System Ordering

The scheduler already separates two systems that read and write the same component. Sometimes you need more than that: a SpawnSystem might need to run before anything that reacts to newly spawned entities, even if the two never touch the same data. That’s what explicit ordering is for.

RunBefore and RunAfter

[RunAfter(typeof(SpawnSystem))]
public sealed partial class ReactToSpawnsSystem : QuerySystem
{
    protected override IQuery DefineQuery(Query query) => query.With<Spawned>();

    public void Update(Time time, in Spawned spawned) { /* ... */ }
}

[RunBefore(typeof(X))]/[RunAfter(typeof(X))] declare that this system must run in a strictly earlier or later stage than X. Stack as many as you need, one attribute per target.

Note

An edge like this can force two systems into separate stages even when neither reads nor writes anything the other touches. That’s a real scheduling cost, reach for ordering only when the sequence actually matters.

Ordering against a whole phase

Naming every system in a phase to order against it doesn’t scale. Define a marker instead, and order against that:

public sealed class InputProcessed : MarkerSystem { }

[RunBefore(typeof(InputProcessed))]
public sealed partial class PlayerMoveInputSystem : QuerySystem { /* ... */ }

[RunBefore(typeof(InputProcessed))]
public sealed partial class PlayerActionInputSystem : QuerySystem { /* ... */ }

[RunAfter(typeof(InputProcessed))]
public sealed partial class AIDecisionSystem : QuerySystem { /* ... */ }

AIDecisionSystem now runs after every input system, whatever they turn out to be. Add a third input system later and it’s covered automatically, nothing about AIDecisionSystem changes.

Note

MarkerSystem is never registered or instantiated. It exists purely as a Type the ordering graph can target.

Ordering a system added at runtime

AddSystem<T>() returns a chainable registration, for systems added outside the attribute-based path:

world.AddSystem<LateJoinerSystem>().After<InputProcessed>();

Tip

For how the stage graph, phases, and cadence actually work, see Scheduling.

Next

Ordering controls sequence. For controlling how often a system runs at all, see Timestep, Pause & Timescale.