Sample CodeiOS, iPadOS, Mac Catalyst, macOSReviewed 2026-07-21View on Apple Developer

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At a glance

Item	Summary
Purpose	Explore Foundation formatters for dates, measurements, names, and numbers in a localized SwiftUI interface.
Architecture	A cross-platform catalog whose navigation root composes self-contained formatter demonstration views.
State style	Each feature view owns its formatter settings, often in a nested private `FormatterState` value.

Project structure

├── Shared/
│   ├── FormattersApp.swift
│   ├── ContentView.swift
│   ├── Dates/*View.swift
│   ├── Names/*View.swift
│   ├── MeasurementsView.swift
│   └── NumbersView.swift
├── iOS/Utilities.swift
└── macOS/Utilities.swift

Almost all source is shared. Two same-signature utility functions adapt pasteboard writes to UIKit or AppKit at compile time.

Overall architecture

flowchart LR
    App["FormattersApp"] --> Nav["ContentView navigation"]
    Nav --> Dates["Dates views"]
    Nav --> Measures["MeasurementsView"]
    Nav --> Names["NamesView"]
    Nav --> Numbers["NumbersView"]
    Dates --> Foundation["Foundation formatter objects"]
    Measures --> Foundation
    Names --> Foundation
    Numbers --> Foundation
    Dates --> Pasteboard["platform Utilities.swift"]

Reference code

Shared/ContentView.swift:10 — the root is a thin catalog router with no formatter state of its own.

struct ContentView: View {
    // ...
}

Ownership and state

classDiagram
    ContentView *-- DatesView : navigation destination
    ContentView *-- MeasurementsView : destination
    ContentView *-- NamesView : destination
    ContentView *-- NumbersView : destination
    NumbersView *-- FormatterState : State
    FormatterState *-- NumberFormatter : private
    DateFormatterView *-- FormatterState : State
    FormatterState *-- DateFormatter : private

Ownership evidence

Shared/NumbersView.swift:88 — a private nested value owns the mutable formatter and keeps it synchronized through property observers.

    private struct FormatterState {
        // ...
    }

Each destination owns its interaction state; there is no shared app model. A nested state wrapper is the mutation authority for its formatter configuration and derived output. SwiftUI owns view lifetime and re-rendering.

Class and protocol design

Type group	Responsibility
`FormattersApp` / `ContentView`	Composition root and catalog navigation.
Date, measurement, name, and number views	Demonstrate one formatter family and own controls.
Nested `FormatterState` values	Keep a mutable Foundation formatter consistent with UI selections.
`Monogram` and small helpers	Reusable presentation for formatted results.
Platform `writeToPasteboard` functions	Hide UIKit/AppKit pasteboard differences.

There is no app-defined protocol layer. SwiftUI View composition and concrete Foundation formatter APIs are the intended teaching surface.

Access control

Boundary	Effect and rationale
`private @State` selections	Only the demonstration view mutates its controls.
`private` nested formatter state and formatter object	Prevents other features from depending on one demo’s configuration mechanics.
Implicit `internal` views and utilities	Shared source is available to app targets without becoming a public library.
Platform utility functions share a signature	Call sites stay platform-neutral while target membership selects the implementation.

Shared/Dates/DateFormatterView.swift:138 shows the private date state type; iOS/Utilities.swift:10 and macOS/Utilities.swift:10 show the target-specific adapter boundary.

Logic ownership and placement

Logic	Owner
Catalog navigation	`ContentView`
Formatter configuration and derived strings	Feature-local state or view
Domain sample values	The feature file that demonstrates them
Cross-platform pasteboard write	Target-specific `Utilities.swift`
Visual monogram generation	Name component view/helper

Design patterns

Pattern	Evidence	Purpose
Catalog	`Shared/ContentView.swift:12`	Keeps unrelated formatter examples independently navigable.
View-local state	`Shared/NumbersView.swift:11`	Gives each example isolated lifetime and mutation.
State wrapper	`Shared/Dates/DateFormatterView.swift:144`	Synchronizes several selections with one formatter instance.
Platform adapter	`iOS/Utilities.swift:10`, `macOS/Utilities.swift:10`	Presents one call shape over two pasteboard APIs.

Naming conventions

Feature files and views are named after Foundation types: DateFormatterView, RelativeDateTimeFormatterView, and NumbersView.
Nested FormatterState communicates local scope rather than a reusable domain model.
Platform-specific files keep the same Utilities.swift and function name to make target substitution invisible to callers.

Architecture takeaways

A documentation catalog benefits from self-contained feature views, not global state.
Wrap mutable formatter configuration close to the controls that drive it.
Use same-signature platform adapters for tiny UIKit/AppKit differences.
Do not infer a service or MVVM layer where the source intentionally demonstrates direct framework APIs.

Source map

Source	Role
`Shared/FormattersApp.swift:10`	Composition root
`Shared/ContentView.swift:10`	Catalog navigation
`Shared/NumbersView.swift:10`	Representative local formatter state
`Shared/Dates/DateFormatterView.swift:13`	Date formatting example
`macOS/Utilities.swift:10`	AppKit pasteboard adapter

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