React Hook - Server-Sent Events (SSE)

README.MD

Server-Sent Events (SSE) are commonly used in real-time applications where the server needs to push updates to the client.

Here are a few use cases:

1. Real-time notifications: SSE can be used to push notifications to the client in real-time. For example, in a social media application, the server can push notifications about new posts, likes, comments, etc.
2. Live news updates: In a news application, the server can push live news updates to the client.
3. Real-time analytics: In an analytics dashboard, the server can push real-time data updates to the client.
4. Chat applications: In a chat application, the server can push new messages to the client in real-time.
5. Online multiplayer games: In an online multiplayer game, the server can push game state updates to the client in real-time.
6. Stock price updates: In a stock trading application, the server can push real-time stock price updates to the client.

useSse - Server-Sent Events Hook

useSse is a custom React hook for handling Server-Sent Events (SSE) in your application.

Usage

Here's a basic example of how to use the useSse hook:

import { useEffect } from 'react';
import useSse from './useSse';

const MyComponent = () => {
  const {
    connectionState,
    connectionError,
    addListener,
    getEventData,
    closeConnection,
  } = useSse('https://my-api.com/events', { withCredentials: true });

  // register the listener once, clean it up on unmount
  useEffect(() => {
    const remove = addListener('myEvent', data => {
      console.log('Received data:', data);
    });

    return () => {
      remove?.();          // detach the listener
      closeConnection();   // close the SSE connection
    };
  }, [addListener, closeConnection]);

  // grab the latest payload for this event on every render
  const data = getEventData('myEvent');

  return (
    <div>
      <p>Connection state: {connectionState}</p>
      {connectionError && <p>Error: {connectionError.message}</p>}
      {data && <p>Last payload: {data}</p>}
    </div>
  );
};

export default MyComponent;

API

The useSse hook returns an object with these properties:

• connectionState: current state of the SSE connection (CONNECTING, OPEN, CLOSED)
• connectionError: an Event object describing the most recent connection error, or null if none occurred
• addListener(eventName, handler): registers a listener for the specified event; the handler receives the event data string. The function returns a cleanup callback you can call (or return from a useEffect) to remove the listener when needed
• getEventData(eventName): returns the latest payload string for the given event name. Because it comes from React state, a component that reads this value will automatically re-render whenever the payload changes
• closeConnection(): closes the underlying EventSource and updates connectionState to CLOSED

serverExample.js

const express = require('express');
const SSE = require('sse-stream');

const app = express();
const sse = SSE();

app.get('/events', sse, (req, res) => {
  setInterval(() => {
    const data = Math.random().toString(); // Generate random data
    res.sse.event('MyEvent', data);
  }, 1000);
});

app.listen(3000, () => {
  console.log('Server is running on port 3000');
});

useSse.ts

import { useEffect, useState, useCallback } from 'react';

interface EventData {
  [key: string]: string;
}

const useSse = (url: string, options?: EventSourceInit) => {
  const [connectionState, setConnectionState] = useState('CONNECTING');
  const [connectionError, setConnectionError] = useState<Event | null>(null);
  const [eventSource, setEventSource] = useState<EventSource | null>(null);
  const [eventData, setEventData] = useState<EventData>({});

  // open the connection once and tidy up on unmount or when url / options change
  useEffect(() => {
    const es = new EventSource(url, options);
    setEventSource(es);

    es.onopen = () => setConnectionState('OPEN');
    es.onerror = (err: Event) => {
      setConnectionState('CLOSED');
      setConnectionError(err);
    };

    return () => es.close();
  }, [url, options]);

  // register a listener for a specific event name
  const addListener = useCallback(
    (eventName: string, handler: (data: string) => void) => {
      if (!eventSource) return;

      const listener = (evt: MessageEvent) => {
        setEventData(prev => ({
          ...prev,
          [eventName]: evt.data,
        }));
        handler(evt.data);
      };

      eventSource.addEventListener(eventName, listener);
      return () => eventSource.removeEventListener(eventName, listener);
    },
    [eventSource],
  );

  // simple getter: no hooks inside so rules of hooks are respected
  const getEventData = useCallback(
    (eventName: string) => eventData[eventName],
    [eventData],
  );

  const closeConnection = useCallback(() => eventSource?.close(), [eventSource]);

  return {
    connectionState,
    connectionError,
    addListener,
    getEventData,
    closeConnection,
  };
};

export default useSse;

@TomRadford Thanks for pointing that out
I pulled the hooks out of the nested callback, added a cleanup for listeners, and updated the README and example
Let me know if you spot anything else