demo.rs

struct PositionComponent {
    x: f32;
    y: f32;
}

impl Component for PositionComponent {
    type Storage = VecStorage<PositionComponent>;
}

enum RenderShape {
    Sprite(String),
    Box(f32, f32)
}

struct RenderComponent {
    shape: RenderShape
}

impl Component for RenderComponent {
    type Storage = VecStorage<RenderComponent>;
}


struct RenderSystem;
impl specs::System<()> for RenderSystem {
    fn run(&mut self, arg: specs::RunArg, _: ()) {
        let (positions, renderables) = arg.fetch(|w| {
            (
                w.read::<PositionComponent>(),
                w.read::<RenderComponent>()
            )});
        
        let renderer = create_renderer_for_frame(); // Could be a member of RenderSystem or whatever
        
        for (pos, ren) in (positions, renderables).join() {
            render.render_object(pos, ren);
        }
    }
}