how to add a timer to a Toga app (python) running a task every interval, using Toga's event loop?

Question

My toga app needs to run a task every X seconds. I prefer a multi-platform solution where possible.

I asked an AI and its solution used an external dependency ("schedule") + additional loop + threading:

def create_timer(self, interval, callback):
    def run_callback():
        schedule.every(interval).seconds.do(callback)
        while True:
            schedule.run_pending()
            threading.Event().wait(interval)  
    timer_thread = threading.Thread(target=run_callback)
    timer_thread.daemon = True  # Ensure thread terminates with app
    timer_thread.start()
    return timer_thread

I rather avoid external modules and adding loops (for performance, minimize resource usage, and simplicity).

How to implment a timer / interval based on Toga's own event loop?

Answer 1

I never used toga but I found information that it uses async loop and that it has functions

• call_later(delay, callback, *args),
• call_at(when, callback, *args)

So I took some example app from documentation and I created working example which displays current time using call_later().

I use it in change() to execute the same function again after some time
(similar to .after(milliseconds, callback) in tkinter)

I don't know how to access app.loop (and label) in this function
so I send app (and label) as parameters.
But works also loop = asyncio.get_event_loop()

import toga
#import datetime
import time
#import asyncio

def change(app, widget):
    # if code needs more time then better set next execution first
    loop = app.loop
    #loop = asyncio.get_event_loop()
    loop.call_later(1, change, app, widget)
    
    #widget.text = datetime.datetime.now().strftime('%Y.%m.%d %H.%M.%S')
    widget.text = time.strftime('%Y.%m.%d %H.%M.%S')

    #app.loop.call_later(1, change, app, widget)  # here it would need `1 - time_of_function_execution`

def build(app):
    box = toga.Box()

    label = toga.Label("?")
    box.add(label)

    # execute first time without delay
    #app.loop.call_soon(change, app, label)
    #app.loop.call_later(0, change, app, label)
    change(app, label)
    
    return box

def main():
    return toga.App("Timer", "org.beeware.toga.tutorial", startup=build)


if __name__ == "__main__":
    main().main_loop()

---

The same using class MyApp(toga.App) like in documentation for App

because all is in class so I can use self. to access loop and label and I don't have to send it as parameters.

import toga
import time


class MyApp(toga.App):

    def startup(self):
        self.main_window = toga.MainWindow()
        self.main_window.content = toga.Box()
        
        self.label = toga.Label("?")
        self.main_window.content.add(self.label)

        # execute first time without delay
        #self.loop.call_soon(self.change, self.label)
        #self.loop.call_later(0, self.change, self.label)
        self.change()
        
        self.main_window.show()

    def change(self):
        # if code needs more time then better set next execution first
        self.loop.call_later(1, self.change)
    
        self.label.text = time.strftime('%Y.%m.%d %H.%M.%S')

        
if __name__ == '__main__':
    app = MyApp("Timer", "org.beeware.toga.tutorial")
    app.main_loop()