Collisions example for an Entity in RealityKit

CustomBox+Collisions.swift

extension CustomBox {
  func addCollisions() {
    guard let scene = self.scene else {
      return
    }
    // Add the subscription for when this cube 
    collisionSubs.append(scene.subscribe(to: CollisionEvents.Began.self, on: self) { event in
      // Get both CustomBox entities, if either entityA or entityB isn't a CustomBox
      // then return becasue this is not the collision we're looking for
      guard let boxA = event.entityA as? CustomBox, let boxB = event.entityB as? CustomBox else {
        return
      }

      // Change the material color on the entity
      boxA?.model?.materials = [SimpleMaterial(color: .red, isMetallic: false)]
      boxB?.model?.materials = [SimpleMaterial(color: .red, isMetallic: false)]
    })
    collisionSubs.append(scene.subscribe(to: CollisionEvents.Ended.self, on: self) { event in
      guard let boxA = event.entityA as? CustomBox, let boxB = event.entityB as? CustomBox else {
        return
      }

      boxA?.model?.materials = [SimpleMaterial(color: .green, isMetallic: false)]
      boxB?.model?.materials = [SimpleMaterial(color: .green, isMetallic: false)]
    })
  }
}