title: Reasons for CollisionEvent not firing in RealityKit scene?
body: <p>Event subscription has been added to a <code>scene</code>of an <code>ARView</code> in <code>RealityKit</code>:</p>
  <pre><code>arView.scene.subscribe(to: CollisionEvents.Began.self) { event in
      print(&quot;Some collision occured&quot;)
  }
  </code></pre>
  <p><code>Entities</code> are visually colliding in the <code>scene</code> but the <code>CollisionEvent</code> is not being fired. Here are 2 example hierarchies of objects which are clearly colliding:</p>
  <p>▿ 'backboard' : __RKPhysics, children: 1<br />
  ...⟐ PhysicsMotionComponent<br />
  ...⟐ Transform<br />
  ...⟐ CollisionComponent<br />
  ...⟐ SynchronizationComponent<br />
  ...⟐ PhysicsBodyComponent<br />
  ...▿ 'simpBld_root' : ModelEntity<br />
  ......⟐ Transform<br />
  ......⟐ ModelComponent<br />
  ......⟐ SynchronizationComponent</p>
  <p>With <code>CollisionComponent</code>:</p>
  <blockquote>
  <p>CollisionComponent(shapes: [RealityKit.ShapeResource], mode: RealityKit.CollisionComponent.Mode.default, filter: RealityKit.CollisionFilter(group: RealityKit.CollisionGroup(rawValue: 1), mask: RealityKit.CollisionGroup(rawValue: 1)))</p>
  </blockquote>
  <p>▿ 'ball' : ModelEntity
  ...⟐ Transform<br />
  ...⟐ CollisionComponent<br />
  ...⟐ ModelComponent<br />
  ...⟐ SynchronizationComponent<br />
  ...⟐ PhysicsBodyComponent</p>
  <p>With <code>CollisionComponent</code>:</p>
  <blockquote>
  <p>CollisionComponent(shapes: [RealityKit.ShapeResource], mode: RealityKit.CollisionComponent.Mode.default, filter: RealityKit.CollisionFilter(group: RealityKit.CollisionGroup(rawValue: 1), mask: RealityKit.CollisionGroup(rawValue: 4294967295)))</p>
  </blockquote>
  <p>Named <code>Entities</code>have been added to the seen as part of an <code>AnchorEntity</code>:</p>
  <pre><code>let anchorEntity = AnchorEntity(anchor: anchor)
  anchorEntity.addChild(backboardEntity)
  anchorEntity.addChild(ballEntity)
  arView.scene.anchors.append(anchorEntity)
  </code></pre>
  <p>Why are no <code>CollisionEvents</code> not fired?</p>