Supporting desktop-class features in your iPad app

At a glance

Project structure

Source bundle/
└── DesktopClassMarkdownEditor/
    └── DesktopClassMarkdownEditor/
        ├── AppDelegate.swift
        ├── Views/
        │   ├── OutlineViewController.swift
        │   ├── EditorViewController.swift
        │   ├── SplitView.swift
        │   ├── SplitViewController.swift
        │   ├── DocumentBrowserViewController.swift
        │   ├── EditorView.swift
        │   └── PreviewView.swift
        ├── Document Management/
        │   └── MarkDownDocument.swift
        ├── SceneDelegate.swift
        └── Markdown/
            ├── MarkdownTag.swift
            └── Outline.swift

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 25 source declaration(s).

Overall architecture

flowchart LR
    N1["AppDelegate"]
    N2["DocumentBrowserViewController"]
    N3["MarkdownDocument"]
    N4["UIKit APIs"]
    N1 --> N2
    N2 --> N3
    N3 --> N4

Reference code

DesktopClassMarkdownEditor/DesktopClassMarkdownEditor/AppDelegate.swift:10 — architecture anchor

@main
class AppDelegate: UIResponder, UIApplicationDelegate {
    // ...
    func application(_ application: UIApplication,
                     didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        return true
    }
    // ...
}

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 UIKit.

Ownership and state

classDiagram
    OutlineViewController o-- OutlineViewControllerDelegate : delegate
    OutlineViewController *-- Array : outlineElements
    OutlineViewController o-- UICollectionViewDiffableDataSource : dataSource
    EditorViewController *-- EditorView : editorTextView

Ownership evidence

DesktopClassMarkdownEditor/DesktopClassMarkdownEditor/Views/OutlineViewController.swift:19 — stored dependency or nearest verified ownership anchor

class OutlineViewController: UIViewController, UICollectionViewDelegate {
    // ...
    weak var delegate: OutlineViewControllerDelegate?
    // ...
}

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

DesktopClassMarkdownEditor/DesktopClassMarkdownEditor/Views/EditorViewController.swift:336 — representative execution boundary

                DispatchQueue.main.async {
                    self.previewView.webView.loadHTMLString(parsedDocument.html, baseURL: Bundle.main.resourceURL)
                    self.delegate?.editor(self, didParse: parsedDocument)
                    self.syncScrollPositionIfNecessary()
                }

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

DesktopClassMarkdownEditor/DesktopClassMarkdownEditor/Views/OutlineViewController.swift:10 — representative type boundary

protocol OutlineViewControllerDelegate: AnyObject {
    func outline(_ outlineView: OutlineViewController, didChoose element: OutlineElement)
    func outline(_ outlineView: OutlineViewController, didSwapTagsFor elements: [OutlineElement], withTag: MarkdownTag)
    func outline(_ outlineView: OutlineViewController, didDuplicate elements: [OutlineElement])
    func outline(_ outlineView: OutlineViewController, didDelete elements: [OutlineElement])
}

The source explicitly defines local protocol relationships: SplitViewController → EditorViewControllerDelegate, SplitViewController → OutlineViewControllerDelegate.

Access control

Reference code

DesktopClassMarkdownEditor/DesktopClassMarkdownEditor/Document Management/MarkDownDocument.swift:113 — representative boundary

    private func updateTextContent() {
        text = parsedDocument!.textContent()
        // ...
    }

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: DocumentBrowserViewController, EditorViewController, OutlineViewController, SplitViewController; Delegate: AppDelegate, EditorViewControllerDelegate, OutlineViewControllerDelegate, SceneDelegate; Document: MarkdownDocument, ParsedDocument; View: EditorView, PreviewView, SplitView.
• Protocols: OutlineViewControllerDelegate, EditorViewControllerDelegate.
• Methods: application, outline, loadView, viewDidLoad, configureDataSource, updateSnapshot, collectionView, tagSpecificActions.
• Files: DesktopClassMarkdownEditor/DesktopClassMarkdownEditor/AppDelegate.swift, DesktopClassMarkdownEditor/DesktopClassMarkdownEditor/Views/OutlineViewController.swift, DesktopClassMarkdownEditor/DesktopClassMarkdownEditor/Views/EditorViewController.swift, DesktopClassMarkdownEditor/DesktopClassMarkdownEditor/Views/SplitView.swift, DesktopClassMarkdownEditor/DesktopClassMarkdownEditor/Views/SplitViewController.swift, DesktopClassMarkdownEditor/DesktopClassMarkdownEditor/SceneDelegate.swift.

Architecture takeaways

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