SSE NextJs & Flutter

sse.dart

import 'dart:async';

import 'package:meta/meta.dart';
import 'package:universal_html/html.dart' as html;

class Sse {
  final html.EventSource eventSource;
  final StreamController<String> streamController;

  Sse._internal(this.eventSource, this.streamController);

  factory Sse.connect({
    @required Uri uri,
    bool withCredentials = false,
    bool closeOnError = true,
  }) {
    final streamController = StreamController<String>();
    final eventSource = html.EventSource(uri.toString(), withCredentials: withCredentials);

    eventSource.addEventListener('message', (html.Event message) {
      streamController.add((message as html.MessageEvent).data as String);
    });

    ///close if the endpoint is not working
    if (closeOnError) {
      eventSource.onError.listen((event) {
        eventSource?.close();
        streamController?.close();
      });
    }
    return Sse._internal(eventSource, streamController);
  }

  Stream get stream => streamController.stream;

  bool isClosed() => this.streamController == null || this.streamController.isClosed;

  void close() {
    this.eventSource?.close();
    this.streamController?.close();
  }
}

sse.js

const express = require('express');
const app = express();
const PORT = 3000;

app.get('/events', (req, res) => {
  // Set SSE headers
  res.setHeader('Content-Type', 'text/event-stream');
  res.setHeader('Cache-Control', 'no-cache');
  res.setHeader('Connection', 'keep-alive');
  // Initial message
  res.write('data: Welcome to Server-Sent Events!\n\n');
  // Send messages every 5 seconds
  const intervalId = setInterval(() => {
    const message = { time: new Date().toISOString() };
    res.write(`data: ${JSON.stringify(message)}\n\n`);
  }, 5000);
  // Clean up on disconnect
  req.on('close', () => {
    clearInterval(intervalId);
    res.end();
  });
});

app.listen(PORT, () => {
  console.log(`SSE server running on http://localhost:${PORT}`);
});