As this page is outdated, please refer to Bevy's official migration guides while reading, to cover the differences: 0.9 to 0.10, 0.10 to 0.11, 0.11 to 0.12, 0.12 to 0.13, 0.13 to 0.14.

I apologize for the inconvenience. I will update the page as soon as I find the time.

Generic Systems

Bevy systems are just plain rust functions, which means they can be generic. You can add the same system multiple times, parametrized to work on different Rust types or values.

Generic over Component types

You can use the generic type parameter to specify what component types (and hence what entities) your system should operate on.

This can be useful when combined with Bevy states. You can do the same thing to different sets of entities depending on state.

Example: Cleanup

One straightforward use-case is for cleanup. We can make a generic cleanup system that just despawns all entities that have a certain component type. Then, trivially run it on exiting different states.

use bevy::ecs::component::Component;

fn cleanup_system<T: Component>(
    mut commands: Commands,
    q: Query<Entity, With<T>>,
) {
    for e in q.iter() {
        commands.entity(e).despawn_recursive();
    }
}

Menu entities can be tagged with cleanup::MenuExit, entities from the game map can be tagged with cleanup::LevelUnload.

We can add the generic cleanup system to our state transitions, to take care of the respective entities:

/// Marker components to group entities for cleanup
mod cleanup {
    use bevy::prelude::*;
    #[derive(Component)]
    pub struct LevelUnload;
    #[derive(Component)]
    pub struct MenuClose;
}

#[derive(Debug, Clone, Eq, PartialEq, Hash)]
enum AppState {
    MainMenu,
    InGame,
}

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_state(AppState::MainMenu)
        // add the cleanup systems
        .add_system_set(SystemSet::on_exit(AppState::MainMenu)
            .with_system(cleanup_system::<cleanup::MenuClose>))
        .add_system_set(SystemSet::on_exit(AppState::InGame)
            .with_system(cleanup_system::<cleanup::LevelUnload>))
        .run();
}

Using Traits

You can use this in combination with Traits, for when you need some sort of varying implementation/functionality for each type.

Example: Bevy's Camera Projections

(this is a use-case within Bevy itself)

Bevy has a CameraProjection trait. Different projection types like PerspectiveProjection and OrthographicProjection implement that trait, providing the correct logic for how to respond to resizing the window, calculating the projection matrix, etc.

There is a generic system fn camera_system::<T: CameraProjection + Component>, which handles all the cameras with a given projection type. It will call the trait methods when appropriate (like on window resize events).

The Bevy Cookbook Custom Camera Projection Example shows this API in action.

Using Const Generics

Now that Rust has support for Const Generics, functions can also be parametrized by values, not just types.

fn process_layer<const LAYER_ID: usize>(
    // system params
) {
    // do something for this `LAYER_ID`
}

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_system(process_layer::<1>)
        .add_system(process_layer::<2>)
        .add_system(process_layer::<3>)
        .run();
}

Note that these values are static / constant at compile-time. This can be a severe limitation. In some cases, when you might suspect that you could use const generics, you might realize that you actually want a runtime value.

If you need to "configure" your system by passing in some data, you could, instead, use a Resource or Local.

Note: As of Rust 1.65, support for using enum values as const generics is not yet stable. To use enums, you need Rust Nightly, and to enable the experimental/unstable feature (put this at the top of your main.rs or lib.rs):

#![feature(adt_const_params)]