bevy/examples/app/custom_loop.rs

use bevy::prelude::*;
use std::{io, io::BufRead};

struct Input(String);

/// This example demonstrates you can create a custom runner (to update an app manually). It reads
/// lines from stdin and prints them from within the ecs.
fn my_runner(mut app: App) {
    app.initialize();

    println!("Type stuff into the console");
    for line in io::stdin().lock().lines() {
        {
            let mut input = app.resources.get_mut::<Input>().unwrap();
            input.0 = line.unwrap();
        }
        app.update();
    }
}

fn print_system(input: Res<Input>) {
    println!("You typed: {}", input.0);
}

fn main() {
    App::build()
        .add_resource(Input(String::new()))
        .set_runner(my_runner)
        .add_system(print_system)
        .run();
}