I'm trying to pass a changing value to my CustomShader in a RealityKit scene, but I'm not able to get the graphics to reflect the change. I'm able to set an initial value of the material.custom.value, and the graphics do use that, but I'm not able to change it once the scene is displayed. The callback that does the per-frame changing is definitely getting called. Any ideas?

My Swift code:

import SwiftUI
import RealityKit
import Combine

// A class with a per-frame callback to change a custom uniform for the geometry shader
class TestData {
    var material: CustomMaterial? = nil
    var theta = Float(0)
    
    func SetupCallback( _ arView: ARView) {
        _ = arView.scene.subscribe(to: SceneEvents.Update.self) { _ in
            // *** This value does get changed, but it's not reflected in the geometry shader
            self.material!.custom.value = simd_float4( 0.5 * sin( self.theta ), 0, 0, 0 )
//            print("x offset = ", self.material!.custom.value[0])
            self.theta += 0.05
        }
    }
}

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

struct ARViewContainer: UIViewRepresentable {
    
    let testData = TestData()
    
    func makeUIView(context: Context) -> ARView {
        let arView = ARView(frame: .zero)

        // Create a simple box
        let box = MeshResource.generateBox(size: 0.3)
        
        // Create a custom materal with a Geometry shader -
        let mtlLibrary = MTLCreateSystemDefaultDevice()!.makeDefaultLibrary()!
        let geometryShader = CustomMaterial.GeometryModifier(
           named: "moveVertices", in: mtlLibrary
        )
        testData.material = try! CustomMaterial(
            from: SimpleMaterial(color: .orange, isMetallic: false),
            geometryModifier: geometryShader )
        // *** This initial value does make it to the Geometry shader
        testData.material!.custom.value = simd_float4(0.5, 0, 0, 0)
        
        // Create an entity with the box and custom material with geometry modifier
        let entity = ModelEntity(mesh: box, materials: [testData.material!])
        
        // Create an anchor in front of us and add the box to it
        let originAnchor = AnchorEntity(world: simd_float3( 0, -1, -1 ))
        originAnchor.addChild(entity)
        arView.scene.anchors.append(originAnchor)

        // Set up the per-frame callback subscription
        testData.SetupCallback(arView)

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

#if DEBUG
struct ContentView_Previews : PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}
#endif

and my GeometryModifier shader

#include <metal_stdlib>
#include <RealityKit/RealityKit.h>
using namespace metal;

[[visible]]
void moveVertices(realitykit::geometry_parameters params)
{
    // Interpret custom_parameter as a position offset
    float4 posOffset = float4( params.uniforms().custom_parameter());
    params.geometry().set_model_position_offset(float3(posOffset));
}