socket_io_lite

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A lightweight, zero-dependency Socket.IO client for Dart & Flutter. It speaks the standard Socket.IO wire protocol, so it connects to any real Socket.IO server — Node.js, NestJS (@nestjs/websockets), or your own — in production and on localhost during development.

Works on every Flutter platform: Android · iOS · Web · Windows · macOS · Linux. It is a pure-Dart package (no third-party dependencies), built entirely on the Dart SDK's own WebSocket, so it runs anywhere Flutter runs.

Protocol support: targets Engine.IO v4, so it works with any Socket.IO v3 or v4 server — the reference socket.io for Node.js, or any protocol-compatible implementation in other languages and frameworks (Go, Python, Java, and more). The client connects over WebSocket transport with ?EIO=4.

Why another Socket.IO client?

Most existing clients pull in extra packages and lag behind the current protocol. socket_io_lite focuses on being small and honest:

  • Zero dependencies — only the Dart/Flutter SDK. Nothing to audit, nothing to fall out of date.
  • Standard-compatible — talks the same Engine.IO/Socket.IO frames as the official JS client, so real servers can't tell the difference.
  • Pure Dart, all platforms — WebSocket from the SDK, with a conditional import so the same API works on native and Web.
  • Heartbeat handled for you — responds to the server's ping/pong and treats a missed heartbeat as a dropped connection.
  • Auto-reconnect — exponential backoff out of the box, with events you can listen to.
  • Acknowledgements — emitWithAck returns a Future that completes with the server's callback value.
  • Testable core — the packet parser is pure (string in, object out), so it is unit-tested without a network.

Getting started

Add it with a single command:

flutter pub add socket_io_lite

Then import it:

import 'package:socket_io_lite/socket_io_lite.dart';

Usage

final socket = SocketIoLite.connect('ws://localhost:3000');

socket.onConnect((_) => print('connected'));
socket.onDisconnect((_) => print('disconnected'));

// Listen for an event
socket.on('chat:message', (data) {
  print('received: $data');
});

// Emit an event
socket.emit('chat:message', {'text': 'halo'});

// Emit and wait for the server's acknowledgement
final reply = await socket.emitWithAck('chat:message', {'text': 'halo'});
print('ack: $reply');

// Clean up
socket.dispose();

More listeners

// Fire once, then auto-remove
socket.once('welcome', (data) => print(data));

// Catch every event (great for debugging/logging)
socket.onAny((event, data) => print('$event -> $data'));

// Respond when the server emits an event expecting an acknowledgement;
// the return value is sent back as the ack.
socket.onAck('ping', (data) => {'pong': true});

Multiple arguments

emit is variadic, matching Socket.IO's emit(event, ...args). Pass as many positional arguments as the server expects:

socket.emit('move', x, y);                       // ["move", x, y]
socket.emit('chat:message', roomId, text, tags); // three args

When an incoming event carries several arguments, the handler receives them as a List:

socket.on('move', (data) {
  final x = data[0];
  final y = data[1];
});

This matches any server that uses multi-argument events. One common case is WebRTC signaling — where the plugin is a drop-in signaling channel for flutter_webrtc (media flows over WebRTC; you might play HLS via video_player):

// emit('offer', id, description)  →  ["offer", id, description] on the wire
socket.emit('offer', peerId, {'type': 'offer', 'sdp': sdp});
socket.emit('candidate', peerId, candidate.toMap());

socket.on('answer', (data) {
  final id = data[0];
  final description = data[1];
});

Reconnection

Enabled by default with exponential backoff. Tune it via connect:

final socket = SocketIoLite.connect(
  'ws://localhost:3000',
  reconnection: true,           // default
  reconnectionAttempts: null,   // null = unlimited
  reconnectionDelay: Duration(seconds: 1),
  reconnectionDelayMax: Duration(seconds: 5),
);

socket.onReconnect((attempt) => print('reconnected after $attempt tries'));
socket.onReconnectAttempt((attempt) => print('retrying #$attempt'));
socket.onReconnectFailed(() => print('gave up'));

Namespaces & auth

Connect straight to a namespace:

final admin = SocketIoLite.connect(
  'ws://localhost:3000',
  namespace: '/admin',
  auth: {'token': 'secret'},   // sent with the CONNECT packet
  query: {'v': '1'},           // appended to the handshake URL
);

Or share one WebSocket across several namespaces with of():

final chat  = SocketIoLite.connect('ws://localhost:3000'); // root '/'
final admin = chat.of('/admin');   // same connection, second namespace
final notes = chat.of('/notes', auth: {'token': 'secret'});

admin.on('kick', (data) => ...);
notes.emit('mark-read', 42);

of() returns the same instance for a given namespace, so calling it again is cheap. The underlying connection closes automatically once the last namespace is disposed.

Connecting to localhost during development

localhost inside an emulator/simulator does not mean your PC. Use the right host for your target:

Environment Server URL
Android Emulator ws://10.0.2.2:3000
iOS Simulator ws://localhost:3000
Physical device ws://<your-PC-LAN-IP>:3000 (same Wi-Fi)
Flutter Web / Desktop ws://localhost:3000

On Android, non-TLS (ws:// / http://) traffic is blocked by default. For debug builds only, allow cleartext in AndroidManifest.xml:

<application android:usesCleartextTraffic="true" ... >

In production use wss://your-domain.com.

Example NestJS server

@WebSocketGateway({ cors: { origin: '*' } })
export class ChatGateway {
  @SubscribeMessage('chat:message')
  handleMessage(@MessageBody() data: any) {
    return { echo: data }; // return value becomes the ACK
  }
}

How it works

Socket.IO is a thin protocol layered on top of a WebSocket:

  • Engine.IO handles the transport and heartbeat. Every frame is prefixed with one digit — 0 open (handshake), 2 ping, 3 pong, 4 message.
  • Socket.IO rides inside the 4 (message) frames. The next digit is the packet type — 0 CONNECT, 1 DISCONNECT, 2 EVENT, 3 ACK.

So an event on the wire looks like 42["chat:message",{"text":"halo"}]: 4 = Engine.IO message, 2 = Socket.IO EVENT, then a JSON array of [eventName, ...args]. socket_io_lite encodes and decodes these frames for you and exposes a simple on/emit API.

Compatibility

Any Socket.IO server that speaks Engine.IO v4, in any language or framework.

Server Supported
Socket.IO v4 (any language/framework) ✅
Socket.IO v3 (any language/framework) ✅
Socket.IO v2 / Engine.IO v3 ❌ (use an EIO v4 server)
Raw WebSocket servers (non–Socket.IO) ❌ (different protocol)

Platforms

Platform Transport
Android · iOS · Win/mac/Linux dart:io WebSocket
Web dart:html WebSocket

On the web this uses dart:html (to stay dependency-free), which compiles with the standard JS web build. The Wasm build (flutter build web --wasm) is not supported yet.

License

MIT — see LICENSE.

Libraries

socket_io_lite
A lightweight, zero-dependency Socket.IO client for Dart & Flutter.