Display text with a custom layout

At a glance

Project structure

TextLayout/
├── AppDelegate.swift
├── BaseViewController.swift
├── CircleTextContainer.swift
├── CircleTextViewController.swift
├── CircleTextViewController+Ellipsis.swift
├── ExclusivePathViewController.swift
├── TwoColumnsViewController.swift
└── Base.lproj/Main.storyboard

Structure observations

• BaseViewController contains only behavior shared by all text demonstrations: access to shared storage and keyboard-inset handling.
• The +Ellipsis file keeps NSLayoutManagerDelegate glyph substitution separate from the circle screen’s lifecycle setup.
• Each feature controller constructs its own layout manager, text container, and text view instead of sharing presentation objects.

Overall architecture

flowchart LR
    Bundle["Sample.rtf"] --> App["AppDelegate.textStorage"]
    App --> Base["BaseViewController"]
    Base --> Circle["CircleTextViewController"]
    Base --> Columns["TwoColumnsViewController"]
    Base --> Exclusion["ExclusivePathViewController"]
    Circle --> CircleContainer["CircleTextContainer"]
    Circle --> GlyphDelegate["NSLayoutManagerDelegate extension"]
    Columns --> TwoContainers["Two NSTextContainers"]
    Exclusion --> Path["Movable exclusion path"]
    CircleContainer --> TextKit["TextKit layout"]
    GlyphDelegate --> TextKit
    TwoContainers --> TextKit
    Path --> TextKit

Reference code

TextLayout/CircleTextViewController.swift:18 — the circle feature builds a complete TextKit presentation stack around the shared storage.

let textContainer = CircleTextContainer(size: .zero)
textContainer.widthTracksTextView = true

let layoutManager = NSLayoutManager()
layoutManager.addTextContainer(textContainer)
textStorage.addLayoutManager(layoutManager)

textView = UITextView(frame: CGRect.zero, textContainer: textContainer)

Interpretation

The shared value is document content, not view state. Each tab applies a different layout strategy by attaching a separate layout manager and containers to the same NSTextStorage, so edits can be reflected across demonstrations.

Ownership and state

classDiagram
    AppDelegate *-- NSTextStorage : creates shared instance
    BaseViewController o-- NSTextStorage : lazy reference
    BaseViewController *-- AnyCancellable : keyboard subscription
    BaseViewController <|-- CircleTextViewController
    BaseViewController <|-- TwoColumnsViewController
    BaseViewController <|-- ExclusivePathViewController
    CircleTextViewController *-- NSLayoutManager : creates
    NSLayoutManager *-- CircleTextContainer : retains
    CircleTextViewController o-- NSRange : glyph state
    ExclusivePathViewController *-- NSKeyValueObservation : retains tokens
    ExclusivePathViewController *-- UIBezierPath : exclusion geometry

Ownership evidence

TextLayout/AppDelegate.swift:13 — the app delegate lazily creates the single storage object from the bundled RTF.

lazy var textStorage: NSTextStorage = {
    guard let fileURL = Bundle.main.url(forResource: "Sample", withExtension: "rtf") else {
        fatalError("Failed to find Sample.rtf in the app bundle.")
    }
    let options = [NSAttributedString.DocumentReadingOptionKey.documentType:
                   NSAttributedString.DocumentType.rtf]
    guard let textStorage = try? NSTextStorage(url: fileURL, options: options,
                                               documentAttributes: nil) else {
        fatalError("Failed to create a text storage from \(fileURL).")
    }
    return textStorage
}()

Class and protocol design

There is no app-defined protocol. Protocol-oriented behavior comes from narrow UIKit conformances (UITextViewDelegate, UITabBarControllerDelegate, and NSLayoutManagerDelegate) attached to the type that owns the related state.

Access control

Reference code

TextLayout/ExclusivePathViewController.swift:13 — observation lifetimes and path geometry are deliberately controller-private.

private var kvoContentOffset: NSKeyValueObservation?
private var kvoTextViewPosition: NSKeyValueObservation?
private var panInitialImageCenter = CGPoint()

private lazy var circlePath: UIBezierPath = {
    UIBezierPath(ovalIn: CGRect(x: 0, y: 0, width: 100, height: 100))
}()

Logic ownership and placement

Design patterns

Naming conventions

• Screen types use outcome names: CircleTextViewController, TwoColumnsViewController, and ExclusivePathViewController.
• TextKit graph objects keep framework vocabulary: textStorage, layoutManager, and textContainer.
• trigger...IfNeeded and restore...IfNeeded communicate guarded, idempotent glyph transitions.
• The +Ellipsis suffix identifies a capability extension rather than another owner.

Architecture takeaways

• Share content storage when several layouts must reflect the same editable document, but keep presentation graphs independent.
• Put layout mathematics in the TextKit subclass or delegate callback UIKit already queries.
• Retain observation/subscription tokens beside the UI state they synchronize and cancel them at lifecycle boundaries.
• Treat cross-file internal visibility as a file-organization tradeoff, not as a public API decision.

Source map

• Official Apple documentation