---
title: simulationRootEntity
framework: realitykit
role: symbol
role_heading: Instance Property
path: realitykit/physicssimulationevents/didsimulate/simulationrootentity
---

# simulationRootEntity

The root simulation entity associated with the simulation that raised the event.

## Declaration

```swift
let simulationRootEntity: Entity?
```

## Discussion

Discussion This entity is nil if the physics simulation is the default global one that covers all entities in the hierarchy. Use PhysicsSimulationComponent to construct a custom physics simulation for a subtree of entities that you have access to.  The event from this custom simulation reports a non-nil root simulation entity.