freECS

freecs is a table-based ECS library for Rust, in about ~600 lines

A macro is used to define the world and its components, and generates the entity component system as part of your source code at compile time.

The internal implementation is minimal and does not use object orientation, generics, traits, or dynamic dispatch.

Quick Start

Add this to your Cargo.toml:

[dependencies]

freecs = "0.5.1"



# (optional) add rayon if you want to parallelize systems

rayon = "^1.10.0"

And in main.rs:

use freecs::{ecs, table_has_components};
use rayon::prelude::*;

ecs! {
    World {
        position: Position => POSITION,
        velocity: Velocity => VELOCITY,
        health: Health => HEALTH,
    }
    Resources {
        delta_time: f32
    }
}

pub fn main() {
    let mut world = World::default();

    // Spawn entities with components
    let _entity = world.spawn_entities(POSITION | VELOCITY, 1)[0];

    // Or use the entity builder
    let entity = EntityBuilder::new()
        .with_position(Position { x: 1.0, y: 2.0 })
        .spawn(&mut world, 1)[0];

    // Read arbitrary components
    let position = world.get_component::<Position>(entity, POSITION);
    println!("Position: {:?}", position);

    // Or use the shorthand methods
    let position = world.get_position(entity);
    println!("Position: {:?}", position);

    world.set_position(entity, Position { x: 1.0, y: 2.0 });

    // Mutate a component
    if let Some(position) = world.get_position_mut(entity) {
        position.x += 1.0;
    }

    // Get an entity's component mask
    let _component_mask = world.component_mask(entity).unwrap();

    // Add a new component to an entity
    world.add_components(entity, HEALTH);

    // Query all entities
    let _entities = world.get_all_entities();

    // Query all entities with a specific component
    let _players = world.query_entities(POSITION | VELOCITY | HEALTH);

    // Query the first entity with a specific component,
    // returning early instead of checking remaining entities
    let _first_player_entity = world.query_first_entity(POSITION | VELOCITY | HEALTH);

    // Remove a component from an entity
    world.remove_components(entity, HEALTH);

    // Systems are functions that iterate over
    // the component tables and transform component data.
    // This function invokes two systems in parallel
    // for each table in the world filtered by component mask.
    systems::run_systems(&mut world);

    // Despawn entities, freeing their table slots for reuse
    world.despawn_entities(&[entity]);
}

use components::*;
mod components {
    #[derive(Default, Debug, Clone)]
    pub struct Position {
        pub x: f32,
        pub y: f32,
    }

    #[derive(Default, Debug, Clone)]
    pub struct Velocity {
        pub x: f32,
        pub y: f32,
    }

    #[derive(Default, Debug, Clone)]
    pub struct Health {
        pub value: f32,
    }
}

mod systems {
    use super::*;

    pub fn run_systems(world: &mut World) {
        // systems can be simple functions that use queries and component accessors
        example_system(world);

        // or you can iterate over the tables in the world directly
        let delta_time = world.resources.delta_time;
        world.tables.par_iter_mut().for_each(|table| {
            if table_has_components!(table, POSITION | VELOCITY | HEALTH) {
                update_positions_system(&mut table.position, &table.velocity, delta_time);
            }
            if table_has_components!(table, HEALTH) {
                health_system(&mut table.health);
            }
        });
    }

    fn example_system(world: &mut World) {
        for entity in world.query_entities(POSITION | VELOCITY) {
            if let Some(position) = world.get_component_mut::<Position>(entity, POSITION) {
                position.x += 1.0;
            }
        }
    }

    // The system itself can also access components in parallel
    #[inline]
    pub fn update_positions_system(positions: &mut [Position], velocities: &[Velocity], dt: f32) {
        positions
            .par_iter_mut()
            .zip(velocities.par_iter())
            .for_each(|(pos, vel)| {
                pos.x += vel.x * dt;
                pos.y += vel.y * dt;
            });
    }

    #[inline]
    pub fn health_system(health: &mut [Health]) {
        health.par_iter_mut().for_each(|health| {
            health.value *= 0.98;
        });
    }
}

Systems

Systems can iterate over tables, but for most use cases you can just write a function that queries entities:

pub fn update_global_transforms_system(world: &mut World) {
    world
        .query_entities(LOCAL_TRANSFORM | GLOBAL_TRANSFORM)
        .into_iter()
        .for_each(|entity| {
            // The entities we queried for are guaranteed to have
            // a local transform and global transform here
            let new_global_transform = query_global_transform(world, entity);
            let global_transform = world.get_global_transform_mut(entity).unwrap();
            *global_transform = GlobalTransform(new_global_transform);
        });
}

pub fn query_global_transform(world: &World, entity: EntityId) -> nalgebra_glm::Mat4 {
    let Some(local_transform) = world.get_local_transform(entity) else {
        return nalgebra_glm::Mat4::identity();
    };
    if let Some(Parent(parent)) = world.get_parent(entity) {
        query_global_transform(world, *parent) * local_transform
    } else {
        local_transform
    }
}

Change Detection

freecs provides an opt-in change detection system that allows you to track when components are modified. This is useful for systems that only need to process entities when their data has changed.

// Get mutable access and modify a component
if let Some(pos) = world.get_component_mut::<Position>(entity, POSITION) {
    pos.x += velocity.x * dt;
    pos.y += velocity.y * dt;
}

// Explicitly mark the component as changed
world.mark_changed(entity, POSITION);

// Later, process change events
while let Some(event) = world.try_next_event() {
    match event {
        Event::ComponentChanged { kind, entity } => {
            println!("Component {:b} changed for entity {:?}", kind, entity);
        }
    }
}

// You can also clear the event queue
world.clear_events();

You can mark multiple components as changed in a single call:

// Mark both position and velocity as changed
world.mark_changed(entity, POSITION | VELOCITY);

The event queue is stored in the world's Resources struct and is automatically available when you create a world with the ecs! macro.

Entity Builder

An entity builder is generated automatically:

let mut world = World::default();

let entities = EntityBuilder::new()
    .with_position(Position { x: 1.0, y: 2.0 })
    .spawn(&mut world, 2);

assert_eq!(world.get_position(entities[0]).unwrap().x, 1.0);
assert_eq!(world.get_position(entities[1]).unwrap().y, 2.0);

License