Flutter - Creating a StatefulWidget

main.dart

import 'package:flutter/material.dart';

void main() {
  runApp(new MyApp());
}

class SandyShores extends StatelessWidget {
  static String routeName = "sandyShores";

  @override
  Widget build(BuildContext context) {
    return new Center(child: new Text("You've landed on the Sandy Shores!"));
  }

}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    var routes = <String, WidgetBuilder> {
      SandyShores.routeName : (BuildContext context) => new SandyShores(),
    };

    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(
        // This is the theme of your application.
        //
        // Try running your application with "flutter run". You'll see
        // the application has a blue toolbar. Then, without quitting
        // the app, try changing the primarySwatch below to Colors.green
        // and press "r" in the console where you ran "flutter run".
        // We call this a "hot reload". Notice that the counter didn't
        // reset back to zero -- the application is not restarted.
        primarySwatch: Colors.blue,
      ),
      //home: new MyHomePage(title: 'Flutter Demo Home Page'),
        home: new ColorPage(),
      routes: routes,
    );
  }
}

class ColorPage extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => new ColorPageState();
}

class ColorPageState extends State<ColorPage> {

  var _color = Colors.blue[400];

  @override
  Widget build(BuildContext context) {
    var item2 = new Container(width: 50.0, height: 50.0, color: _color);

    var buttonText = new Text('Click');

    var item1 = new RaisedButton(child: buttonText, onPressed: _onButonPressed);

    var items = [item1, item2];

    var row = new Row(children: items);

    var center = new Center(child: row);

    Scaffold scaffold = new Scaffold(body: center);

    return scaffold;
  }


  void _onButonPressed() {

    setState(() {
      if (_color == Colors.blue[400]) {
        _color = Colors.red[400];
      } else {
        _color = Colors.blue[400];
      }
    });

  }

}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  // This widget is the home page of your application. It is stateful,
  // meaning that it has a State object (defined below) that contains
  // fields that affect how it looks.

  // This class is the configuration for the state. It holds the
  // values (in this case the title) provided by the parent (in this
  // case the App widget) and used by the build method of the State.
  // Fields in a Widget subclass are always marked "final".

  final String title;

  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      // This call to setState tells the Flutter framework that
      // something has changed in this State, which causes it to rerun
      // the build method below so that the display can reflect the
      // updated values. If we changed _counter without calling
      // setState(), then the build method would not be called again,
      // and so nothing would appear to happen.
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    var headerText = new Text('Headers');

    var header = new DrawerHeader(child: headerText);

    var item1Text = new Text('Sandy Shores');

    var item1Icon = new Icon(Icons.settings);

    var item1 = new DrawerItem(child: item1Text, icon: item1Icon, onPressed: _onItem1OnPressed);

    var children = [header, item1];

    var listView = new ListView(children: children);

    var drawer = new Drawer(child: listView);

    // This method is rerun every time setState is called, for instance
    // as done by the _incrementCounter method above.
    // The Flutter framework has been optimized to make rerunning
    // build methods fast, so that you can just rebuild anything that
    // needs updating rather than having to individually change
    // instances of widgets.
    return new Scaffold(
        drawer: drawer,
      appBar: new AppBar(
        // Here we take the value from the MyHomePage object that
        // was created by the App.build method, and use it to set
        // our appbar title.
        title: new Text(config.title),
      ),
      body: new Center(
        child: new Text(
          'Button tapped $_counter time${ _counter == 1 ? '' : 's' }.',
        ),
      ),
      floatingActionButton: new FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: new Icon(Icons.add),
      ), // This trailing comma tells the Dart formatter to use
      // a style that looks nicer for build methods.
    );
  }


  void _onItem1OnPressed() {
    Navigator.popAndPushNamed(context, SandyShores.routeName);
  }

}

https://www.youtube.com/watch?v=bqMnyIFP0dY - Video on Creating a StatefulWidget in Flutter