run_in_executor not working on android

[cozimus]

Describe the bug

Refering to what reported here and here, the best approach to run a CPU-intensive task in backgroud is to use run_in_executor, that's works perfectly on Windows, but does not on Android.

Steps to reproduce

A minimum (not)-working example to reproduce the error is the following:

import toga, time

class MainApp(toga.App):
    def startup(self):

        self.main_window = toga.MainWindow(
            title=self.formal_name, size=(360, 654), resizable=True
        )
        self.label = toga.Label("")
        button = toga.Button("click here", on_press=self.click_handler)
        self.main_box = toga.Box(children=[button, self.label])
        self.main_window.content = self.main_box
        self.main_window.show()

        self.loop.create_task(self.loop_task())

    def click_handler(self, widget):
        self.label.text = "Clicked"

    async def loop_task(self):
        while True:
            await self.loop.run_in_executor(None, time.sleep, 1)
            self.label.text = time.time()


def main():
    return MainApp("Example App", "com.example")

Expected behavior

If your run it on Windows (via briefcase dev or briefcase run windows) it does the job update the label every second, but if I run it on android briefcase run android, the application freezes after the first render and don't respond

Screenshots

Environment

• Operating System: Windows 11
• Python version: 3.11.5
• Software versions:
  • Briefcase: 0.3.17
  • Toga: 0.4.2

[freakboy3742]

Thanks for the report. Right now, run_in_executor() isn't implemented on Android.

Interestingly, that code references the fact that "in the future", we may be able to restore run_in_executor(). Unless I'm missing something, it appears "the future" may be now. In my testing, restoring the default executor behavior implements the expected behavior. I've pushed #2481 to implement this.