An ECS, or Entity Component System is a design pattern popular in simulations, visualizations and game-development. It eschews rich objects and complex inheritence hierarchies.

Instead:

• Entities: A simple bag of facets.
• Facets: Simple data-only objects.
• Queries: Track entities which have specific facets.
• Systems: Process the facets of entities matched by queries.

React ECS helps you build your simulations and games using this pattern, in standard React JS fashion (hooks, components, etc)

Let's make a simple simulation that updates a cube based on a Spinning facet, using React ECS' ThreeJS integration @react-ecs/three.

This facet just tracks how much its entity should spin.

class Spin extends Facet<Spin> {
    amount? = new Vector3(1, 1, 1);
}

Systems use queries to track the entities they work upon.

This system uses a query to find entities with both the ThreeView and Spin facets. ThreeView is facet provided by @react-ecs/three to visually display entities in a ThreeJS scene.

This system updates the entity's 3D rotation based on the Spin facet:

const SpinSystem = () => {
    const query = useQuery((e) => e.hasAll(ThreeView, Spin));

    return useSystem((dt) => {
        query.loop([ThreeView, Spin], (e, [view, spin]) => {
            const rot = view.object3d.rotation;
            rot.x += spin.amount.x * dt;
            rot.y += spin.amount.y * dt;
            rot.z += spin.amount.z * dt;
        });
    });
};

Now we can create a component to tie it all together. For more information see our documentation and examples.

const CoolSim = () => {
  const ECS = useECS()
  useAnimationFrame(ECS.update)

  return (
    <Canvas>
      <ECS.Provider>
        <SpinSystem />
        <Entity>
          <Spin />
          <ThreeView>
            <Box />
          </ThreeView>
        </Entity>
      </ECS.Provider>
    </Canvas>
  )
}