Creating a 3D painting space

At a glance

Project structure

Source bundle/
└── Painting/
    ├── App/
    │   └── EntryPoint.swift  # EntryPoint
    ├── Components/
    │   └── ClosureComponent.swift  # ClosureComponent, ClosureSystem
    ├── Views/
    │   ├── MainView.swift  # MainView
    │   └── PaintingView.swift  # PaintingView
    ├── Trackers/
    │   └── PaintingHandTracking.swift  # PaintingHandTracking
    ├── Canvas/
    │   ├── PaintingCanvas.swift  # PaintingCanvas
    │   └── Stroke.swift  # Stroke
    └── Extensions/
        └── Entity.swift

Structure observations

• The runtime boundary is WindowGroup + ImmersiveSpace; the pruned tree lists only files that explain lifecycle, state, or framework integration.
• App code is split into role-named views, models, managers, providers, components, or systems.

Overall architecture

flowchart LR
    EntryPoint_1["EntryPoint"]
    WindowGroup___ImmersiveSpace_2["WindowGroup + ImmersiveSpace"]
    MainView_3["MainView"]
    PaintingHandTracking_4["PaintingHandTracking"]
    RealityView_entity_graph_5["RealityView entity graph"]
    ARKit_providers___RealityKit_6["ARKit providers + RealityKit"]
    EntryPoint_1 --> WindowGroup___ImmersiveSpace_2
    WindowGroup___ImmersiveSpace_2 --> MainView_3
    MainView_3 --> PaintingHandTracking_4
    PaintingHandTracking_4 --> RealityView_entity_graph_5
    RealityView_entity_graph_5 --> ARKit_providers___RealityKit_6

Reference code

Painting/App/EntryPoint.swift:11 — the app or executable entry declares the outer scene lifecycle.

struct EntryPoint: App {
    // ...
}

The diagram is a responsibility flow, not a claim that every adjacent node directly calls the next. It keeps scene ownership, shared state, RealityKit content, and framework-provider work at separate levels.

Ownership and state

classDiagram
    PaintingHandTracking *-- ARKitSession : arSession
    PaintingHandTracking *-- HandTrackingProvider : handTracking
    PaintingCanvas *-- Entity : root
    Stroke *-- Entity : entity

Ownership evidence

Painting/Trackers/PaintingHandTracking.swift:15 — representative stored state or the nearest verified lifecycle anchor.

    let arSession = ARKitSession()
    
    /// The `HandTrackingProvider` for hand tracking.
    let handTracking = HandTrackingProvider()
    
    /// The current left hand anchor the app detects.
    @Published var latestLeftHand: HandAnchor?

Ownership here is deliberately narrow: @Environment and weak references are shared links, initialized @State or stored services are lifecycle ownership, and a RealityView content closure owns additions to its entity graph without making the SwiftUI view a reference-type owner.

Class and protocol design

The source defines no local substitution protocol in the reviewed boundary. Its protocol use is framework-facing (App, View, RealityKit/ARKit protocols, or platform adapters), so this document does not label the whole app protocol-oriented.

Access control

No reviewed declaration uses fileprivate, private(set), public, open; unmodified Swift declarations are internal.

Reference code

Painting/Canvas/PaintingCanvas.swift:36 — representative visibility boundary.

    private func addBox(size: SIMD3<Float>, position: SIMD3<Float>) -> Entity {
        // ...
    }

Logic ownership and placement

Design patterns

Naming conventions

• Role suffixes are evidence, not decoration: View: MainView, PaintingView.
• ECS names pair data and behavior: components ClosureComponent; systems ClosureSystem.
• Protocols: no app-defined protocol in the reviewed source.
• Commands use verb-led methods: update, startTracking.
• Files generally match their primary type; Views, Models, Managers, Providers, Components, Systems, and Packages folders describe architectural roles where present.

Architecture takeaways

• Treat EntryPoint as the owner of scene declarations, not as the owner of every RealityKit entity created later.
• Keep view-local interaction in SwiftUI, but move provider sessions, playback resources, shared game state, or transport state into a stable owner when their lifetime exceeds one render pass.
• Run ARKit providers for the scene lifetime and consume their asynchronous updates in cancellable tasks; map anchors to entities at the boundary.
• Use RealityKit components for per-entity data and systems for repeated simulation instead of a monolithic view model.
• Model immersive-space open, transition, and close states explicitly so windows and immersive content cannot drift apart.

Source map

• Official Apple documentation