Dispatchqueue background thread update not working on iOS 12 - swift4

Question

I have the following piece of code that works perfectly on 11.4.1 but fails on 12
let background = DispatchQueue(label:"task")
var debugMeshNode = SCNNode()
let myKit = MyKit()
func renderer(_ renderer: SCNSceneRenderer, didAdd node: SCNNode, for anchor: ARAnchor) {
self.background.async {
let node = self.myKit.extractNode(anchor:anchor)
self.debugMeshNode.addChildNode(node) // no node added on UI in iOS12
}
}
func session(_ session: ARSession, didUpdate frame: ARFrame) {
self.background.async {
self.myKit.process(frame: frame)
}
}
Could anyone point my mistake here?
UPDATE
The code seems to work if I add a print statement in the block like so,
func renderer(_ renderer: SCNSceneRenderer, didAdd node: SCNNode, for anchor: ARAnchor) {
self.background.async {
let node = self.myKit.extractNode(anchor:anchor)
self.debugMeshNode.addChildNode(node) // no node added on UI in iOS12
print("sample")
}
}

Answer 1

Originally from here, I used this
func guaranteeMainThreadSynchronousExecution(_ block: () -> ()) {
if Thread.isMainThread {
block()
} else {
DispatchQueue.main.sync {
block()
}
}
}
and updated my code like so,
func renderer(_ renderer: SCNSceneRenderer, didAdd node: SCNNode, for anchor: ARAnchor) {
self.guaranteeMainThreadSynchronousExecution {
self.background.async {
let node = self.myKit.extractNode(anchor:anchor)
self.debugMeshNode.addChildNode(node) // no node added on UI in iOS12
}
}
}
Then it works flawlessly. Hope this helps someone.