Connecting Flask Socket.IO Server and Flutter


I know this is an older question but hopefully this answer will help someone down the road.

The issue may be caused by the lack of support for polling in flutter.


In Flutter env. it only works with dart:io websocket, not with dart:html websocket, so in this case you have to add 'transports': ['websocket'] when creates the socket instance.

IO.Socket socket ='http://localhost:3000', <String, dynamic>{
    'transports': ['websocket'],
    'extraHeaders': {'foo': 'bar'} // optional

The default Flask development server doesn't support websockets so you'll need to use another server. Thankfully it's simple to get eventlet working with Flask. All you should have to do is install the eventlet package using pip.

pip install eventlet

Once eventlet is installed socketio will detect and use it when running the server.

You can use chrome to double check what transport method is being used. Open your chrome dev tools Ctrl+Shift+I in Windows and go to the Network tab. On each network request you should see either transport=polling or transport=websocket

isa türk
Author by

isa türk

Updated on December 18, 2022


  • isa türk
    isa türk 5 months

    Basically, I have a socket io flask code:

    import cv2
    import numpy as np
    from flask import Flask, render_template
    from flask_socketio import SocketIO, emit
    from threading import Lock,Timer as tmr
    from engineio.payload import Payload
    import base64 
    from PIL import Image
    import io
    app = Flask(__name__)
    app.config['SECRET_KEY'] = 'secret!'
    socketio = SocketIO(app)
    def connect():
        print("a client connected")
    def disconnect():
        print('Client disconnected')
    def hello():
        return render_template('index.html')
    if __name__ == '__main__':,port=port,host= '')

    This code is working fine when I try to connect it to javascript

    However in flutter I can not achieve that

    I have stateful Widget that has a button in it And I am using this function to connect to my socket when the button is pressed

    import 'package:socket_io_client/socket_io_client.dart' as IO;
    IO.Socket socket;
      connectToSocket() {
      socket ="", <String, dynamic>{
        'transports': ['websocket', 'polling'],

    I can not connect it please help me.

    • tplusk
      tplusk about 3 years
      How is it you are starting the flask server here? From the main or elsewhere?
    • tplusk
      tplusk about 3 years
      Also - where are you calling connectToSocket() ?
  • isa türk
    isa türk almost 3 years
    Thank you sir. I am sure that it will help a lot of people including me :)
  • Dhinakaran
    Dhinakaran about 2 years
    Thank you. After installing eventlet working fine
  • Admin
    Admin about 2 years
    Hi, I am trying to run flask socketio app on localhost, but i can't run it. it says running on server http://[: : 1]/5000 when i give'localhost', and flutter doesn't connect to flask socket too. I am trying since past 4 days now. I can't find a fix:/. Please help. I have eventlet installed