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
        ├── SceneDelegate.swift
        └── Markdown/
            ├── MarkdownTag.swift
            ├── Outline.swift
            └── Parser.swift

Structure observations

• Document lifecycle is isolated in MarkdownDocument and DocumentBrowserViewController.
• SplitViewController is the composition and coordination root for editor/outline presentation.
• Parser/model files are independent of UIKit presentation; the editor schedules parsing and preview updates.

Overall architecture

flowchart LR
    BROWSER["DocumentBrowserViewController"] --> DOC["MarkdownDocument"]
    BROWSER --> SPLIT["SplitViewController"]
    SPLIT --> EDITOR["EditorViewController"]
    SPLIT --> OUTLINE["OutlineViewController"]
    EDITOR --> PARSER["Parser and ParsedDocument"]
    PARSER --> PREVIEW["PreviewView and outline updates"]

Reference code

DesktopClassMarkdownEditor/DesktopClassMarkdownEditor/Views/DocumentBrowserViewController.swift:82 — document presentation composition

    func presentDocument(_ documentURL: URL) {
        let document = MarkdownDocument(fileURL: documentURL)
        let splitVC = SplitViewController(document: document)

        splitVC.transitioningDelegate = self
        splitVC.modalPresentationStyle = .fullScreen
        splitVC.editorViewController.documentBrowser = self
        present(splitVC, animated: true)
    }

Interpretation

The browser creates the document session and composition root. The split controller connects child controllers through narrow local delegate protocols; the editor owns parsing/preview mechanics, while the outline sends user commands back through the split controller to the document.

Ownership and state

classDiagram
    DocumentBrowserViewController *-- MarkdownDocument : creates per URL
    DocumentBrowserViewController *-- SplitViewController : presents
    SplitViewController *-- EditorViewController : let
    SplitViewController *-- OutlineViewController : let
    EditorViewController *-- Parser : let
    EditorViewController o-- EditorViewControllerDelegate : weak

Ownership evidence

DesktopClassMarkdownEditor/DesktopClassMarkdownEditor/Views/SplitViewController.swift:13 — split-controller object graph

    let document: MarkdownDocument
    let editorViewController: EditorViewController
    let outlineViewController: OutlineViewController

    init(document: MarkdownDocument) {
        self.document = document
        editorViewController = EditorViewController(document: document)
        outlineViewController = OutlineViewController()
    }

Composition denotes source-visible construction and retained value state. Aggregation denotes a stored or injected collaborator whose exclusive lifetime the source does not prove.

Class and protocol design

DesktopClassMarkdownEditor/DesktopClassMarkdownEditor/Views/EditorViewController.swift:11 — editor collaboration contract

protocol EditorViewControllerDelegate: AnyObject {
    func editor(_ editorViewController: EditorViewController, didParse document: ParsedDocument)
}

The local editor and outline delegate protocols are real capability seams. They decouple child controllers from their concrete parent, but do not make every model or view protocol-based.

Access control

Reference code

DesktopClassMarkdownEditor/DesktopClassMarkdownEditor/Markdown/MarkdownTag.swift:39 — tag data and private transformer

    let openMarkdown: String
    let midMarkdown: String
    let closeMarkdown: String
    let htmlTag: String

    let options: Options
    let outlineRepresentation: OutlineRepresentation?
    private let htmlTransformer: ((MarkdownTag, String) -> String)?

The source has no Swift open declaration here: init(open:) is an initializer parameter, not an access modifier, and the local prompt variable is likewise not open. Core feature types remain module-internal.

Logic ownership and placement

Design patterns

Naming conventions

• Types use domain/role names (MarkdownDocument, Parser, OutlineElement, EditorViewController).
• Local protocols name the collaborator role with a Delegate suffix.
• Commands use concrete verbs (presentDocument, updatePreview, swapTags, duplicate, delete).
• Folders separate Document Management, Markdown transformation, Utilities, and Views.

Architecture takeaways

• Make the per-document split controller the composition root, not the application delegate.
• Use narrow weak delegates for sibling-view synchronization and keep document mutation on the document model.
• Do not infer access modifiers from parameter names such as open; verify declaration syntax.
• Keep parser/preview work behind a dedicated editor-owned pipeline.

Source map

• Official Apple documentation