too_many_systems.md

Too Many Systems

Systems are important in the Bevy engine. When our project is getting larger and larger, we may need to add many systems. For example, in our tutorial for The Default Scheduler For Systems, we added 10 systems.

fn main() {
    App::new()
        .add_systems(First, first)
        .add_systems(Last, last)
        .add_systems(PostStartup, post_startup)
        .add_systems(PostUpdate, post_update)
        .add_systems(PreStartup, pre_startup)
        .add_systems(PreUpdate, pre_update)
        .add_systems(RunFixedUpdateLoop, run_fixed_update_loop)
        .add_systems(Startup, startup)
        .add_systems(StateTransition, state_transition)
        .add_systems(Update, update)
        .run();
}

This makes the main function long and unclear.

To solve this problem, we can move these codes that add systems into a Plugin.

struct MyPlugin;

impl Plugin for MyPlugin {
    fn build(&self, app: &mut App) {
        app.add_systems(First, first)
            .add_systems(Last, last)
            .add_systems(PostStartup, post_startup)
            .add_systems(PostUpdate, post_update)
            .add_systems(PreStartup, pre_startup)
            .add_systems(PreUpdate, pre_update)
            .add_systems(RunFixedUpdateLoop, run_fixed_update_loop)
            .add_systems(Startup, startup)
            .add_systems(StateTransition, state_transition)
            .add_systems(Update, update);
    }
}

Our main function is simplified.

fn main() {
    App::new().add_plugins(MyPlugin).run();
}

The full code is as follows:

use bevy::app::{
    App, First, Last, Plugin, PostStartup, PostUpdate, PreStartup, PreUpdate, RunFixedUpdateLoop,
    Startup, StateTransition, Update,
};

fn main() {
    App::new().add_plugins(MyPlugin).run();
}

struct MyPlugin;

impl Plugin for MyPlugin {
    fn build(&self, app: &mut App) {
        app.add_systems(First, first)
            .add_systems(Last, last)
            .add_systems(PostStartup, post_startup)
            .add_systems(PostUpdate, post_update)
            .add_systems(PreStartup, pre_startup)
            .add_systems(PreUpdate, pre_update)
            .add_systems(RunFixedUpdateLoop, run_fixed_update_loop)
            .add_systems(Startup, startup)
            .add_systems(StateTransition, state_transition)
            .add_systems(Update, update);
    }
}

fn first() {
    println!("First");
}

fn last() {
    println!("Last");
}

fn post_startup() {
    println!("PostStartup");
}

fn post_update() {
    println!("PostUpdate");
}

fn pre_startup() {
    println!("PreStartup");
}

fn pre_update() {
    println!("PreUpdate");
}

fn run_fixed_update_loop() {
    println!("RunFixedUpdateLoop");
}

fn startup() {
    println!("Startup");
}

fn state_transition() {
    println!("StateTransition");
}

fn update() {
    println!("Update");
}

We get the same output.

PreStartup
Startup
PostStartup
First
PreUpdate
StateTransition
RunFixedUpdateLoop
Update
PostUpdate
Last

➡️ Next: Faster Compile Time

📘 Back: Table of contents