Skills
skills/dpearson2699/swift-ios-skills/swift-formatstyle

swift-formatstyle

Installation
SKILL.md

Swift FormatStyle

Format values for human-readable display using the FormatStyle protocol and Foundation's concrete format styles. Replaces legacy Formatter subclasses with a type-safe, composable, cacheable API.

Docs: FormatStyle

Contents

Quick Reference

Type Style Access Example
Int, Double .number 42.formatted(.number.precision(.fractionLength(2))) → "42.00"
Currency .currency(code:) 29.99.formatted(.currency(code: "USD")) → "$29.99"
Percent .percent 0.85.formatted(.percent) → "85%"
Date .dateTime Date.now.formatted(.dateTime.month().day().year())
Date range .interval (date1..<date2).formatted(.interval)
Relative date .relative(presentation:unitsStyle:) date.formatted(.relative(presentation: .named)) → "yesterday"
Duration .time(pattern:) Duration.seconds(3661).formatted(.time(pattern: .hourMinuteSecond)) → "1:01:01"
Duration .units(allowed:width:) Duration.seconds(90).formatted(.units(allowed: [.minutes, .seconds])) → "1 min, 30 sec"
Measurement .measurement(width:) Measurement(value: 72, unit: UnitTemperature.fahrenheit).formatted(.measurement(width: .abbreviated))
PersonNameComponents .name(style:) name.formatted(.name(style: .short)) → "Tom"
[String] .list(type:width:) ["A","B","C"].formatted(.list(type: .and)) → "A, B, and C"
Byte count .byteCount(style:) Int64(1_048_576).formatted(.byteCount(style: .memory)) → "1 MB"
URL .url url.formatted(.url.scheme(.never).host().path())

Numbers

// Default locale-aware formatting
let n = 1234567.formatted()  // "1,234,567" (en_US)

// Precision
1234.5.formatted(.number.precision(.fractionLength(0...2)))  // "1,234.5"
1234.5.formatted(.number.precision(.significantDigits(3)))    // "1,230"

// Rounding
1234.formatted(.number.rounded(rule: .down, increment: 100)) // "1,200"

// Grouping
1234567.formatted(.number.grouping(.never))                   // "1234567"

// Notation
1_200_000.formatted(.number.notation(.compactName))           // "1.2M"
42.formatted(.number.notation(.scientific))                    // "4.2E1"

// Sign display
(-42).formatted(.number.sign(strategy: .always()))            // "+42" / "-42"

// Locale override
42.formatted(.number.locale(Locale(identifier: "de_DE")))     // "42"

Docs: IntegerFormatStyle, FloatingPointFormatStyle

Currency

29.99.formatted(.currency(code: "USD"))   // "$29.99"
29.99.formatted(.currency(code: "EUR"))   // "€29.99"
29.99.formatted(.currency(code: "JPY"))   // "¥30"

// Customize precision
let style = FloatingPointFormatStyle<Double>.Currency(code: "USD")
    .precision(.fractionLength(0))
1234.56.formatted(style)  // "$1,235"

Percentages

0.85.formatted(.percent)                                      // "85%"
0.8567.formatted(.percent.precision(.fractionLength(1)))       // "85.7%"
42.formatted(.percent)                                         // "42%"  (integer)

Dates

let now = Date.now

// Components
now.formatted(.dateTime.year().month().day())           // "Apr 22, 2026"
now.formatted(.dateTime.hour().minute())                // "4:30 PM"
now.formatted(.dateTime.weekday(.wide).month(.wide).day()) // "Wednesday, April 22"

// Predefined styles
now.formatted(date: .long, time: .shortened)            // "April 22, 2026 at 4:30 PM"
now.formatted(date: .abbreviated, time: .omitted)       // "Apr 22, 2026"

// ISO 8601
now.formatted(.iso8601)                                 // "2026-04-22T16:30:00Z"

// Relative
let yesterday = Calendar.current.date(byAdding: .day, value: -1, to: .now)!
yesterday.formatted(.relative(presentation: .named))    // "yesterday"
yesterday.formatted(.relative(presentation: .numeric))  // "1 day ago"

// Interval
(date1..<date2).formatted(.interval.month().day().hour().minute())

// Components (countdown-style)
(date1..<date2).formatted(.components(style: .wide, fields: [.day, .hour]))
// "2 days, 5 hours"

Docs: Date.FormatStyle, Date.RelativeFormatStyle, Date.IntervalFormatStyle

Anchored Relative Dates (iOS 18+)

Date.AnchoredRelativeFormatStyle formats relative to a fixed anchor date rather than the current moment.

Docs: Date.AnchoredRelativeFormatStyle

Durations

Duration (iOS 16+) has two format styles:

Docs: Duration.TimeFormatStyle, Duration.UnitsFormatStyle

TimeFormatStyle — compact separator-based

let d = Duration.seconds(3661)

d.formatted(.time(pattern: .hourMinuteSecond))       // "1:01:01"
d.formatted(.time(pattern: .hourMinute))             // "1:01"
d.formatted(.time(pattern: .minuteSecond))           // "61:01"

// Fractional seconds
Duration.seconds(3.75).formatted(
    .time(pattern: .minuteSecond(padMinuteToLength: 2, fractionalSecondsLength: 2))
)  // "00:03.75"

UnitsFormatStyle — labeled units

Duration.seconds(3661).formatted(
    .units(allowed: [.hours, .minutes, .seconds], width: .abbreviated)
)  // "1 hr, 1 min, 1 sec"

Duration.seconds(90).formatted(
    .units(allowed: [.minutes, .seconds], width: .wide)
)  // "1 minute, 30 seconds"

Duration.seconds(90).formatted(
    .units(allowed: [.minutes, .seconds], width: .narrow)
)  // "1m 30s"

// Limit unit count
Duration.seconds(3661).formatted(
    .units(allowed: [.hours, .minutes, .seconds], width: .abbreviated, maximumUnitCount: 2)
)  // "1 hr, 1 min"

Measurements

let temp = Measurement(value: 72, unit: UnitTemperature.fahrenheit)
temp.formatted(.measurement(width: .wide))        // "72 degrees Fahrenheit"
temp.formatted(.measurement(width: .abbreviated))  // "72°F"
temp.formatted(.measurement(width: .narrow))       // "72°"

let dist = Measurement(value: 5, unit: UnitLength.kilometers)
dist.formatted(.measurement(width: .abbreviated, usage: .road))  // "3.1 mi" (en_US)

Docs: Measurement.FormatStyle

Person Names

var name = PersonNameComponents()
name.givenName = "Thomas"
name.familyName = "Clark"
name.middleName = "Louis"
name.namePrefix = "Dr."
name.nickname = "Tom"
name.nameSuffix = "Esq."

name.formatted(.name(style: .long))        // "Dr. Thomas Louis Clark Esq."
name.formatted(.name(style: .medium))      // "Thomas Clark"
name.formatted(.name(style: .short))       // "Tom"
name.formatted(.name(style: .abbreviated)) // "TC"

Style resolution follows priority: script → user preferences → locale → developer setting.

Docs: PersonNameComponents.FormatStyle

Lists

["Alice", "Bob", "Charlie"].formatted(.list(type: .and))
// "Alice, Bob, and Charlie"

["Alice", "Bob", "Charlie"].formatted(.list(type: .or))
// "Alice, Bob, or Charlie"

// With member formatting
[1, 2, 3].formatted(.list(memberStyle: .number, type: .and))
// "1, 2, and 3"

// Narrow width
["A", "B", "C"].formatted(.list(type: .and, width: .narrow))
// "A, B, C"

Docs: ListFormatStyle

Byte Counts

Int64(1_048_576).formatted(.byteCount(style: .memory))   // "1 MB"
Int64(1_048_576).formatted(.byteCount(style: .file))      // "1 MB"
Int64(1_048_576).formatted(.byteCount(style: .binary))    // "1 MiB"

Docs: ByteCountFormatStyle

URLs

let url = URL(string: "https://example.com/path?q=1")!
url.formatted()
// "https://example.com/path?q=1"

url.formatted(.url.scheme(.never).host().path())
// "example.com/path"

url.formatted(.url.scheme(.always).host(.never).path())
// "https:///path"

Docs: URL.FormatStyle

SwiftUI Integration

Text accepts a format: parameter, keeping formatting out of the view model.

// Inline format style
Text(price, format: .currency(code: "USD"))
Text(date, format: .dateTime.month().day().year())
Text(duration, format: .units(allowed: [.minutes, .seconds]))

// Timer-style (live updating)
Text(.now, style: .timer)
Text(.now, style: .relative)
Text(timerInterval: start...end)

Prefer Text(_:format:) over string interpolation — it allows SwiftUI to re-render only the formatted value and supports accessibility scaling.

Custom FormatStyle

Conform to FormatStyle for domain-specific formatting. Conform to ParseableFormatStyle if you also need parsing.

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

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

// Usage
let followers = 12_500
Text(followers, format: .abbreviatedCount)  // "12.5K"

Common Mistakes

Mistake Fix
Using legacy NumberFormatter / DateFormatter in new code Use FormatStyle (iOS 15+). Foundation caches format style instances automatically.
String interpolation for formatted numbers in Text Use Text(value, format:) for locale correctness and accessibility
Hardcoding locale in format styles Omit .locale() to inherit the user's current locale by default
Using .time(pattern:) for labeled duration display Use .units(allowed:width:) for "1 hr, 30 min" style output
Creating Formatter instances in body or tight loops FormatStyle instances are value types cached by Foundation; safe to create inline
Formatting Duration with DateComponentsFormatter Use Duration.TimeFormatStyle or Duration.UnitsFormatStyle directly
Ignoring usage: parameter for measurements Specify .road, .asProvided, etc. for locale-aware unit conversion

Review Checklist

  • FormatStyle used instead of legacy Formatter subclasses for iOS 15+ targets
  • Text(_:format:) used instead of pre-formatting strings for SwiftUI text
  • No hardcoded locale unless explicitly needed (e.g., server communication)
  • Duration formatting uses Duration.TimeFormatStyle or Duration.UnitsFormatStyle
  • Currency codes are ISO 4217 strings, not hardcoded symbols
  • Measurement formatting includes usage: for user-facing display
  • Custom FormatStyle types conform to Codable + Hashable for caching

References

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