constructor(parent) {
    super(parent)
    this._sourcesDirty = true

    // Required props:
    this.xrSession = null
    this.xrReferenceSpace = null

    // Separate subtree for the XRInputSourceFacade instances:
    this._xrInputSourceSubtree = new ParentFacade(this)

    this._onInputSourcesChange = e => {
      this._sourcesDirty = true
      this.afterUpdate()
    }
  }
A container facade, placed at the root of the scene, that manages the tracking of
`XRInputSource`s and the rendering of their related scene objects.