Swift FormatStyle

Use Foundation's type-safe FormatStyle APIs for user-facing display and ParseableFormatStyle when the same convention must accept input. Route String Catalogs, plurals, localized copy, bundles, and RTL layout to ios-localization; formatting still requires locale review even when no text is translated.

Contents

• [Workflow](#workflow)
• [Quick Reference](#quick-reference)
• [Selection and Availability](#selection-and-availability)
• [Parsing](#parsing)
• [SwiftUI Integration](#swiftui-integration)
• [Custom FormatStyle](#custom-formatstyle)
• [Common Mistakes](#common-mistakes)
• [Review Checklist](#review-checklist)
• [References](#references)

Workflow

1. Identify the value's semantic type and whether output is display-only or must round-trip through parsing.
2. Select the narrowest built-in style and keep the user's current locale unless a wire format explicitly requires another locale.
3. Render the exact UI with representative locales such as enUS, deDE, arSA, and jaJP.
4. Check separators, numbering systems, calendars, currency and unit conventions, text direction, and layout.
5. If output or parsing fails, fix the style or surrounding copy, restore the input fixture, and rerun the same locale matrix.

Load [FormatStyle Recipes](references/formatstyle-recipes.md) when implementation needs concrete modifiers, date intervals, relative dates, duration patterns, measurements, names, lists, byte counts, or URL component controls.

Quick Reference

Selection and Availability

• Prefer FormatStyle over legacy NumberFormatter, DateFormatter, DateComponentsFormatter, and manual interpolation for new iOS 15+ code.
• Duration format styles and URL.FormatStyle require iOS 16+.
• Date.AnchoredRelativeFormatStyle requires iOS 18+ and formats relative to a fixed anchor rather than the current moment.
• Omit .locale(...) for normal UI so the style inherits the user's locale. Use a fixed locale only for an explicit protocol or test fixture.
• Treat relative date output as standalone text unless the entire sentence is localized around it.

Docs: FormatStyle

Parsing

Use the matching parseable style when users edit or import formatted values:

let style = Decimal.FormatStyle.Currency(code: "USD")
    .locale(Locale(identifier: "en_US"))

let value = try Decimal("$3,500.63", format: style)
let display = value.formatted(style)

The fixed locale above is appropriate only because the input contract is explicitly en_US. For normal UI input, use the user's locale and test round trips with representative decimal separators, currency placement, calendars, and numbering systems.

SwiftUI Integration

Prefer Text(_:format:) so SwiftUI owns the formatted value:

Text(price, format: .currency(code: currencyCode))
Text(date, format: .dateTime.month().day().year())
Text(duration, format: .units(allowed: [.minutes, .seconds]))

For every user-facing formatted Text, preview or test the exact screen across the locale matrix. Use Text(.now, style: .timer), Text(.now, style: .relative), or Text(timerInterval:) for live-updating time displays rather than manually scheduling string refreshes.

Custom FormatStyle

Create a custom style only when built-in composition cannot express the domain convention. FormatStyle refines Codable and Hashable; keep styles as reusable values and add ParseableFormatStyle only when input must round-trip.

struct AbbreviatedCountStyle: FormatStyle {
    func format(_ value: Int) -> String {
        switch value {
        case ..<1_000: "\(value)"
        case 1_000..<1_000_000: String(format: "%.1fK", Double(value) / 1_000)
        default: String(format: "%.1fM", Double(value) / 1_000_000)
        }
    }
}

extension FormatStyle where Self == AbbreviatedCountStyle {
    static var abbreviatedCount: Self { .init() }
}

Custom styles still need locale and accessibility review; a compact English suffix may not be suitable for every locale.

Common Mistakes

Review Checklist

• Semantic value type and display-versus-parse requirement identified
• Built-in FormatStyle used where it can express the convention
• Availability gated beside iOS 16+ and iOS 18+ APIs
• Currency uses an ISO 4217 code; exact decimal values use Decimal
• User locale inherited unless a protocol explicitly fixes it
• Relative date text stands alone or the full sentence is localized
• URL components and measurement usage: are deliberate
• SwiftUI uses Text(_:format:) for formatted values
• Exact rendered UI and parse round trips pass the representative-locale matrix

References

• Detailed recipes and modifiers: [references/formatstyle-recipes.md](references/formatstyle-recipes.md)
• Apple docs: FormatStyle · ParseableFormatStyle · Date.FormatStyle · Duration.TimeFormatStyle · URL.FormatStyle