Real time countdown implementation

[ysomad]

Hello! Working on typing speed test app using flask-socketio and I have an issue with countdown implementation.

I have keydown event in which im checking is typing test started or no, storing is_started in session so on next keydown emits I have 2 cases:

1. Start typing test if is_started is not set (when typing test is started countdown should start and emit countdown event to client
2. Emit keydown_reply which is sending pressed key back to client

What I achieve: implemented countdown using Celery
Problems:

1. Celery cannot use main socket io instance and because of that it's impossible to emit events to a specific socket (or i did not find any information how to do it)
2. On page refresh countdown still going and I need manually to revoke it

Question:
Is there any ideas how to implement countdown so it can be independent (and emitting events back to the client on background) from keydown event handler and so that it is possible to get his state in the same handler?

# tasks.py

@celery.task
def start_countdown(seconds: int) -> bool:
    socketio = create_socketio(flask_app, False)
    while seconds > 0:
        socketio.emit('typing_test_countdown', seconds)
        seconds -= 1
        socketio.sleep(1)
    return True

# events.py

@socketio.on('keydown')
def on_keydown(key: str):
    cfg = session.get('config')
    typing_test_finished = False

    if session.get('is_started') in (None, False):
        # Start typing test
        session['is_started'] = True
        result = start_countdown.delay(cfg.duration) # starts celery task with countdown
        typing_test_finished = result.get()

        emit('keydown_reply', {
            'key': key, 
            'correct': False, # not implemented
        })
    else:
        # Process typing test  
        emit('keydown_reply', {
            'key': key, 
            'correct': False, # not implemented
        })

    if typing_test_finished is True:
        # Send typing test results
        emit('get_result', {}) # not implemented
        typing_test_finished = False
        session['is_started'] = False

[miguelgrinberg]

My main question is why do you need the server to count down the seconds. Wouldn't it be easier for the client to count the time on its own? The server has to keep track of when the countdown expires, of course, but I don't see why it has to control the client side timer, since both can count seconds on their own and they'll be very close, if not exactly the same.

Celery cannot use main socket io instance and because of that it's impossible to emit events to a specific socket (or i did not find any information how to do it)

See the section on emitting from an external process in the documentation.

On page refresh countdown still going and I need manually to revoke it

You can use the disconnect handler to do any necessary clean up work. This is invoked after the user refreshes the page.

[ysomad]

That's true, I thought it would be more precisely for some reason but I'll stick with counting on client ofc, thank you!

I've done celery countdown issue but now I'm switching to python-socketio since I don't need Flask at all on this project.

Sorry for a long response, I was on sick leave.