Writing a system

The "S" in ECS

The basics

Subclass SystemBase, list the components you care about, and implement Process. This one regenerates health on anything with a HealthComponent:

HYP_CLASS(NoScriptBindings, Serialize = false)
class RegenSystem final : public SystemBase
{
    HYP_OBJECT_BODY(RegenSystem);

public:
    void Process(float delta, Span<Handle<Scene>> scenes) override;

private:
    SystemComponentDescriptors GetComponentDescriptors() const override
    {
        return {
            ComponentDescriptor<HealthComponent, ComponentAccess::READ_WRITE> {}
        };
    }
};

The implementation:

void RegenSystem::Process(float delta, Span<Handle<Scene>> scenes)
{
    if (!GetWorld()->GetGameState().IsSimulating())
    {
        return;
    }

    for (Scene* scene : scenes)
    {
        for (auto [entity, healthComponent] : scene->GetEntityManager()->GetEntitySet<HealthComponent>().GetScopedView(GetComponentInfos()))
        {
            if (healthComponent.health < 100.0f)
            {
                healthComponent.health += 5.0f * delta;
            }
        }
    }
}

The IsSimulating() check skips processing while editing, so it only runs during play.

Component access

Systems can run in parallel. Saying which components you only READ and which you WRITE lets the engine work out what's safe to run at the same time. Only ask for WRITE access where you need it.

Adding it to the world

Systems live on the world, not on a scene, and run over every scene in it:

GetWorld()->AddSystemT<RegenSystem>();

Subsystems

For world-wide logic that isn't tied to entities, like a day/night clock or a matchmaking queue, use a subsystem. Subclass Subsystem, fill in OnAddedToWorld(), OnRemovedFromWorld() and Update(float delta), then:

GetWorld()->AddSubsystem<MySubsystem>();

A world has at most one of each subsystem type. Retrieve it with GetSubsystem<MySubsystem>().