sse_channel

sse_channel wraps Server-Sent Events (SSE) endpoints with the StreamChannel API so you can treat bi-directional SSE as a first-class stream/sink pair on both dart:io and dart:html.

Quick start

import 'dart:convert';

import 'package:sse_channel/sse_channel.dart';

Future<void> main() async {
  final channel = SseChannel.connect(Uri.parse('https://sse.dev/test'));

  await channel.ready;

  channel.stream.listen(
    (event) {
      print('[${event.event}] ${event.data} (id: ${event.id})');
    },
    onError: (error, stackTrace) {
      if (error is SseChannelException) {
        print('SSE failure: ${error.message}');
      }
    },
  );

  channel.sink.add(jsonEncode({'type': 'ping'}));
  await channel.sink.close();
}

Why use it?

• Spec-aligned client. The IO implementation tracks Last-Event-ID, honors retry: hints, sets Cache-Control: no-cache, and reconnects automatically using the WHATWG SSE rules.
• Rich events. Streams emit MessageEvent instances so you retain the payload, event name, and identifier metadata that servers send.
• Consistent errors. Both IO and HTML channels surface failures via the same SseChannelException type, making it simple to detect connection issues.
• Test friendly. The HTML channel exposes HtmlSseChannel.test, and the IO implementation can be exercised with in-process servers; the repository ships with unit and integration tests you can mirror.

Testing

Run the VM test suite (includes IO integration coverage):

$ dart test

Run browser tests (exercises the HTML channel using Chrome by default):

$ dart test -p chrome

Use HtmlSseChannel.test in your own applications to inject a mock stream and sink when you need to verify browser-side code without a live SSE endpoint.

On the IO side you can now import package:sse_channel/io.dart and construct IOSseChannel with your own StreamChannel<String?> to reuse the package's reconnection logic while driving it with a fake transport during tests.

Platform support

• dart:io – uses package:http to establish SSE connections and POST responses back to the server.
• dart:html – delegates to package:sse in the browser and adapts events to the shared MessageEvent model.

Resources

• Server-Sent Events specification (WHATWG HTML)
• StreamChannel API docs