swift-formatstyle

swift-formatstyle is an engineering reference for using Foundation's type-safe FormatStyle and ParseableFormatStyle APIs to format and parse locale-aware values in Swift. It solves the common iOS problem of correctly displaying and round-tripping numbers, decimals, currencies, percentages, dates, durations, measurements, person names, lists, byte counts, and URLs across locales — replacing legacy Formatter subclasses like NumberFormatter and DateFormatter and error-prone manual string interpolation.

The skill provides a workflow (identify the value's semantic type, pick the narrowest built-in style, render against representative locales such as en_US, de_DE, ar_SA, and ja_JP, and check separators, numbering systems, calendars, currency/unit conventions, and text direction), plus a quick-reference table mapping each value type to its default style and key constraints. It documents availability gates (Duration and URL format styles require iOS 16+, Date.AnchoredRelativeFormatStyle requires iOS 18+), SwiftUI integration via Text(_:format:), parsing with ParseableFormatStyle, and building custom format styles. A bundled formatstyle-recipes.md reference gives concrete modifiers for precision, rounding, grouping, notation, currency codes, date intervals, relative dates, durations, measurements, names, lists, byte counts, and URL component controls; it also cautions that URL formatting is display formatting, not sanitization or a security boundary. Localization concerns like String Catalogs, plurals, and RTL layout are routed to a separate ios-localization skill.

It targets Swift and iOS developers modernizing formatting code, designing reusable format APIs, and testing user-facing output across locales.