Simple example for starting/stopping a (background) task with nicegui #5030
Replies: 1 comment 3 replies
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Hi @malleite2025, In order to avoid CPU-bound tasks blocking the main UI thread, you need to run them on a separate process. This can be done using NiceGUI's import time
from multiprocessing import Manager, Queue
from nicegui import run, ui
def calculate(ctrl_q: Queue) -> None:
while ctrl_q.empty() or ctrl_q.get_nowait() != 'STOP':
print('calculating...')
time.sleep(0.5)
@ui.page('/')
def main_page():
manager = Manager()
ctrl_q = manager.Queue()
async def start() -> None:
await run.cpu_bound(calculate, ctrl_q)
def stop() -> None:
ctrl_q.put('STOP')
ui.button('start', on_click=start)
ui.button('stop', on_click=stop)
ui.run() |
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Dear @falkoschindler Thank you very much for the code, I was able to run it with a few modifications (eg. flushing the queue in case the user clicks many times the stop button and keeping only one instance of the task running in case the start button is pressed many times). This worked as a script, the way you sent. But I would like to implement this in a class (the application will have many tabs and configurations so it is better for modularization). With this it may be necessary that a ui element gets its content updated inside another class method. I tried the code bellow, but if I try to scope any ui (e.g. label or number) as "self" for accessing it inside a class method I get a crash (also bellow). The general idea is that several ui s (labels, numbers, plots) are updated based on the async task "calculated" .... What would be the correct way to do this in nicegui ? Thank you for all the help, and for all the nice work with nicegui ! import time
from multiprocessing import Manager, Queue, RLock
import multiprocessing
from nicegui import run, ui
class ss() :
def __init__(self,ctrl_q) :
self.running = False
self.ctrl_q = ctrl_q
self.k = 0
def calculate(self) -> None:
while self.ctrl_q.empty() or self.ctrl_q.get_nowait() != 'STOP':
self.k += 1
print(f'calculating... {self.k}')
#this does not work
#self.label.text = f'calculating... {self.k}'
time.sleep(0.5)
async def start(self) -> None:
global running
print ("START",self.ctrl_q.qsize())
print (multiprocessing.active_children())
while self.ctrl_q.qsize() != 0 :
self.ctrl_q.get_nowait()
if not self.running :
self.running = True
await run.cpu_bound(self.calculate)
def stop(self) -> None:
print ("STOP")
self.ctrl_q.put('STOP')
self.running = False
self.k = 0
def my_controls (self) :
start = ui.button('start', on_click=self.start)
stop = ui.button('stop', on_click=self.stop)
with ui.card() :
lb = f'calculating... {self.k}'
#This does not work
#self.label = ui.label(lb)
label = ui.label(lb)
@ui.page('/')
def main_page():
manager = Manager()
ctrl_q = manager.Queue()
x = ss(ctrl_q)
x.my_controls()
ui.run()Terminal output of the crash : |
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See https://github.com/zauberzeug/nicegui/wiki/FAQs#why-is-my-long-running-function-blocking-ui-updates:
In your case you're sending See https://github.com/zauberzeug/nicegui/blob/main/examples/progress/main.py for an example on how to use another queue to send progress updates back to the main process. |
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Dear @falkoschindler , |
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First Check
Example Code
Description
Hi,
I am struggling a bit trying to have a very simple application where a "start" button starts a background task to do some work continuously and a "stop" button stops this task . I understand I should use asyncio otherwise the task blocks the execution of the gui (as happens with the attached code) , but I am not being able to make it work. Is there a similar example on how to do this correctly ? Thank you
NiceGUI Version
2.21.1
Python Version
3.13
Browser
Chrome
Operating System
Linux
Additional Context
No response
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