Drawing in the air and on surfaces with a spatial stylus

At a glance

Project structure

Source bundle/
└── ARKit-SurfaceInking/
    └── ARKit-SurfaceInking/
        ├── SurfaceInkingApp.swift
        ├── Drawing/
        │   ├── DrawingInputProvider.swift
        │   ├── Style/
        │   │   └── BrushStyleProvider.swift
        │   ├── Mesh/
        │   │   └── BrushComponent.swift
        │   ├── DrawingDocument.swift
        │   └── DrawingSource.swift
        ├── Models/
        │   ├── AppModel.swift
        │   ├── StylusModel.swift
        │   └── BrushState.swift
        └── Views/
            ├── DeveloperSettingsView.swift
            ├── StylusDrawingSettingsView.swift
            └── BrushTypeView.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, Swift.
• The verified tree contains 4 project/configuration file(s) and 49 source declaration(s).

Overall architecture

flowchart LR
    N1["SurfaceInkingApp"]
    N2["StylusDrawingSettingsView"]
    N3["AppModel"]
    N4["BrushStyleProvider"]
    N5["RealityKit / SwiftUI APIs"]
    N1 --> N2
    N2 --> N3
    N3 --> N4
    N4 --> N5

Reference code

ARKit-SurfaceInking/ARKit-SurfaceInking/SurfaceInkingApp.swift:11 — architecture anchor

@main
struct SurfaceInkingApp: App {
    // ...
    private let appModel = AppModel()
    // ...
}

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 visionOS.

Ownership and state

classDiagram
    StylusAnchorInput *-- SIMD3 : position
    StylusAnchorInput o-- TimeInterval : timestamp
    StylusInputState o-- StylusAnchorInput : initialAnchorInput
    StylusInputState *-- Int : initialChangedButtonInputIndex

Ownership evidence

ARKit-SurfaceInking/ARKit-SurfaceInking/Drawing/DrawingInputProvider.swift:25 — stored dependency or nearest verified ownership anchor

struct StylusAnchorInput {
    let position: SIMD3<Float>
    let timestamp: TimeInterval
}

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

ARKit-SurfaceInking/ARKit-SurfaceInking/Drawing/DrawingDocument.swift:13 — representative execution boundary

@MainActor
class DrawingDocument {
    // ...
    let rootEntity: Entity
    // ...
}

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

ARKit-SurfaceInking/ARKit-SurfaceInking/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

ARKit-SurfaceInking/ARKit-SurfaceInking/Drawing/CurveProcessing/SmoothCurveSampler.swift:31 — representative boundary

public struct SmoothCurveSampler {
    // ...
    private(set) var curve: DrawingMeshGenerator
    // ...
}

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

Naming conventions

• Types: App: SurfaceInkingApp; Component: BrushComponent; Document: DrawingDocument; Generator: DrawingMeshGenerator; Model: AppModel, StylusModel; Provider: BrushStyleProvider, DrawingInputProvider; System: BrushSystem; View: BrushStyleView, BrushTypeView, DeveloperDrawingMediumSettingsView, DeveloperSettingsView, ImmersiveDrawingView.
• Protocols: HermiteInterpolant.
• Methods: isButtonPressed, getButtonPressure, getDrawingMedium, getChangedButtonInputIndex, updateButtonInputs, pruneButtonInputs, getCurrentDrawingMedium, getAnchorInput.
• Files: ARKit-SurfaceInking/ARKit-SurfaceInking/SurfaceInkingApp.swift, ARKit-SurfaceInking/ARKit-SurfaceInking/Drawing/DrawingInputProvider.swift, ARKit-SurfaceInking/ARKit-SurfaceInking/Models/AppModel.swift, ARKit-SurfaceInking/ARKit-SurfaceInking/Models/StylusModel.swift, ARKit-SurfaceInking/ARKit-SurfaceInking/Drawing/Style/BrushStyleProvider.swift, ARKit-SurfaceInking/ARKit-SurfaceInking/Models/BrushState.swift.

Architecture takeaways

• SurfaceInkingApp is the main source-visible entry or composition anchor for this sample.
• Framework work reaches RealityKit, SwiftUI, simd, ARKit 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