Swift Format Style

Write and review Swift code that formats values for display, ensuring modern FormatStyle APIs are used instead of legacy Formatter subclasses or C-style formatting.

Review process:

1. Check for legacy formatting patterns and replace with modern FormatStyle equivalents using references/anti-patterns.md.
2. Validate number, percent, and currency formatting using references/numeric-styles.md.
3. Validate date and time formatting using references/date-styles.md.
4. Validate duration formatting using references/duration-styles.md.
5. Validate measurement, list, person name, byte count, and URL formatting using references/other-styles.md.
6. Check SwiftUI Text views for proper FormatStyle integration using references/swiftui.md.

If doing partial work, load only the relevant reference files.

Core Instructions

• Target iOS 15+ / macOS 12+ minimum for basic FormatStyle. Duration and URL styles require iOS 16+ / macOS 13+.
• Never use legacy Formatter subclasses (DateFormatter, NumberFormatter, MeasurementFormatter, DateComponentsFormatter, DateIntervalFormatter, PersonNameComponentsFormatter, ByteCountFormatter).
• Never use C-style String(format:) for number formatting. Always use .formatted() or FormatStyle directly.
• Never use DispatchQueue for formatting on background threads - FormatStyle types are value types and thread-safe.
• Prefer .formatted() instance method for simple cases, and explicit FormatStyle types for reusable or complex configurations.
• In SwiftUI, use Text(_:format:) instead of Text("\(value.formatted())").
• Use Decimal instead of Float/Double for currency values.
• FormatStyle types are locale-aware by default. Only set locale explicitly when you need a specific locale different from the user's current locale.
• FormatStyle types conform to Codable and Hashable, making them safe to store and compare.

Output Format

If the user asks for a review, organize findings by file. For each issue:

1. State the file and relevant line(s).
2. Name the anti-pattern being replaced.
3. Show a brief before/after code fix.

Skip files with no issues. End with a prioritized summary of the most impactful changes to make first.

If the user asks you to write or fix formatting code, make the changes directly instead of returning a findings report.

Example output:

RecordingView.swift

Line 42: Use Duration.formatted() instead of String(format:) for time display.

// Before
let minutes = Int(duration) / 60
let seconds = Int(duration) % 60
return String(format: "%02d:%02d", minutes, seconds)

// After
Duration.seconds(duration).formatted(.time(pattern: .minuteSecond))

Line 78: Use Text(_:format:) instead of string interpolation.

// Before
Text("\(fileSize.formatted(.byteCount(style: .file)))")

// After
Text(fileSize, format: .byteCount(style: .file))

Summary

1. Legacy formatting (high): C-style String(format:) on line 42 should use Duration.formatted().
2. SwiftUI (medium): Text interpolation on line 78 should use the format: parameter directly.

End of example.

References

• references/anti-patterns.md - legacy patterns to replace: String(format:), DateFormatter, NumberFormatter, and other Formatter subclasses.
• references/numeric-styles.md - number, percent, and currency formatting with rounding, precision, sign, notation, scale, and grouping.
• references/date-styles.md - date/time compositing, ISO 8601, relative, verbatim, HTTP, interval, and components styles.
• references/duration-styles.md - Duration.TimeFormatStyle and Duration.UnitsFormatStyle with patterns, units, width, and fractional seconds.
• references/other-styles.md - measurement, list, person name, byte count, URL formatting, and custom FormatStyle creation.
• references/swiftui.md - SwiftUI Text integration and best practices.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: n0an
• Source: n0an/Swift-FormatStyle-Agent-Skill
• License: MIT

Install and usage instructions live in the source repository linked above.