Scene is modified when rendering callback

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)

            ...

        }
    }