schedule_swap.rs

use bevy::prelude::*;

fn main() {
    // create an alternative Update schedule with a new set of systems
    // and store it in a resource to swap later
    let mut update = Schedule::new(Update);
    update.add_systems(bar);

    let mut app = App::new();
    app
        .insert_resource(AltUpdate(update))
        .add_systems(Update, foo)
        // swapping a schedule cannot happen while currently in that schedule
        // therefore this system in Update will panic
        .add_systems(PostUpdate, flip)
    ;

    app.update();
    println!("second frame");
    app.update();
}

#[derive(Resource, Deref, DerefMut)]
struct AltUpdate(pub Schedule);

fn flip(mut schedules: ResMut<Schedules>, mut alt: ResMut<AltUpdate>) {
    std::mem::swap(schedules.get_mut(Update).unwrap(), &mut *alt);
}

fn foo() {
    println!("foo");
}

fn bar() {
    println!("bar");
}