swift-formatstyle

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Summary

Formats and parses locale-aware numbers, decimals, currencies, percentages, dates, durations, measurements, names, lists, byte counts, and URLs with Foundation FormatStyle and ParseableFormatStyle. Use when replacing legacy Formatter subclasses, designing reusable format APIs, using SwiftUI Text(_:format:), correcting availability, or testing user-facing formatted output across locales.

SKILL.md

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.

Workflow
Quick Reference
Selection and Availability
Parsing
SwiftUI Integration
Custom FormatStyle
Common Mistakes
Review Checklist
References

Workflow

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

Load FormatStyle Recipes when implementation needs concrete modifiers, date intervals, relative dates, duration patterns, measurements, names, lists, byte counts, or URL component controls.

Value	Default style	Important constraint
`Int`, `Double`	`.number`	Configure precision, rounding, grouping, notation, or sign only as required.
`Decimal`	`.number`, `.percent`, `.currency(code:)`	Prefer for exact decimal display and parsing.
Currency	`.currency(code:)`	Pass an ISO 4217 code; never hardcode a symbol.
Percent	`.percent`	Confirm whether the input is a fraction (`0.85`) or whole percent (`85`).
`Date`	`.dateTime`, `.relative`, `.interval`	Relative output should usually stand alone, not be embedded in a sentence.
`Duration`	`.time(pattern:)`, `.units(allowed:width:)`	Requires iOS 16+; choose compact clock output versus labeled units.
`Measurement`	`.measurement(width:usage:)`	`usage:` controls locale-aware conversion policy.
`PersonNameComponents`	`.name(style:)`	Do not manually concatenate name parts.
Collections	`.list(type:width:)`	Let locale rules choose separators and conjunctions.
Byte count	`.byteCount(style:)`	Choose file, memory, decimal, or binary semantics deliberately.
`URL`	`.url`	Requires iOS 16+; query, port, and fragment are opt-in display components.

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)

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

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() }
}

Mistake	Fix
Manual formatting or legacy formatter allocation in view code	Use the matching `FormatStyle` and `Text(_:format:)`.
Hardcoded currency symbol or locale	Pass an ISO currency code and inherit the user locale unless the contract says otherwise.
Binary floating-point for exact decimal input	Use `Decimal.FormatStyle` and its matching parse strategy.
Assuming `URL.formatted()` preserves every component	Opt in to `.port(.always)`, `.query(.always)`, or `.fragment(.always)` only when those components should display.
Relative date output embedded in a larger sentence	Keep it standalone or localize the complete sentence.
Clock-style duration used for prose	Use `.units(allowed:width:)` for labeled output.
Measurement conversion left implicit	Select a `usage:` appropriate to the UI.
One-locale spot check	Run the same exact-screen fixture across representative locales and fix/rerun failures.

swift-formatstyle

Summary

SKILL.md

Swift FormatStyle

Contents

Workflow

Quick Reference

Selection and Availability

Parsing

SwiftUI Integration

Custom FormatStyle

Common Mistakes

Review Checklist

References

Related skills