Demonstrating the GlobalKey used to display the SnackBar element.

Scaffold_SnackBar.dart

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatefulWidget {
  MyApp({Key key}) : super(key: key);
  State createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  int _count = 0;
  final GlobalKey keyScaffold = new GlobalKey();
  @override
  Widget build(BuildContext context) {
    final title = 'Floating App Bar';
    final ThemeData theme = Theme.of(context);
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      title: title,
      home: Scaffold(
        key: keyScaffold,
        appBar: AppBar(
          title: Text('Sample Code'),
        ),
        body: Center(
            child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text('You have pressed the button $_count times.'),
            RaisedButton(
              onPressed: () {
                final snackBar = SnackBar(
                    content: Text('Button\'s been pressed $_count times.'));
                ScaffoldState state = keyScaffold.currentState;
                state.showSnackBar(snackBar);
              },
              elevation: 20.0,
              textColor: theme.canvasColor,
              color: theme.primaryColor,
              child: Text('Display SnackBar'),
            ),
          ],
        )),
        bottomNavigationBar: BottomAppBar(
          child: Container(
            height: 50.0,
          ),
        ),
        floatingActionButton: FloatingActionButton(
          onPressed: () => setState(() {
                _count++;
              }),
          tooltip: 'Increment Counter',
          child: Icon(Icons.add),
        ),
        floatingActionButtonLocation: FloatingActionButtonLocation.centerDocked,
      ),
    );
  }
}