Continuously sending data from Python Flask server to client through Websockets

Question

I am setting up a dashboard which requires a Python Flask server to fetch public transport data once every few seconds or so and send this data to a client webpage through websockets. I am using FlaskSocketIO to achieve this.

I am able to send a message upon the 'connect' event, for example, but I've been unable to find a way to continuously update this message. This is my approach:

from flask import Flask, render_template, url_for, request
from flask_socketio import SocketIO, emit, send
import eventlet
eventlet.monkey_patch()

app = Flask(__name__)
socketio = SocketIO(app, logger=True, engineio_logger=True)

def listen():
    while True:
        message = # some updating message
        emit('message', message)
        socketio.sleep(1)

@app.route('/')
def index():
    return render_template('index.html')

@socketio.on('connect')
def handle_connect():
    listen()

if __name__ == "__main__":
    socketio.run(app, debug=True)

The logger says it sends the message. However, on the client side, no message is recieved. The client side JavaScript is:

var socket = io();

socket.on('message', function(msg) {
    console.log(msg);
});

Caio Filus · Accepted Answer · 2020-02-14 12:38:16Z

i have the same problem with you, in my case i wanted to update sensor dashboard, so i study about Observer Pattern, this link is really good Observer Pattern.Implement a SocketEmitObservator, using the Observer you dont need to create a listener(infinite while loop) and make your code more clean:

class SensorObserver(Observer):
    sensor_id: int
    sid = ''

    def __init__(self, sid,sensor_id=None):
        Observer.__init__(self)
        self.sensor_id = sensor_id
        self.sid = sid
        SensorPublisher().attach(self)

    def update(self, sensorData: DataModel) -> None:
        socket.emit("info",
                        {'sensor': subject},
                        namespace='/api/sector/machine',
                        room=self.sid)

    def __del__(self):
        SensorPublisher().detach(self)

class SensorPublisher(Subject, Singleton):
    _observers: List[SensorObserver] = []
    _sensorData: Dict[str, DataModel] = {}

    def attach(self, observer: SensorObserver) -> None:
        if observer not in self._observers:
            self._observers.append(observer)

    def notify(self, sensorModel: DataModel) -> None:
        for observer in self._observers:
            if sensorModel.sensor_id == observer.sensor_id:
                observer.update(sensorModel)

    def detach(self, observer: SensorObserver) -> None:
        if observer in self._observers:
            ob = self._observers.pop(self._observers.index(observer))
            del ob

    def update(self, sensorModel: DataModel):
        sendorId: str = str(sensorModel.sensor_id)
        self._sensorData[sendorId] = sensorModel
        self.notify(sensorModel)


#### And in the place where i update sensor data i put
SensorPublisher().update(dataStructList)

#### To create a observr just do
@socketio.on('connect')
def handle_connect():
    SensorObserver(self.sid, item)

In my case i use like this, but you can adapt for your problem

Miguel Grinberg · Accepted Answer · 2020-02-15 08:21:00Z

1

You cannot add a while True loop in the connect handler, as that will prevent the connect handler from returning and establishing the connection.

You can do this as a response to another event sent by the client after the connection is made, or you can also start a background task that implements this loop. The connect handler needs to return quickly to allow the connection to happen.

answered Feb 15, 2020 at 8:21

Miguel Grinberg

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