Simulating real-time with socket io

[Srmortal]

In flutter app I have added socket io because of a feature that needs real-time but problem is my api is not yet deployed so I want mock server to simulate real-time communication in rest api I can easily mock responses for any request but for socket io that is my question how I mock it for development and testing stages in flutter side

[zzz6519003]

A good approach is to mock the Socket.IO server, not the client.

For local development, run a tiny local Socket.IO server on your machine and point the Flutter app to it. Your app code will still use the real client library, but the backend is a fake server that emits the same events your UI expects.

Example with Node:

const { Server } = require("socket.io");

const io = new Server(3000, {
  cors: {
    origin: "*"
  }
});

io.on("connection", (socket) => {
  console.log("client connected");

  socket.emit("connected", { ok: true });

  socket.on("join-room", (room) => {
    socket.join(room);
    io.to(room).emit("room-update", {
      room,
      users: 1
    });
  });

  socket.on("send-message", (msg) => {
    socket.broadcast.emit("new-message", {
      user: "mock-user",
      text: msg
    });
  });
});

Then in Flutter:

final socket = io('http://localhost:3000', <String, dynamic>{
  'transports': ['websocket'],
  'autoConnect': false,
});

socket.connect();

socket.on('connected', (data) {
  print(data);
});

socket.emit('join-room', 'room-1');

This is usually much better than trying to “fake” Socket.IO at the app layer, because you are testing the real protocol behavior.

For unit tests, the usual pattern is:

• create a small service/interface around the socket
• inject a fake implementation in tests
• emit fake events from that fake service

Example idea:

abstract class RealTimeService {
  Stream<Map<String, dynamic>> get events;
  void connect();
  void emit(String event, dynamic data);
}

Then your production implementation wraps the socket client, and your test implementation emits mocked events manually.

So in short:

• For dev: run a local mock Socket.IO server
• For tests: inject a fake service / fake socket wrapper
• Don’t try to mock the network protocol itself unless you need to test very low-level behavior

If you want, I can give you a small complete Flutter + Node mock server example for a typical chat or live status feature.