Socket.io React component doesn't update state when receiving targeted room messages from Flask backend

Backend
1

I’m working with a Flask-SocketIO backend that handles private chat rooms. When users join a room, my Python server sends back room details using a targeted emit:

server.emit('room_info', {'channel': channel_id, 'target_user': target_user}, to=session_id)

The session_id represents a private channel created for each connected user. On the React side, I’m trying to store this data in component state:

function MessagePanel({ userID, authToken }) {
  const [messages, setMessages] = useState([]);
  const [channelsData, setChannelsData] = useState(new Map());
  const [activeChannel, setActiveChannel] = useState("");
  const [inputText, setInputText] = useState("");
  let socketConnection = null;

  useEffect(() => {
    socketConnection = io(SERVER_URL);
  });

  useEffect(() => {
    socketConnection.on("broadcast_message", (payload) => {
      setMessages((prev) => [...prev, payload]);
    });

    socketConnection.on("room_info", (payload) => {
      console.log(`connected to ${payload.channel} for ${payload.target_user}`);
      setActiveChannel((prev) => payload.channel);
      setChannelsData((prev) => prev.set(payload.target_user, payload.channel));
    });

    return () => socketConnection.disconnect();
  }, []);

  const handleSendMessage = () => {
    if (inputText.trim().length === 0) return;

    socketConnection.emit("broadcast_send", {
      sender: userID,
      content: inputText,
      created_at: Date.now(),
    });
    setInputText("");
  };

  const connectToRoom = (targetUser) => {
    socketConnection.emit("room_join", {
      auth: authToken,
      current_user: userID,
      target: targetUser,
    });
  };

  const disconnectFromRoom = (targetUser) => {
    socketConnection.emit("room_exit", {
      auth: authToken,
      current_user: userID,
      target: targetUser,
    });
    
    const updatedChannels = channelsData;
    updatedChannels.delete(targetUser);
    setChannelsData(updatedChannels);
  };

  return (...);
}

The weird thing is that React doesn’t trigger re-renders when the server sends targeted messages, but I can see them in browser dev tools. However, the broadcast messages work perfectly. When I remove the to=session_id parameter from my Python code, everything works fine. What could cause this behavior when using targeted socket emissions?

2

I’ve hit this same issue. Your socket gets recreated every render because that first useEffect has no dependencies. So those event listeners in the second useEffect? They’re stuck on an old, dead connection. Either move the socket setup outside your component or use useRef to keep the same instance around. Also, you’re directly mutating that Map in disconnectFromRoom instead of creating a new one. React won’t re-render because it checks reference equality - it thinks nothing changed.

3

Hmm, that’s interesting - are you sure the targeted messages are actually reaching the client? Can you see the room_info console.log firing when you receive those targeted emits? Also wondering if there’s something weird happening with session management on the Flask side?

4

looks like a timing issue with your useEffect dependency. put the socket listeners inside the same useEffect where you set up the connection - don’t keep them separate. also, socketConnection gets recreated every render, which’ll cause issues.