How to update streamlits UI using a thread

Using Streamlit
1

Hello I have the following code with a countdown timer and I’m trying to use a thread to update the timer_placeholder widget but it doesn’t work. It render the first second only. I can see the code running but the UI does not.

import streamlit as st
import threading
import time
import datetime
from datetime import timedelta
from functools import wraps
from streamlit.runtime.scriptrunner import add_script_run_ctx, get_script_run_ctx

# Constants
INTERVIEW_DURATION = 60

def run_in_thread(func):
    @wraps(func)
    def wrapper(*args, **kwargs):
        thread = threading.Thread(target=func, args=args, kwargs=kwargs)
        ctx = get_script_run_ctx()  
        add_script_run_ctx(thread, ctx)
        thread.start()
    return wrapper

@run_in_thread
def countdown_timer(timer_placeholder):
    """Runs a countdown timer in a separate thread."""
    print("Spawning new thread", st.session_state.get('remaining_time', INTERVIEW_DURATION))
    while st.session_state.get('remaining_time', INTERVIEW_DURATION) > 0:
        elapsed_time = (datetime.datetime.now() - st.session_state.start_time).total_seconds()
        st.session_state.remaining_time = max(0, INTERVIEW_DURATION - int(elapsed_time))
        timer_placeholder.info(
            f"⏳ Time remaining: {str(timedelta(seconds=st.session_state.remaining_time))}"
        )
        if st.session_state.remaining_time <= 0:
            timer_placeholder.warning("⏰ Time's up!")
            break  # Exit the loop if time is up
        
        time.sleep(1)  # Sleep for 1 second

    print("Ended")
    timer_placeholder.write("Finished")

st.title("Countdown Timer")

if "remaining_time" not in st.session_state:
    st.session_state.remaining_time = INTERVIEW_DURATION
if "start_time" not in st.session_state:
    st.session_state.start_time = datetime.datetime.now()

timer_placeholder = st.empty()

if st.button("Start Timer"):
    # Reset the timer state
    st.session_state.remaining_time = INTERVIEW_DURATION
    st.session_state.start_time = datetime.datetime.now()
    countdown_timer(timer_placeholder)

if st.session_state.remaining_time > 0:
    timer_placeholder.info(
        f"⏳ Time remaining: {str(timedelta(seconds=st.session_state.remaining_time))}"
    )