_getSceneDef() {
    const scene = super._getSceneDef()
    const {xrSession, xrReferenceSpace} = this
    if (xrSession && xrReferenceSpace) {
      scene.objects = emptyArray.concat(
        scene.objects,
        {
          key: 'xrInputMgr',
          facade: XRInputSourceManager,
          xrSession,
          xrReferenceSpace
        }
      )
    }
    return scene
  }
@override to add VR controllers manager object