Bevy ECS composition

When to use

Load this when gameplay data should be split across small components and systems, instead of one type that owns everything. The Quick Start ECS page is the source for the names below.

ECS shape

• Components are Rust structs that derive Component. The ECS page splits Person and Name so more than one kind of entity can carry a name.
• Entities are spawned with Commands in a Startup system. The page spawns a tuple of components: commands.spawn((Person, Name(...))).
• Systems are functions. Query<&Name, With<Person>> reads names on entities that also have Person. Query<&mut Name, With<Person>> writes. Register with add_systems.
• Schedule: Startup runs once. Update runs afterward. Systems run in parallel unless you call .chain(). The page chains update_people before greet_people and leaves an unrelated system outside the chain.

Spawn a tuple of components. A parent and child use ChildOf. Change detection uses Changed.

Composition choices

Need	Bevy idiom
Has-a parts on one object	Several Component structs on one entity
Behavior	A system function over a Query
Spawn at start	Commands in a Startup system
Ordered steps	.chain() on the systems that depend on each other
Parent and child	ChildOf on the child entity

Scope

• Tuple spawn, ChildOf, Changed, and .chain() are in the examples below.
• Editor widget trees are out of scope. Composition here is entities, components, and systems.

[package]
name = "ecs-composition"
version = "0.1.0"
edition = "2021"

[dependencies]
bevy = "0.20"

People carry a name. Greeting runs after the rename, in one chain.

#[derive(Component)]
struct Person;

#[derive(Component)]
struct Name(String);

fn spawn_people(mut commands: Commands) {
    commands.spawn((Person, Name("Ada".to_string())));
    commands.spawn(Name("crate".to_string()));
}