Display SnackBar in Flutter

I want to display a simple SnackBar inside Flutter's Stateful widget. My application creates new instance of MaterialApp with a stateful widget called MyHomePage.

I try to show the SnackBar in showSnackBar() method. But it fails with The method showSnackBar was called on null.

What's wrong with this code?

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)
    ));
  }
}

SOLUTION:

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

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

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

class _MyHomePageState extends State<MyHomePage> {

  @override
  void initState() {
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    showInSnackBar("Some text");
    return new Padding(
        padding: const EdgeInsets.all(16.0),
        child: new Scaffold(
          body: new Text("Simple Text")
        )
    );
  }

  void showInSnackBar(String value) {
    Scaffold.of(context).showSnackBar(new SnackBar(
        content: new Text(value)
    ));
  }
}

There's three problems. The first is that you don't have a Scaffold anywhere, and the Scaffold widget is the one that knows how to show snack bars. The second is that you have a key for getting a hold of the scaffold, but you've put it on a Padding instead (and Paddings don't have any knowledge of snack bars). The third is that you've used the key before the widget that it's associated with has had a chance to be initialised, since initState is called before build.

The simplest solution is to change the home line in your MyApp widget to:

home: new Scaffold(body: new MyHomePage()),

...and then remove all mention of _scaffoldKey and instead use Scaffold.of(context) where you currently have _scaffoldKey.currentState.