Is there a way to programmatically change the material of an Entity that was created in Reality Composer? - swift

Question

I want to change the color of an entity programmatically after it was created in Reality Composer.
As Reality Composer does not create a ModelEntity (it creates a generic Entity), it does not appear that I have access to change its color. When I typecast to a ModelEntity, I now have access to the ModelComponent materials. However, when I try to add that to the scene I get a Thread 1: signal SIGABART error. Could not cast value of type 'RealityKit.Entity' (0x1fcebe6e8) to 'RealityKit.ModelEntity' (0x1fceba970). Sample code below.
import UIKit
import RealityKit
class ViewController: UIViewController {
#IBOutlet var arView: ARView!
override func viewDidLoad() {
super.viewDidLoad()
// Load the "Box" scene from the "Experience" Reality File
let boxAnchor = try! Experience.loadBox()
// Typecast Steelbox as ModelEntity to change its color
let boxModelEntity = boxAnchor.steelBox as! ModelEntity
// Remove materials and create new material
boxModelEntity.model?.materials.removeAll()
let blueMaterial = SimpleMaterial(color: .blue, isMetallic: false)
boxModelEntity.model?.materials.append(blueMaterial)
// Add the box anchor to the scene
arView.scene.anchors.append(boxAnchor)
}
}

Answer 1

Model entity is stored deeper in RealityKit's hierarchy, and as you said, it's Entity, not ModelEntity. So use downcasting to access mesh and materials:
import UIKit
import RealityKit
class ViewController: UIViewController {
#IBOutlet var arView: ARView!
override func viewDidLoad() {
super.viewDidLoad()
let boxScene = try! Experience.loadBox()
print(boxScene)
let modelEntity = boxScene.steelBox?.children[0] as! ModelEntity
let material = SimpleMaterial(color: .green, isMetallic: false)
modelEntity.model?.materials = [material]
let anchor = AnchorEntity()
anchor.scale = [5,5,5]
modelEntity.setParent(anchor)
arView.scene.anchors.append(anchor)
}
}