executing_multiple_systems_simultaneously.md

Executing Multiple Systems Simultaneously

One of the power of Bevy is to run multiple systems simultaneously. To do so, we put multiple systems in a pair of parentheses and pass them to the add_systems method.

use bevy::app::{App, Startup};

fn main() {
    App::new().add_systems(Startup, (hello_a, hello_b)).run();
}

fn hello_a() {
    println!("Hello A!");
}

fn hello_b() {
    println!("Hello B!");
}

Since the two systems run in parallel, there is no guarantee which one would be executed first. The output could be either

Hello A!
Hello B!

or

Hello B!
Hello A!

To test these systems and see if they really run in parallel, we can make them sleep for a while.

use std::{thread, time::Duration};

// ...

fn hello_a() {
    thread::sleep(Duration::from_secs(1));
    println!("Hello A!");
}

fn hello_b() {
    thread::sleep(Duration::from_secs(1));
    println!("Hello B!");
}

We will see Hello A! and Hello B! appears at the same time.

The number of systems in a pair of parentheses is limited to 20 (see IntoSystemConfigs implementations). Yet, we can have more parentheses inside.

add_systems(Startup, (hello_a, (hello_b, hello_c)))

The added parentheses can be recursive, so the number of systems we can have can be treated as unlimited conceptually.

➡️ Next: Executing Multiple Systems In Order

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