flutter showsnackbar

const snackBar = SnackBar(
  content: Text('Yay! A SnackBar!'),
);

// Find the ScaffoldMessenger in the widget tree
// and use it to show a SnackBar.
ScaffoldMessenger.of(context).showSnackBar(snackBar);

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: new MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key}) : super(key: key);

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

class _MyHomePageState extends State<MyHomePage> {

  final GlobalKey<ScaffoldState> _scaffoldKey = new GlobalKey<ScaffoldState>();

  @override
  void initState() {
    super.initState();
    showInSnackBar("Some text");
  }

  @override
  Widget build(BuildContext context) {
    return new Padding(
            key: _scaffoldKey,
            padding: const EdgeInsets.all(16.0),
            child: new Text("Simple Text")
    );
  }

  void showInSnackBar(String value) {
    _scaffoldKey.currentState.showSnackBar(new SnackBar(
        content: new Text(value)
    ));
  }
}