Flutter/Dart Websocket server

a_websocket_server.dart

/// A simple websocket server Flutter app that can ping a connected client 
/// thanks to Simon Lightfoot @devangelslondon and Craig Labenz @craig_labenz
/// #boringshow https://youtu.be/AaQzV1LTmo0?si=2xjcfH0FA5tt4nyW

import 'dart:io';

import 'package:flutter/material.dart';

import 'package:shelf/shelf.dart';
import 'package:shelf/shelf_io.dart';
import 'package:shelf_router/shelf_router.dart' as router;
import 'package:shelf_web_socket/shelf_web_socket.dart';
import 'package:web_socket_channel/web_socket_channel.dart';

void main() {
  runApp(const MainApp());
}

final _router = router.Router()..get('/ws', webSocketHandler(_handleWebSocket));
WebSocketChannel? wsc;

Future<void> _startServer() async {
  final ip = InternetAddress.anyIPv4;

  final handler = const Pipeline().addMiddleware(logRequests()).addHandler(_router);

  final port = int.parse(Platform.environment['PORT'] ?? '8081');
  final server = await serve(handler, ip, port);
  debugPrint('Server listening on port ${server.port}');
}

void _handleWebSocket(WebSocketChannel webSocket) {
  debugPrint("client connected");
  webSocket.stream.listen((message) {
    debugPrint("echo to client $message");
    webSocket.sink.add("echo $message");
  }, onDone: () {
    debugPrint('client connection closed');
  });

  wsc = webSocket;
}

class MainApp extends StatefulWidget {
  const MainApp({super.key});

  @override
  State<MainApp> createState() => _MainAppState();
}

class _MainAppState extends State<MainApp> {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        backgroundColor: Colors.white,
        body: Center(
          child: Column(
            children: [
              const Text('--- SERVER ---'),
              const SizedBox(height: 8),
              const ElevatedButton(
                onPressed: _startServer,
                child: Text("start server"),
              ),
              const SizedBox(height: 8),
              ElevatedButton(
                  onPressed: () {
                    wsc?.sink.add('ping from server');
                  },
                  child: const Text("ping client"))
            ],
          ),
        ),
      ),
    );
  }
}

pubspec.yaml

name: websocket_server
description: "Websocket server"
publish_to: "none"
version: 0.1.0

environment:
  sdk: ">=3.2.6 <4.0.0"

dependencies:
  flutter:
    sdk: flutter
  shelf: ^1.4.1
  shelf_router: ^1.1.4
  shelf_web_socket: ^2.0.0
  web_socket_channel: ^2.4.0

dev_dependencies:
  flutter_test:
    sdk: flutter
  flutter_lints: ^2.0.0

flutter:
  uses-material-design: true