park bevy app main thread until there's work to do. full version: https://github.com/evanrelf/indigo/blob/59a733f173c0e0ce754a0da7d0b1a590aa05621b/crates/indigo-bevy/src/park.rs

park_mini.rs

// defining the plugin:

use bevy::{prelude::*, tasks::IoTaskPool};

pub struct Parker(parking::Parker);

#[derive(Resource)]
pub struct Unparker(pub parking::Unparker);

fn park_system(parker: NonSend<Parker>) {
    parker.0.park();
}

pub struct ParkPlugin;

impl Plugin for ParkPlugin {
    fn build(&self, app: &mut App) {
        let (parker, unparker) = parking::pair();
        app.insert_non_send_resource(Parker(parker))
            .insert_resource(Unparker(unparker))
            .add_systems(Last, park_system);
    }
}

// using the plugin:

fn some_other_system(unparker: Res<Unparker>) {
    let task_pool = IoTaskPool::get();
    let unparker = unparker.0.clone();
    let task = task_pool.spawn(async move {
        // feed some input to the bevy app here
        todo!();
        // wake up the main thread to let bevy loose on the new inputs
        unparker.unpark();
    });
    task.detach();
}