Completion of concurrent tasks after run_forever.

[draustin]

It seems that run_forever has the effect of calling QApplication.exec() while simultaneously running the event loop tasks. When exec returns, the event loop stops and run_forever returns. At this point I would like to send a stop signal to the other tasks to perform cleanup actions, and then restart the loop and run it until the tasks are complete. I came up with a solution, pasted below. This issue is a suggestion to elaborate the docstring / add an example, and to help others.

Thanks for your work.

import sys

from PyQt5 import QtWidgets
import qasync
import asyncio


class Widget(QtWidgets.QWidget):
    """A widget with a single label."""

    def __init__(self):
        QtWidgets.QWidget.__init__(self)

        self._label = QtWidgets.QLabel()
        b0 = QtWidgets.QHBoxLayout()
        self.setLayout(b0)
        b0.addWidget(self._label)

    def set_text(self, text):
        """Set the text of the label."""
        self._label.setText(text)


stop_flag = False


async def start_counting(widget):
    try:
        n = 0
        while not stop_flag:
            widget.set_text(str(n))
            await asyncio.sleep(1)
            n += 1
        print('Counting stopped.')
    except BaseException as e:
        print('Counting cancelled.')


app = QtWidgets.QApplication(sys.argv)
widget = Widget()
widget.show()
loop = qasync.QEventLoop(app)
task = loop.create_task(start_counting(widget))
# Runs until last widget closed, like QApplication.exec().
loop.run_forever()
assert not loop.is_running()
# Looking at the __init__.py in qasync, it seems that it stopped running because QApplication.exec() returned.
# However, our counting task is incomplete.
tasks = asyncio.all_tasks()
assert len(tasks) == 1

# Send stop signal. Alternative, can cancel.
stop_flag = True
# task.cancel()

# Restart the loop to finish the tasks. In this example there is only one, but gather them for generality.
gathered = asyncio.gather(*tasks)
loop.run_until_complete(gathered)

# The following does not work.
# loop.run_forever()

print('All done.')

[draustin]

PS this is possible related to #35.

[hosaka]

See my comment on #35 (comment)