wave5-fixed-step-simulation-system-group

Fixed-Step Simulation System Group — Physics Ordering

Intent

Schedule physics-affecting and fixed-rate simulation systems inside FixedStepSimulationSystemGroup with correct ordering relative to PhysicsSystemGroup and its sub-groups.

Use When

• Any system writes PhysicsVelocity, PhysicsCollider, or other physics state for the physics solver
• Simulation logic that must run at a fixed timestep regardless of render frame rate
• Reading physics solver results (collision events, contact data) after the solver completes

Avoid When

• System is rendering-dependent (visual effects, camera) — use SimulationSystemGroup or PresentationSystemGroup
• System reads physics results but doesn't affect physics — can run in SimulationSystemGroup after completing physics dependency

Senior Pattern

// Pre-solve forces — run before physics world is built and solved:
[UpdateInGroup(typeof(FixedStepSimulationSystemGroup))]
[UpdateBefore(typeof(PhysicsSystemGroup))]
[BurstCompile]
public partial struct PreSolveForceSystem : ISystem { ... }

// Post-solve event processing — run after solver emits events:
[UpdateInGroup(typeof(FixedStepSimulationSystemGroup))]
[UpdateAfter(typeof(PhysicsSimulationGroup))]   // PhysicsSimulationGroup, NOT PhysicsSystemGroup
[BurstCompile]
public partial struct PostSolveEventSystem : ISystem { ... }

Execution order inside FixedStepSimulationSystemGroup:

→ [UpdateBefore(PhysicsSystemGroup)]         ← custom force/impulse systems
→ PhysicsSystemGroup
    → PhysicsBuildWorldGroup                 ← builds collision world from transforms + colliders
    → PhysicsSimulationGroup                 ← runs solver, emits trigger/collision events
→ [UpdateAfter(PhysicsSimulationGroup)]      ← event processing (trigger/collision results)

Anti-Patterns

• [UpdateAfter(PhysicsSystemGroup)] for event processing — PhysicsSystemGroup encompasses build+simulation+cleanup; use PhysicsSimulationGroup for reading solver results before cleanup.
• SystemAPI.Time.DeltaTime inside fixed-step group — equivalent to fixedDeltaTime but using fixedDeltaTime makes intent explicit.
• Expensive structural changes (ECB playback) inside the fixed loop — archetype churn causes broad-phase rebuild overhead; defer to SimulationSystemGroup if possible.
• Placing event-response systems in SimulationSystemGroup (render rate) when they must respond to per-physics-tick events — may miss ticks when fixed loop runs twice per render frame.