Ios swift – Continuous-looping animations in RealityKit

I am writing an app that uses RealityKit. I want one of the entities in my AR scene to animate (rotate on its X axis) continuously.

I can get it to spin 180º once on load, and I can add a slider to my UI that controls its rotation, but I can’t get it to animate continuously.

If any of this matters: I am using the latest stable versions of everything as of June 2023, no betas. Ventura 13.4, Xcode 14.3.1, Swift 5.8.1. Targeting iOS 16.4.

Here is a standalone demo of the issue, which I am trying out via Xcode Preview.

import Combine
import RealityFoundation
import RealityKit
import SwiftUI

struct ARKitPOC: View {
    var body: some View {
        ARViewContainer().edgesIgnoringSafeArea(.all)
    }
}

struct ARViewContainer: UIViewRepresentable {
    @State private var animUpdateSubscription: Cancellable?

    func makeUIView (context: Context) -> ARView {
        let redMaterial = SimpleMaterial(color: .red, roughness: 0.05, isMetallic: false)

        let boxResource = MeshResource.generateBox(width: 1.0, height: 1.0, depth: 1.0, cornerRadius: 0.1, splitFaces: false)
        let boxEntity = ModelEntity(mesh: boxResource, materials: [redMaterial])
        boxEntity.name = "box"
        boxEntity.position.z = -2
        
        let anchorEntity = AnchorEntity(world: .zero)
        anchorEntity.name = "anchor"
        anchorEntity.addChild(boxEntity)

        let arView = ARView(frame: .zero)
        arView.scene.addAnchor(anchorEntity)
        arView.cameraMode = .nonAR
        
        rotateForever(scene: arView.scene,
                      entity: boxEntity,
                      angle: -.pi,
                      axis: [0,0,1],
                      duration: 1.0)

        return arView
    }
    
    func updateUIView(_ arView: ARView, context: Context) {
    }

    func rotateForever(scene: RealityFoundation.Scene,
                       entity: HasTransform,
                       angle: Float,
                       axis: SIMD3<Float>,
                       duration: TimeInterval) {
        if animUpdateSubscription == nil {
            animUpdateSubscription = scene.subscribe(to: AnimationEvents.PlaybackCompleted.self,
                                                     on: entity,
                                                     { _ in
                print("subscription triggered")
                self.rotateForever(scene: scene,
                                   entity: entity,
                                   angle: angle,
                                   axis: axis,
                                   duration: duration)
            })
            print("subscription:", animUpdateSubscription)
        }

        var transform = entity.transform
        transform.rotation *= simd_quatf(angle: angle, axis: axis)
        entity.move(to: transform, relativeTo: entity.parent, duration: duration, timingFunction: .linear)
    }
}

struct ARKitPOC_Previews: PreviewProvider {
    static var previews: some View {
        ARKitPOC()
    }
}

Here, I render a red cube into the scene and trigger a looping animation that rotates it 180º from its current orientation. Completion of that animation is supposed to result in an event, which should trigger another animation, such that the cube rotates indefinitely.

I’ve been searching, and have found this technique recommended in several places:
https://www.appsloveworld.com/swift/100/89/realitykit-animate-a-transform-on-a-loop
https://stackoverflow.com/a/66916901/535401
https://rozengain.medium.com/quick-realitykit-tutorial-2-looping-animations-gestures-ee518b06b7f6

However, the call to scene.subscribe(to:on:_:) is returning nil, and the event handler I provided never gets invoked, so the animation only ever happens once.

The use of scene.subsribe required that I import Combine, which leads me to believe that this chunk of code:

            animUpdateSubscription = scene.subscribe(to: AnimationEvents.PlaybackCompleted.self,
                                                     on: entity,
                                                     { _ in

is shorthand for this Combine-style subscription:

            animUpdateSubscription = scene.publisher(for: AnimationEvents.PlaybackCompleted.self, on: entity)
                                          .sink(receiveValue: { _ in

I get exactly the same behavior with that substitution.

Apple’s documentation says nothing about the possibility of a nil return value. Ditto for the proposed Combine equivalent. What am I missing here? (see update below)

Update

By changing to the publisher(...).sink(...) approach, then chaining a call to print("publisher(for:)") between those, I can see the following in my log output:

publisher(for:): receive subscription: (RealityFoundation.REEventDispatcher<__C.REAnimationHasCompletedEvent>.(unknown context at $10b680970).EventSubscription<RealityKit.Scene.CorePublisher<__C.REAnimationHasCompletedEvent>.(unknown context at $10b686788).Inner<Combine.Publishers.CompactMap<RealityKit.Scene.CorePublisher<__C.REAnimationHasCompletedEvent>, RealityKit.AnimationEvents.PlaybackCompleted>.(unknown context at $10a11e878).Inner<Combine.Publishers.Print<RealityKit.Scene.Publisher<RealityKit.AnimationEvents.PlaybackCompleted>>.(unknown context at $10a119ff8).Inner<Combine.Subscribers.Sink<RealityKit.AnimationEvents.PlaybackCompleted, Swift.Never>>>>>)
publisher(for:): request unlimited
publisher(for:): receive cancel

So, a valid subscription is getting created, but then it’s getting canceled almost immediately. How do I figure out what’s doing that? Is the Swift.Never or the unknown context near the end of the first line meaningful?