World Systems

A world system runs over the whole world, not a single entity. Where an entity system is a script attached to one entity that runs that entity’s behavior, a world system is created once per world and iterates entities itself, once per frame, just like an engine (C++) system. The two are different tools.

	Entity system	World system
Instance	one per entity (C# Script component)	one per world
`this` is	that entity’s script	the system (no entity)
Iterates	nothing (it is the entity)	entities you query through the context
Good for	one actor’s behavior	a rule applied across many entities

Reach for a world system when a behavior is really a rule over a set of entities (spin everything tagged a certain way, drain every Health below zero) rather than logic that belongs to one actor.

Anatomy of a system

A world system is a class that derives from EntitySystem and carries an [EntitySystem] attribute declaring its stage and priority. The attribute is what makes the type discovered and run; one instance is created per world automatically.

using LuminaSharp;
using Lumina;

namespace Game;

[EntitySystem(Stage = EUpdateStage.PrePhysics, Priority = 128)]
public sealed class SpinSystem : EntitySystem
{
    public override void OnUpdate(SystemContext Context)
    {
        float Dt = Context.DeltaTime;
        Context.Registry.View<STransformComponent>().Each((Entity E, STransformComponent Transform) =>
        {
            Transform.AddYaw(90.0f * Dt);
        });
    }
}

Hooks

Override only what you need. All three receive the per-tick SystemContext.

Method	When it runs
`OnStartup(SystemContext)`	Once, when the system is created for a world.
`OnUpdate(SystemContext)`	Every frame, in its stage.
`OnTeardown(SystemContext)`	Once, when the world (or this system) is torn down.

The system’s World property is also available, for the full World API.

Stages and priority

A world system runs in one update stage, and systems within a stage run in priority order. These mirror the engine’s own pipeline, and they are the same pre- and post-physics phases an entity system can opt into.

`EUpdateStage`	Runs
`FrameStart`	Start of frame, before anything else.
`PrePhysics`	Before the physics step (input, movement intent). The default.
`DuringPhysics`	Alongside the physics step.
`PostPhysics`	After physics resolves (react to results).
`FrameEnd`	End of frame.
`Paused`	The stage that ticks while the editor is idle (not playing).

Priority is an int, lower runs first, and 128 is the default (medium).

[EntitySystem(Stage = EUpdateStage.PostPhysics, Priority = 0)]   // runs early in PostPhysics

The system context

SystemContext mirrors the C++ system context, exposing time, entity creation, and access to the live world’s component store.

Member	Does
`Context.DeltaTime` / `Context.Time`	Seconds this frame / total world time.
`Context.Registry`	The component store. Author views with `Registry.View<...>()`.
`Context.Create()` / `Context.Destroy(entity)`	Make or destroy an entity.
`Context.SetEntityLocation(entity, v)`	Set an entity’s world-space location.
`Context.DrawDebugLine(start, end, color)`	One-frame debug line (Dev/Debug only).

Iterating entities with views

Registry.View<...>() is an entt-style typed view, every entity that has all of the listed components. Iterate it with .Each(...) or foreach, and pass an Exclude<...>() filter as the argument to skip entities that also have a component.

public override void OnUpdate(SystemContext Context)
{
    EntityRegistry Registry = Context.Registry;

    // Every entity with both components, excluding the frozen ones.
    var View = Registry.View<STransformComponent, SVelocityComponent>(
        EntityRegistry.Exclude<SFrozenTag>());

    foreach ((Entity E, STransformComponent Transform, SVelocityComponent Velocity) in View)
    {
        Transform.Translate(Velocity.Value * Context.DeltaTime);
    }
}

Views support arity 1–4 and an exclude filter of up to 3 types. The wrappers a view hands back are valid only for the current iteration step. Read out any field you need to keep, don’t store the wrapper.

Subsystems

Everything outside the entity set (physics, navigation, networking, debug drawing) is reached through the system’s World property, exactly as in entity systems, via World.Physics, World.Navigation, World.Net, World.Draw. See The World API.