Working with WebSockets

In addition to normal HTTP requests, we can connect to servers using WebSockets. WebSockets allow for two-way communication with a server without polling.

In this example, we’ll connect to a test server provided by websocket.org. The server will simply send back the same message we send to it!

Directions

  1. Connect to a WebSocket server
  2. Listen for messages from the server
  3. Send Data to the Server
  4. Close the WebSocket connection

1. Connect to a WebSocket server

The web_socket_channel package provides the tools we’ll need to connect to a WebSocket server.

The package provides a WebSocketChannel that allows us to both listen for messages from the server as well as push messages to the server.

In Flutter, we can create a WebSocketChannel that connects to a server in one line:

final channel = IOWebSocketChannel.connect('ws://echo.websocket.org');

2. Listen for messages from the server

Now that we’ve established a connection, we can listen to messages from our server.

After we send a message to the test server, it will send the same message back.

How do we listen for messages and display them? In this example, we’ll use a StreamBuilder Widget to listen for new messages and a Text Widget to display them.

StreamBuilder(
  stream: widget.channel.stream,
  builder: (context, snapshot) {
    return Text(snapshot.hasData ? '${snapshot.data}' : '');
  },
);

How does this work?

The WebSocketChannel provides a Stream of messages from the server.

The Stream class is a fundamental part of the dart:async package. It provides a way to listen to async events from a data source. Unlike Future, which returns a single async response, the Stream class can deliver many events over time.

The StreamBuilder Widget will connect to a Stream and ask Flutter to rebuild every time it receives an event using the given builder function!

3. Send Data to the Server

In order to send data to the server, we’ll add messages to the sink provided by the WebSocketChannel.

channel.sink.add('Hello!');

How does this work

The WebSocketChannel provides a StreamSink to push messages to the server.

The StreamSink class provides a general way to add sync or async events to a data source.

4. Close the WebSocket connection

After we’re done using the WebSocket, we’ll want to close the connection! To do so, we can close the sink.

channel.sink.close();

Complete example

import 'package:flutter/foundation.dart';
import 'package:web_socket_channel/io.dart';
import 'package:flutter/material.dart';
import 'package:web_socket_channel/web_socket_channel.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final title = 'WebSocket Demo';
    return MaterialApp(
      title: title,
      home: MyHomePage(
        title: title,
        channel: IOWebSocketChannel.connect('ws://echo.websocket.org'),
      ),
    );
  }
}

class MyHomePage extends StatefulWidget {
  final String title;
  final WebSocketChannel channel;

  MyHomePage({Key key, @required this.title, @required this.channel})
      : super(key: key);

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

class _MyHomePageState extends State<MyHomePage> {
  TextEditingController _controller = TextEditingController();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Padding(
        padding: const EdgeInsets.all(20.0),
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.start,
          children: <Widget>[
            Form(
              child: TextFormField(
                controller: _controller,
                decoration: InputDecoration(labelText: 'Send a message'),
              ),
            ),
            StreamBuilder(
              stream: widget.channel.stream,
              builder: (context, snapshot) {
                return Padding(
                  padding: const EdgeInsets.symmetric(vertical: 24.0),
                  child: Text(snapshot.hasData ? '${snapshot.data}' : ''),
                );
              },
            )
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _sendMessage,
        tooltip: 'Send message',
        child: Icon(Icons.send),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }

  void _sendMessage() {
    if (_controller.text.isNotEmpty) {
      widget.channel.sink.add(_controller.text);
    }
  }

  @override
  void dispose() {
    widget.channel.sink.close();
    super.dispose();
  }
}

Web Sockets Demo