Drawing with PencilKit

At a glance

Project structure

Source bundle/
├── PencilKitDraw/
│   ├── AppDelegate.swift
│   ├── DataModel.swift
│   ├── DrawingViewController.swift
│   ├── PencilKitDrawNavigationController.swift
│   ├── SignatureViewController.swift
│   ├── ThumbnailCollectionViewController.swift
│   ├── ThumbnailCollectionViewCell.swift
│   └── Base.lproj/
│       ├── LaunchScreen.storyboard
│       └── Main.storyboard
├── Configuration/
│   └── SampleCode.xcconfig
└── PencilKitDraw.xcodeproj/
    ├── .xcodesamplecode.plist
    └── project.pbxproj

Structure observations

• Architecturally prominent files are ranked from entry points and role-named declarations; resource-only paths are omitted.
• Primary languages: Swift.
• The verified tree contains 6 project/configuration file(s) and 9 source declaration(s).

Overall architecture

flowchart LR
    N1["AppDelegate"]
    N2["DataModelController"]
    N3["DataModel"]
    N4["DataModelControllerObserver"]
    N5["PencilKit APIs"]
    N1 --> N2
    N2 --> N3
    N3 --> N4
    N4 --> N5

Reference code

PencilKitDraw/AppDelegate.swift:17 — architecture anchor

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
    var window: UIWindow?
}

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 PencilKit.

Ownership and state

classDiagram
    AppDelegate o-- UIWindow : window
    DataModel *-- Array : defaultDrawingNames
    DataModel *-- CGFloat : canvasWidth
    DataModel *-- Array : drawings

Ownership evidence

PencilKitDraw/AppDelegate.swift:19 — stored dependency or nearest verified ownership anchor

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
    var window: UIWindow?
}

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

PencilKitDraw/DataModel.swift:60 — representative execution boundary

class DataModelController {
    // ...
    private let thumbnailQueue = DispatchQueue(label: "ThumbnailQueue", qos: .background)
    // ...
}

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

PencilKitDraw/DataModel.swift:37 — representative type boundary

protocol DataModelControllerObserver {
    /// Invoked when the data model changes.
    func dataModelChanged()
}

The source explicitly defines local protocol relationships: ThumbnailCollectionViewController → DataModelControllerObserver.

Access control

Reference code

PencilKitDraw/DataModel.swift:60 — representative boundary

class DataModelController {
    // ...
    private let thumbnailQueue = DispatchQueue(label: "ThumbnailQueue", qos: .background)
    // ...
}

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: Controller: DataModelController, DrawingViewController, PencilKitDrawNavigationController, SignatureViewController, ThumbnailCollectionViewController; Delegate: AppDelegate; Model: DataModel; Observer: DataModelControllerObserver.
• Protocols: DataModelControllerObserver.
• Methods: dataModelChanged, updateDrawing, generateAllThumbnails, generateThumbnail, updateThumbnail, didChange, saveDataModel, loadDataModel.
• Files: PencilKitDraw/AppDelegate.swift, PencilKitDraw/DataModel.swift, PencilKitDraw/DrawingViewController.swift, PencilKitDraw/PencilKitDrawNavigationController.swift, PencilKitDraw/SignatureViewController.swift, PencilKitDraw/ThumbnailCollectionViewController.swift.

Architecture takeaways

• AppDelegate is the main source-visible entry or composition anchor for this sample.
• Framework work reaches UIKit, PencilKit 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