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} else if (snapshot.hasError) {
return Text('${snapshot.error}');
}
}
// By default, show a loading spinner.
return const CircularProgressIndicator();
},
),
),
),
);
}
}<code_end>
<topic_end>
<topic_start>Communicate with WebSockets
In addition to normal HTTP requests,
you can connect to servers using WebSockets.
WebSockets allow for two-way communication with a server
without polling.In this example, connect to a
test WebSocket server sponsored by Lob.com.
The server sends back the same message you send to it.
This recipe uses the following steps:<topic_end>
<topic_start>
1. Connect to a WebSocket server
The web_socket_channel package provides the
tools you need to connect to a WebSocket server.The package provides a WebSocketChannel
that allows you to both listen for messages
from the server and push messages to the server.In Flutter, use the following line to
create a WebSocketChannel that connects to a server:
<code_start>final channel = WebSocketChannel.connect(
Uri.parse('wss://echo.websocket.events'),
);<code_end>
<topic_end>
<topic_start>
2. Listen for messages from the server
Now that you’ve established a connection,
listen to messages from the server.After sending a message to the test server,
it sends the same message back.In this example, use a StreamBuilder
widget to listen for new messages, and a
Text widget to display them.
<code_start>StreamBuilder(
stream: channel.stream,
builder: (context, snapshot) {
return Text(snapshot.hasData ? '${snapshot.data}' : '');
},
)<code_end>
<topic_end>
<topic_start>
How this works
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 connects to a Stream
and asks Flutter to rebuild every time it
receives an event using the given builder() function.<topic_end>
<topic_start>
3. Send data to the server
To send data to the server,
add() messages to the sink provided
by the WebSocketChannel.
<code_start>channel.sink.add('Hello!');<code_end>
<topic_end>
<topic_start>
How this works
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.<topic_end>
<topic_start>
4. Close the WebSocket connection
After you’re done using the WebSocket, close the connection:
<code_start>channel.sink.close();<code_end>
<topic_end>
<topic_start>
Complete example
<code_start>import 'package:flutter/material.dart';
import 'package:web_socket_channel/web_socket_channel.dart';
void main() => runApp(const MyApp());
class MyApp extends StatelessWidget {
const MyApp({super.key});
@override
Widget build(BuildContext context) {
const title = 'WebSocket Demo';
return const MaterialApp(
title: title,
home: MyHomePage(
title: title,
),
);
}
}
class MyHomePage extends StatefulWidget {
const MyHomePage({
super.key,
required this.title,
});
final String title;
@override
State<MyHomePage> createState() => _MyHomePageState();
}