swift-format-style

Original：🇺🇸 English

Translated

Writes and reviews Swift FormatStyle code, replacing legacy Formatter subclasses and C-style String(format:) with modern .formatted() APIs. Use when formatting numbers, dates, durations, measurements, lists, names, byte counts, or URLs.

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Sourcen0an/swift-formatstyle-agent-skill

Added on2026-04-11

NPX Install

npx skill4agent add n0an/swift-formatstyle-agent-skill swift-format-style

SKILL.md Content

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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:

Check for legacy formatting patterns and replace with modern FormatStyle equivalents using
```
references/anti-patterns.md
```
.
Validate number, percent, and currency formatting using
```
references/numeric-styles.md
```
.
Validate date and time formatting using
```
references/date-styles.md
```
.
Validate duration formatting using
```
references/duration-styles.md
```
.
Validate measurement, list, person name, byte count, and URL formatting using
```
references/other-styles.md
```
.
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:

State the file and relevant line(s).
Name the anti-pattern being replaced.
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.

swift

// 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.

swift

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

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

Summary

Legacy formatting (high): C-style String(format:) on line 42 should use Duration.formatted().
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.