import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: ScaffoldButton(),
        ),
      ),
    );
  }
}

class ScaffoldButton extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return TextButton(
      onPressed: () {
        ScaffoldMessenger.of(context).showSnackBar(
          SnackBar(
            behavior: SnackBarBehavior.floating,
            width: 320,
            content: const Text('This is a snackbar!'),
            action: SnackBarAction(
              label: 'Close',
              onPressed: () {},
            ),
          ),
        );
      },
      child: Text('Launch snackbar'),
    );
  }
}