bevy/crates/bevy_window/src/system.rs

use legion::prelude::*;
use bevy_app::{Events, AppExit, GetEventReader};
use crate::{WindowId, WindowCloseRequested};

pub fn exit_on_window_close_system(resources: &mut Resources, window_id: Option<WindowId>) -> Box<dyn Schedulable> {
    let mut window_close_requested_event_reader = resources.get_event_reader::<WindowCloseRequested>();
    SystemBuilder::new("exit_on_window_close")
        .read_resource::<Events<WindowCloseRequested>>()
        .write_resource::<Events<AppExit>>()
        .build(move |_, _, (ref window_close_requested_events, ref mut app_exit_events), _| {
            for window_close_requested_event in window_close_requested_events.iter(&mut window_close_requested_event_reader) {
                match window_id.as_ref() {
                    Some(window_id) => if *window_id == window_close_requested_event.id {
                        app_exit_events.send(AppExit);
                    },
                    None => if window_close_requested_event.is_primary {
                        app_exit_events.send(AppExit);
                    }
                }
            }
        })
}