To display snackbar from an alert dialog

main.dart

// Copyright (c) 2019, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'package:flutter/material.dart';

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

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

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

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  final _scaffoldKey = GlobalKey<ScaffoldState>();
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      key: _scaffoldKey,
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[],
        ),
      ),
      floatingActionButton: FloatingActionButton.extended(
        label: Text('Show Dialog'),
        onPressed: () {
          showDialog(
            context: context,
            builder: (context) => AlertDialog(
              title: Text('Tap OK to show Snackbar'),
              actions: [
                FlatButton(
                    child: Text('OK'),
                    onPressed: () {
                      _scaffoldKey.currentState.showSnackBar(
                        SnackBar(
                          content: Text('I am a snackbar'),
                        ),
                      );
                      Navigator.pop(context);
                    }),
              ],
            ),
          );
        },
        tooltip: 'Increment',
        icon: Icon(Icons.error),
      ),
    );
  }
}