New event loop integration system #1371

I would like to change our event-loop integration with Trio's guest mode. Trio's guest mode also works on asyncio thanks to aioguest. For it to work, we just need to have a function that can schedule a callback in the host event-loop in a thread-safe way.

• in asyncio, that would be loop.call_soon_threadsafe(callback),
• in tkinter, that would be tk.after(0, callback).
• Qt supports Trio's guest mode, see here.
• ...

For instance, here is how it looks like when the host event-loop is tkinter's:

from tkinter import Tk

from anyio import sleep
from anyioutils import start_guest_run


def run_sync_soon_threadsafe(fn):
    tk.after(0, fn)

def done_callback(outcome):
    print("done")

async def foo():
    for i in range(10):
        await sleep(0.1)
        print(i)

tk = Tk()
start_guest_run(foo, run_sync_soon_threadsafe=run_sync_soon_threadsafe, done_callback=done_callback)
tk.mainloop()

Before calling tk.mainloop(), we call start_guest_run() which runs foo as a task in tkinter's event-loop.

Thoughts?