I'm porting over some code that uses ARKit to Swift 6 (with Complete Strict Concurrency Checking enabled).
Some methods on ARSCNViewDelegate, namely Coordinator.renderer(_:didAdd:for:) among at least one other is causing a consistent crash. On Swift 5 this code works absolutely fine.
The above method consistently crashes with _dispatch_assert_queue_fail. My assumption is that in Swift 6 a trap has been inserted by the compiler to validate that my downstream code is running on the main thread.
In Implementing a Main Actor Protocol That’s Not @MainActor, Quinn “The Eskimo!” seems to address scenarios of this nature with 3 proposed workarounds yet none of them seem feasible here.
For #1, marking ContentView.addPlane(renderer:node:anchor:) nonisolated and using @preconcurrency import ARKit compiles but still crashes :(
For #2, applying @preconcurrency to the ARSCNViewDelegate conformance declaration site just yields this warning: @preconcurrency attribute on conformance to 'ARSCNViewDelegate' has no effect
For #3, as Quinn recognizes, this is a non-starter as ARSCNViewDelegate is out of our control.
The minimal reproducible set of code is below. Simply run the app, scan your camera back and forth across a well lit environment and the app should crash within a few seconds. Switch over to Swift Language Version 5 in build settings, retry and you'll see the current code works fine.
import ARKit
import SwiftUI
struct ContentView: View {
@State private var arViewProxy = ARSceneProxy()
private let configuration: ARWorldTrackingConfiguration
@State private var planeFound = false
init() {
configuration = ARWorldTrackingConfiguration()
configuration.worldAlignment = .gravityAndHeading
configuration.planeDetection = [.horizontal]
}
var body: some View {
ARScene(proxy: arViewProxy)
.onAddNode { renderer, node, anchor in
addPlane(renderer: renderer, node: node, anchor: anchor)
}
.onAppear {
arViewProxy.session.run(configuration)
}
.onDisappear {
arViewProxy.session.pause()
}
.overlay(alignment: .top) {
if !planeFound {
Text("Slowly move device horizontally side to side to calibrate")
} else {
Text("Plane found!")
.bold()
.foregroundStyle(.green)
}
}
}
private func addPlane(renderer: SCNSceneRenderer, node: SCNNode, anchor: ARAnchor) {
guard let planeAnchor = anchor as? ARPlaneAnchor,
let device = renderer.device,
let planeGeometry = ARSCNPlaneGeometry(device: device)
else { return }
planeFound = true
planeGeometry.update(from: planeAnchor.geometry)
let material = SCNMaterial()
material.isDoubleSided = true
material.diffuse.contents = UIColor.white.withAlphaComponent(0.65)
planeGeometry.materials = [material]
let planeNode = SCNNode(geometry: planeGeometry)
node.addChildNode(planeNode)
}
}
struct ARScene {
private(set) var onAddNodeAction: ((SCNSceneRenderer, SCNNode, ARAnchor) -> Void)?
private let proxy: ARSceneProxy
init(proxy: ARSceneProxy) {
self.proxy = proxy
}
func onAddNode(
perform action: @escaping (SCNSceneRenderer, SCNNode, ARAnchor) -> Void
) -> Self {
var view = self
view.onAddNodeAction = action
return view
}
}
extension ARScene: UIViewRepresentable {
func makeUIView(context: Context) -> ARSCNView {
let arView = ARSCNView()
arView.delegate = context.coordinator
arView.session.delegate = context.coordinator
proxy.arView = arView
return arView
}
func updateUIView(_ uiView: ARSCNView, context: Context) {
context.coordinator.onAddNodeAction = onAddNodeAction
}
func makeCoordinator() -> Coordinator {
Coordinator()
}
}
extension ARScene {
class Coordinator: NSObject, ARSCNViewDelegate, ARSessionDelegate {
var onAddNodeAction: ((SCNSceneRenderer, SCNNode, ARAnchor) -> Void)?
func renderer(_ renderer: SCNSceneRenderer, didAdd node: SCNNode, for anchor: ARAnchor) {
onAddNodeAction?(renderer, node, anchor)
}
}
}
@MainActor
class ARSceneProxy: NSObject, @preconcurrency ARSessionProviding {
fileprivate var arView: ARSCNView!
@objc dynamic var session: ARSession {
arView.session
}
}
Any help is greatly appreciated!
