I'm not able to get my changes to material.custom.value to change the behavior of the shader. I'm able to set an initial value, and the shader uses that, but any changes are ignored. I reassigned the CustomMaterial to the ModelComponent as you suggested, and even reassigned the ModelComponent to the ModelEntity, but no effect. The callback changing the value is definitely getting called and setting the value. Help! 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 entity: ModelEntity? = nil   var model: ModelComponent? = nil   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 )       self.model?.materials = [self.material!]       self.entity?.model = self.model! //      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!])     testData.model = ModelComponent(mesh: box, materials: [testData.material!])     testData.entity = ModelEntity()     testData.entity!.model = testData.model!     // Create an anchor in front of us and add the box to it     let originAnchor = AnchorEntity(world: simd_float3( 0, -1, -1 ))     originAnchor.addChild(testData.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: #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)); }