Creating a spatial drawing app with RealityKit

At a glance

Project structure

Source bundle/
└── RealityKitDrawingApp/
    ├── RealityKitDrawingApp.swift
    ├── Brushes/
    │   ├── Solid/
    │   │   └── Style/
    │   │       └── SolidBrushStyleProvider.swift
    │   └── Sparkle/
    │       └── SparkleBrushVertex.h
    ├── Canvas/
    │   ├── DrawingCanvasPlacementView.swift
    │   └── DrawingCanvasVisualizationView.swift
    ├── Document/
    │   ├── AnchorEntityInputProvider.swift
    │   └── DrawingDocument.swift
    ├── Splash Screen/
    │   ├── SplashScreenBackgroundComponent.swift
    │   └── SplashScreenForeground.swift
    └── UI/
        ├── Palette/
        │   ├── BrushTypeView.swift
        │   └── Presets/
        │       └── PresetBrushView.swift
        └── BrushState.swift

Structure observations

• Architecturally prominent files are ranked from entry points and role-named declarations; resource-only paths are omitted.
• Primary languages: C/Objective-C header, Metal, Swift.
• The verified tree contains 4 project/configuration file(s) and 68 source declaration(s).

Overall architecture

flowchart LR
    N1["RealityKitDrawingApp"]
    N2["SplashScreenView"]
    N3["DrawingDocument"]
    N4["AnchorEntityInputProvider"]
    N5["RealityKit / RealityKitContent APIs"]
    N1 --> N2
    N2 --> N3
    N3 --> N4
    N4 --> N5

Reference code

RealityKitDrawingApp/RealityKitDrawingApp.swift:11 — architecture anchor

@main
struct RealityKitDrawingApp: App {
    // ...
    private static let configureCanvasWindowId: String = "ConfigureCanvas"
    private static let splashScreenWindowId: String = "SplashScreen"
    private static let immersiveSpaceWindowId: String = "ImmersiveSpace"

    // ...
}

Interpretation

The arrows summarize the source-visible entry, role-named types or folders, and framework direction; when nodes come from structural folders, the sequence is a high-level interpretation rather than proof that every adjacent node calls the next. Ownership is claimed only where the next section cites a stored property or assignment. The diagram is intentionally limited to the dominant path into RealityKit.

Ownership and state

classDiagram
    Settings *-- Float : thickness
    Settings o-- ThicknessType : thicknessType
    Settings *-- SIMD3 : color
    Settings *-- Float : metallic

Ownership evidence

RealityKitDrawingApp/Brushes/Solid/Style/SolidBrushStyleProvider.swift:29 — stored dependency or nearest verified ownership anchor

    struct Settings: Equatable, Hashable {
        var thickness: Float = 0.005
        // ...
    }

Composition arrows indicate a source-visible construction expression or locally owned value state; aggregation means the owner stores or receives a dependency without proving exclusive lifetime ownership.

Concurrency, scheduling, and thread safety

Evidence limit: actor isolation, async/await, or Task creation does not by itself prove background-thread execution; Sendable conformance alone does not prove thread-safe mutation.

@MainActor/MainActor.run, DispatchQueue.main, and RunLoop.main are reported as distinct isolation, queue, and event-loop mechanisms. A plain Task is kept separate from Task.detached; neither is labeled as a background thread.

Reference code

RealityKitDrawingApp/Brushes/Solid/CurveExtruder.swift:36 — representative execution boundary

struct CurveExtruder {
    // ...
    @MainActor
    private var sampleCapacity: Int {
        let vertexCapacity = lowLevelMesh?.vertexCapacity ?? 0
        let indexCapacity = lowLevelMesh?.indexCapacity ?? 0

        // Each sample adds `shape.count` vertices.
        let sampleVertexCapacity = vertexCapacity / shape.count

        // Each segment between two samples adds `topology.count` indices.
        let sampleIndexCapacity = indexCapacity / topology.count + 1

        return min(sampleVertexCapacity, sampleIndexCapacity)
    }
    // ...
}

State propagation, frameworks, and dependencies

Evidence limit: an import proves a source-level compilation dependency at the cited line; it does not prove runtime use, architectural adoption, or whether a Swift package is a direct application dependency.

receive(on:) describes downstream delivery scheduling, while subscribe(on:) describes upstream subscription/request/cancel scheduling. An import Combine alone establishes neither behavior nor a Store, reducer, Redux, or other application architecture.

Class and protocol design

RealityKitDrawingApp/Utilities/CurveProcessing.swift:12 — representative type boundary

protocol HermiteInterpolant {
    static func distance(_: Self, _: Self) -> Float
    // ...
}

The source explicitly defines local protocol relationships: Float → HermiteInterpolant, SIMD2 → HermiteInterpolant, SIMD3 → HermiteInterpolant, SIMD4 → HermiteInterpolant.

Access control

Reference code

RealityKitDrawingApp/Brushes/Solid/CurveExtruder.swift:16 — representative boundary

struct CurveExtruder {
    private var lowLevelMesh: LowLevelMesh?
    // ...
}

Swift declarations without a modifier are internal; explicit private, fileprivate, private(set), public, or open entries above are interpreted by language semantics. Objective-C/C samples instead rely on header and implementation boundaries, which are not equivalent to Swift lexical privacy.

Logic ownership and placement

Design patterns

Main application flow

sequenceDiagram
    participant DrawingDocument
    participant ShaderGraphMaterial
    participant DrawingSource
    DrawingDocument->>ShaderGraphMaterial: ShaderGraphMaterial()
    DrawingDocument->>ShaderGraphMaterial: ShaderGraphMaterial()
    DrawingDocument->>DrawingSource: DrawingSource()
    DrawingDocument->>DrawingSource: DrawingSource()

Reference code

RealityKitDrawingApp/Document/DrawingDocument.swift:85 — init()

    @MainActor
    init(rootEntity: Entity, brushState: BrushState, canvas: DrawingCanvasSettings) async {
        self.rootEntity = rootEntity
        self.brushState = brushState
        self.startDate = .now
        self.canvas = canvas

        let leftRootEntity = Entity()
        let rightRootEntity = Entity()
        rootEntity.addChild(leftRootEntity)
        rootEntity.addChild(rightRootEntity)

        var solidMaterial: RealityKit.Material = SimpleMaterial()
        if let material = try? await ShaderGraphMaterial(named: "/Root/Material",
                                                         from: "SolidBrushMaterial",
                                                         in: realityKitContentBundle) {
            solidMaterial = material
        }

        var sparkleMaterial: RealityKit.Material = SimpleMaterial()
        if var material = try? await ShaderGraphMaterial(named: "/Root/SparkleBrushMaterial",
                                                         from: "SparkleBrushMaterial",
                                                         in: realityKitContentBundle) {
            try? material.setParameter(name: "ParticleUVScale", value: .float(8))
            material.writesDepth = false
            sparkleMaterial = material
        }

        leftSource = await DrawingSource(rootEntity: leftRootEntity,
                                         solidMaterial: solidMaterial,
                                         sparkleMaterial: sparkleMaterial)
        rightSource = await DrawingSource(rootEntity: rightRootEntity,
                                          solidMaterial: solidMaterial,
                                          sparkleMaterial: sparkleMaterial)
    }

Naming conventions

• Types: App: RealityKitDrawingApp; Component: DrawingCanvasPlacementComponent, DrawingCanvasVisualizationComponent, HandComponent, SolidBrushComponent, SparkleBrushComponent; Document: DrawingDocument; Generator: SolidDrawingMeshGenerator, SparkleDrawingMeshGenerator; Provider: AnchorEntityInputProvider, SolidBrushStyleProvider, SparkleBrushStyleProvider; System: DrawingCanvasPlacementSystem, DrawingCanvasVisualizationSystem, DrawingSystem, SolidBrushSystem, SparkleBrushSystem; View: BrushTypeView, DrawingCanvasConfigurationView, DrawingCanvasPlacementView, DrawingCanvasVisualizationView, DrawingMeshView.
• Protocols: HermiteInterpolant.
• Methods: setMode, radius, styleInput, update, generateMesh, receive, generateTexture, setTextureSize.
• Files: RealityKitDrawingApp/RealityKitDrawingApp.swift, RealityKitDrawingApp/Brushes/Solid/Style/SolidBrushStyleProvider.swift, RealityKitDrawingApp/Brushes/Sparkle/SparkleBrushVertex.h, RealityKitDrawingApp/Canvas/DrawingCanvasPlacementView.swift, RealityKitDrawingApp/Canvas/DrawingCanvasVisualizationView.swift, RealityKitDrawingApp/Document/AnchorEntityInputProvider.swift.

Architecture takeaways

• RealityKitDrawingApp is the main source-visible entry or composition anchor for this sample.
• Framework work reaches RealityKit, SwiftUI, Collections, RealityKitContent through a deliberately small high-level chain; the detailed API graph remains inside the cited implementation files.
• Stored-property evidence identifies lifecycle collaboration; it does not by itself prove exclusive object ownership.
• Access-control conclusions separate verified language visibility from the likely design rationale.
• Local protocol relationships provide an explicit substitution boundary.

Source map

• Official Apple documentation