Implement scene understanding and visualize meshes with `RealityView`

ContentView.swift

import RealityKit
import SwiftUI

@MainActor
final class MySystem: System {
    // After turning on scene-understanding options, RealityKit automatically generates entities representing real-world geometry with a SceneUnderstandingComponent.
    // https://developer.apple.com/documentation/realitykit/realitykit-scene-understanding#Use-scene-reconstruction-in-iOS-and-macOS
    private static let query = EntityQuery(where: .has(SceneUnderstandingComponent.self) && .has(ModelComponent.self))
    private static let debugMaterial: UnlitMaterial = {
        var material = UnlitMaterial(color: .green)
        material.triangleFillMode = .lines
        return material
    }()

    init(scene: RealityKit.Scene) {}

    func update(context: SceneUpdateContext) {
        for entity in context.entities(matching: Self.query, updatingSystemWhen: .rendering) {
            entity.components[ModelComponent.self]?.materials = [Self.debugMaterial]
        }
    }
}

struct ContentView: View {
    var body: some View {
        RealityView { content in
            content.camera = .spatialTracking

            MySystem.registerSystem()
            let session = SpatialTrackingSession()
            let config = SpatialTrackingSession.Configuration(
                tracking: [],
                sceneUnderstanding: [
                    .occlusion,
                    //.physics,
                    //.collision,
                    .shadow,
                ])
            await session.run(config)
        }
    }
}