Scene is modified when rendering callback - ios

Question

I had the following code which produced the error:
func renderer(_ renderer: SCNSceneRenderer, didAdd node: SCNNode, for anchor: ARAnchor) {
if anchor is ARImageAnchor {
let phoneScene = SCNScene(named: "Phone_01.scn")!
let phoneNode = phoneScene.rootNode.childNode(withName: "parentNode", recursively: true)!
// rotate the phone node
let rotationAction = SCNAction.rotateBy(x: 0, y: 0.5, z: 0, duration: 1)
let inifiniteAction = SCNAction.repeatForever(rotationAction)
phoneNode.runAction(inifiniteAction)
phoneNode.position = SCNVector3(anchor.transform.columns.3.x,anchor.transform.columns.3.y + 0.1,anchor.transform.columns.3.z)
node.addChildNode(phoneNode)
}
}
Scene is modified in a rendering callback of another scene.
So I replaced it with the following:
func renderer(_ renderer: SCNSceneRenderer, didAdd node: SCNNode, for anchor: ARAnchor) {
if anchor is ARImageAnchor {
DispatchQueue.global().async {
let phoneScene = SCNScene(named: "Phone_01.scn")!
let phoneNode = phoneScene.rootNode.childNode(withName: "parentNode", recursively: true)!
DispatchQueue.main.async {
// rotate the phone node
let rotationAction = SCNAction.rotateBy(x: 0, y: 0.5, z: 0, duration: 1)
let inifiniteAction = SCNAction.repeatForever(rotationAction)
phoneNode.runAction(inifiniteAction)
phoneNode.position = SCNVector3(anchor.transform.columns.3.x,anchor.transform.columns.3.y + 0.1,anchor.transform.columns.3.z)
node.addChildNode(phoneNode)
}
}
}
}
And now the error is gone and everything works OK. My question is: is that the correct solution? Should I switch to background thread to load the scene and then to main thread to add the nodes. Are nodes even added on the main thread?

Answer 1

Try something like this in the method delegate. This was an example of an old proyect.
DispatchQueue.main.async {
if let imageAnchor = anchor as? ARImageAnchor {
let plane = SCNPlane(width: imageAnchor.referenceImage.physicalSize.width, height: imageAnchor.referenceImage.physicalSize.height)
plane.firstMaterial?.diffuse.contents = UIColor(white: 1.0, alpha: 0.5)
let planeNode = SCNNode(geometry: plane)
planeNode.eulerAngles.x = -.pi
node.addChildNode(planeNode)
...
}
}