RuntimeError when showing dialog from async handler in Cocoa

Describe the bug A runtime error “Cannot enter into task while another task is being executed” is raised when showing a dialog from an async callback. This only occurs when there is already a “background task” running, either scheduled manually with asyncio.create_task or through toga with toga.App.add_background_task. The background task then crashes.

To Reproduce

Example code to reproduce, adapted from the handlers example:

Example

import asyncio

import toga
from toga.constants import COLUMN
from toga.style import Pack


class HandlerApp(toga.App):

    async def do_async(self, widget, **kwargs):

        widget.enabled = False
        for i in range(1, 10):
            self.async_label.text = "Iteration {}".format(i)
            if not self.ask_to_continue():
                break

        self.async_label.text = "Ready."
        widget.enabled = True

    def ask_to_continue(self):
        return self.main_window.confirm_dialog('Continue?', 'Press OK to continue.')

    async def do_background_task(self, widget, **kwargs):
        while True:
            self.counter += 1
            self.label.text = "Background: Iteration {}".format(self.counter)
            await asyncio.sleep(1)

    def startup(self):
        self.main_window = toga.MainWindow(title=self.name)

        self.label = toga.Label('Ready.', style=Pack(padding=10))
        self.async_label = toga.Label('Ready.', style=Pack(padding=10))

        self.counter = 0
        self.add_background_task(self.do_background_task)

        btn_style = Pack(flex=1)
        btn_async = toga.Button('Async callback', on_press=self.do_async, style=btn_style)

        box = toga.Box(
            children=[
                self.label,
                btn_async,
                self.async_label,
            ],
            style=Pack(
                flex=1,
                direction=COLUMN,
                padding=10
            )
        )

        self.main_window.content = box
        self.main_window.show()


def main():
    return HandlerApp('Handlers', 'org.beeware.handlers')


if __name__ == '__main__':
    app = main()
    app.main_loop()

This raises the exception:

Exception ignored on calling ctypes callback function: <function CFTimerHandle._cf_timer_callback.<locals>.cf_timer_callback at 0x114c4e670>
Traceback (most recent call last):
  File "/usr/local/lib/python3.8/site-packages/rubicon/objc/eventloop.py", line 155, in cf_timer_callback
    callback(*args)
  File "/usr/local/lib/python3.8/site-packages/rubicon/objc/eventloop.py", line 304, in _callback
    context.run(callback, *args)
RuntimeError: Cannot enter into task <Task pending name='Task-1' coro=<handler_with_cleanup() running at /usr/local/lib/python3.8/site-packages/toga/handlers.py:25> wait_for=<Future finished result=None>> while another task <Task pending name='Task-2' coro=<handler_with_cleanup() running at /usr/local/lib/python3.8/site-packages/toga/handlers.py:25>> is being executed.

Expected behavior The background task should pause while the dialog is blocking the event loop and resume afterwards.

Environment:

• Operating System: macOS Catalina, macOS Big Sur
• Python version: 3.8.5
• Software versions:
  • Toga: 0.3.0.dev23

Additional context This may be an issue with the Cocoa event loop integration in rubicon.objc.

[massenz]

In addition, toga’s asyncio integration is barely documented.

“barely documented” seems even an exaggeration: the “reference” doc for add_background_task(handler) won’t even specify the signature of handler, so much so that I have to reverse engineer it 😃

(incidentally, AFAICT, it gets invoked with two “app variants”? my main class (a toga.App) and a toga_iOS.app.App?)

[kakos-k9]

I would like to know how you did to call another screen, could you show the code?