client.dart

pubspec
    socket_io_client: ^1.0.1

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

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(home: Home());
  }
}

class Home extends StatefulWidget {
  const Home({
    Key key,
  }) : super(key: key);

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

class _HomeState extends State<Home> {
  Socket socket;
  @override
  void initState() {
    super.initState();
    socket = io(
      'http://localhost:3000',
      OptionBuilder().disableAutoConnect().build(),
    );
    socket.onConnect((data) {
      socket.emit('msg', 'test1');
      print('Connected');
      setState(() {});
    });
    socket.onConnectError((data) => print('error $data'));
    socket.onConnectTimeout((data) => print('onConnectTimeout $data'));
    socket.onDisconnect((_) {
      print('Disconnected');
      setState(() {});
    });
    socket.onConnecting((data) => print('Connecting'));
  }

  void connect() => socket.connect();
  void disconnect() => socket.disconnect();

  void _connectDisconect() {
    if (socket.disconnected) {
      print('Connecting');
      connect();
    } else {
      print('Disconnecting');
      disconnect();
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Material App Bar'),
      ),
      body: Center(
        child: ElevatedButton(
          onPressed: _connectDisconect,
          child: Padding(
            padding: EdgeInsets.all(10),
            child: socket.connected ? Text('Disconnect') : Text('Connect'),
          ),
        ),
      ),
    );
  }
}

server.dart

pubspec
      socket_io: ^1.0.0

import 'package:socket_io/socket_io.dart';

void main() {
  var io = Server();
  var nsp = io.of('/some');
  nsp.on('connection', (client) {
    print('connection /some');
    client.on('msg', (data) {
      print('data from /some => $data');
      client.emit('fromServer', "ok 2");
    });
  });
  io.on('connection', (client) {
    print('connection default namespace');
    client.on('msg', (data) {
      print('data from default => $data');
      client.emit('fromServer', "ok");
    });
  });
  io.listen(3000);
}