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

Language Introspector

At a glance

Item	Summary
Purpose	Explore locale, language, script, direction, transcription, and localized formatter behavior.
Architecture	A SwiftUI navigation shell routes through `AppScreen`; each feature owns one or more main-actor observable models and passes them to bindable child views.
Feature boundary	Models own Foundation calculations and formatter mutation; views own layout and user bindings.

Project structure

LanguageIntrospector/
├── LanguageIntrospectorApp.swift
├── ContentView.swift
├── AppScreen.swift
├── Languages{Model,View}.swift
├── DatesView.swift
├── *Formatter{Model,View}.swift
├── {Measurements,Names,Numbers}{Model,View}.swift
└── small presentation helpers

Files pair a model with its feature view. The date screen is a composition of four independent formatter model/view pairs.

Overall architecture

flowchart LR
    App["LanguageIntrospectorApp"] --> Content["ContentView"]
    Content --> Route["AppScreen enum"]
    Route --> Languages["LanguagesView + LanguagesModel"]
    Route --> Dates["DatesView + four formatter models"]
    Route --> Measures["Measurements view/model"]
    Route --> Names["Names view/model"]
    Route --> Numbers["Numbers view/model"]
    Languages --> Foundation["Locale / formatter APIs"]
    Dates --> Foundation

Reference code

LanguageIntrospector/AppScreen.swift:36 — the route enum owns the exact mapping from navigation identity to feature composition.

extension AppScreen {
    // ...
    @MainActor
    // ...
}

Ownership and state

classDiagram
    ContentView *-- AppScreen : navigation selection
    LanguagesView *-- LanguagesModel : State
    DatesView *-- DateFormatterModel : State
    DatesView *-- DateComponentsModel : State
    DatesView *-- DateIntervalFormatterModel : State
    DatesView *-- RelativeDateTimeFormatterModel : State
    DateFormatterView --> DateFormatterModel : Bindable
    DateFormatterModel *-- DateFormatter

Ownership evidence

LanguageIntrospector/DatesView.swift:10 — the feature root owns all formatter-model lifetimes and injects them into children.

struct DatesView: View {
    // ...
}

Feature roots own observable model instances with @State. Child views receive @Bindable access for UI controls. Each model is the mutation authority for its Foundation formatter and derived presentation strings.

Class and protocol design

Type	Responsibility
`AppScreen`	Stable, codable navigation identity plus label/destination mapping.
`LanguagesModel`	Sort language examples against preferred locales and expose the selection.
Formatter model classes	Own formatter objects, inputs, and derived localized output.
Feature views	Own model lifetime and compose the screen.
Bindable child views	Bind controls to one supplied model without owning it.
Small view modifiers/helpers	Reuse presentation without moving feature state.

The app uses Swift observation and SwiftUI protocols; it defines no service/repository protocol because all data is local and deterministic.

Access control

Boundary	Effect and rationale
`private @State` models and navigation fields	Makes the feature/root view the explicit lifetime owner.
`private` sample data, formatter helpers, and update methods	Prevents views from bypassing model invariants.
Model binding properties remain implicit `internal`	Child views in separate files can bind within the app module.
All app types are internal; no `public` API	The source builds an executable, not a reusable package.
`@MainActor` models	Constrains observable UI mutation to the main actor; this is isolation, not access control.

LanguageIntrospector/LanguagesModel.swift:19 shows private sorting logic and LanguageIntrospector/DateFormatterModel.swift:79 shows the private mutation gate.

Logic ownership and placement

Logic	Owner
Navigation identity and destination selection	`AppScreen`
Preferred-language matching and sorting	`LanguagesModel`
Formatter configuration and derived strings	Corresponding formatter model
Model lifetime and feature composition	Top-level feature view
Reusable styling	View modifiers and presentation helpers

Design patterns

Pattern	Evidence	Purpose
Observable model	`LanguageIntrospector/DateFormatterModel.swift:10`	Moves mutable formatter logic out of view layout.
Model-view split	`LanguageIntrospector/DateFormatterView.swift:10`	A bindable view renders and edits a supplied model.
Enum router	`LanguageIntrospector/AppScreen.swift:10`	Keeps navigation labels and destinations exhaustive.
Feature slicing	`LanguageIntrospector/DatesView.swift:10`	Composes several focused formatter models without one global store.

Naming conventions

Model/view pairs share a stem: DateFormatterModel and DateFormatterView.
Feature aggregation drops the implementation suffix: DatesView owns all date-format models.
Derived properties describe output (localizedDateWithPattern, sortedLanguageTexts) while mutation helpers use verbs (updateFormatter).

Architecture takeaways

Give each feature root ownership of its observable model lifetime.
Pass models to children as bindable dependencies rather than reconstructing them.
Use an enum router for a small, closed navigation catalog.
Keep deterministic Foundation formatting logic in focused models, not a global service.

Source map

Source	Role
`LanguageIntrospector/LanguageIntrospectorApp.swift:10`	Composition root
`LanguageIntrospector/AppScreen.swift:10`	Navigation router
`LanguageIntrospector/ContentView.swift:10`	Navigation shell
`LanguageIntrospector/LanguagesModel.swift:11`	Representative observable model
`LanguageIntrospector/DatesView.swift:10`	Feature model ownership
`LanguageIntrospector/DateFormatterView.swift:10`	Bindable child view

Official Apple documentation