaisedButton(
              child: Text("刷新"),
              onPressed: () =&gt; setState(() =&gt; print('setState')), //build method will called, new TestChildWidget() will be called
            )
          ],
        );
    }
  }
  
  class TestChildWidget extends StatefulWidget{
    TestChildWidget(){
       //new TestChildWidget(), this default construtor will be called, but why createState() not called?
    }
    @override
    State&lt;StatefulWidget&gt; createState() {
      // TODO: implement createState
      print('TestChildWidget createState');
  
      return new TestChildWidgetState();
    }
  
  }
  
  class TestChildWidgetState extends State&lt;TestChildWidget&gt;{
    @override
    Widget build(BuildContext context) {
      print('TestChildWidgetState build');
  
      // TODO: implement build
      return Text('1111');
    }
  }
  </code></pre>
  
  <p>in the 'TestPageState', I put a button to call 'setState' method. new TestChildWidget() will create a new  TestChildWidget,default construtor TestChildWidget() is called, but why 'createState()' method in TestChildWidget not called? </p>