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Core Text for TextKit Developers

Core Text for TextKit Developers

Use when working with Core Text for glyph-level access, custom typesetting, hit testing, font tables, or when TextKit 2 lacks the glyph APIs you need.

Reference Skills

Use when working with Core Text for glyph-level access, custom typesetting, hit testing, font tables, or when TextKit 2 lacks the glyph APIs you need.

Family: Text Model And Foundation Utilities

Use this skill when you need glyph-level control — either because TextKit 2 has no glyph APIs, or because your use case (custom typesetting, font tables, per-glyph rendering) requires the Core Text layer directly.

When to Use

Section titled “When to Use”
  • You need glyph positions, advances, or bounding boxes that TextKit 2 hides
  • You are doing custom typesetting or non-standard line breaking
  • You need font table access or OpenType feature control
  • You are rendering text with custom Core Graphics effects per glyph
  • You need hit testing or caret positioning outside of a TextKit text container

Quick Decision

Section titled “Quick Decision”
Need glyph-level access?
    TextKit 1 available? → Use NSLayoutManager glyph APIs
    TextKit 2 only? → Drop to Core Text (this skill)


Need custom line breaking?
    → CTTypesetter


Need to draw text into a CGContext directly?
    → CTLine or CTFrame


Need font metrics, tables, or OpenType features?
    → CTFont


Need inline non-text elements with custom metrics?
    → CTRunDelegate

Core Guidance

Section titled “Core Guidance”

Architecture

Section titled “Architecture”
CTFramesetter (factory)
    → CTTypesetter (line breaking)
        → CTFrame (laid-out region)
            → CTLine (one visual line)
                → CTRun (contiguous glyphs, same attributes)
                    → CGGlyph[] (actual glyph IDs)
                    → CGPoint[] (positions)
                    → CGSize[]  (advances)

Core Text sits directly above Core Graphics. It is a C API using Core Foundation types. Available since iOS 3.2 / macOS 10.5. Both TextKit 1 and TextKit 2 render through Core Text internally — it is the foundation under both, not part of either.

Thread safety: Font objects (CTFont, CTFontDescriptor, CTFontCollection) are thread-safe and can be shared across threads. Layout objects (CTTypesetter, CTFramesetter, CTRun, CTLine, CTFrame) are NOT thread-safe — use them on a single thread only.

CTLine — The Most Common Escape Hatch

Section titled “CTLine — The Most Common Escape Hatch”

For most TextKit developers, CTLine is the entry point. Create one from an attributed string to get glyph information:

let attributedString = NSAttributedString(string: "Hello 👋🏽",
    attributes: [.font: UIFont.systemFont(ofSize: 16)])
let line = CTLineCreateWithAttributedString(attributedString)


// Typographic bounds
var ascent: CGFloat = 0, descent: CGFloat = 0, leading: CGFloat = 0
let width = CTLineGetTypographicBounds(line, &ascent, &descent, &leading)
let height = ascent + descent + leading


// Hit testing: point → string index
let index = CTLineGetStringIndexForPosition(line, CGPoint(x: 50, y: 0))


// Caret positioning: string index → x offset
let offset = CTLineGetOffsetForStringIndex(line, 3, nil)


// Image bounds (actual rendered pixels, not typographic)
let ctx = UIGraphicsGetCurrentContext()!
let imageBounds = CTLineGetImageBounds(line, ctx)

CTRun — Glyph-Level Access

Section titled “CTRun — Glyph-Level Access”

Each CTRun is a contiguous sequence of glyphs sharing the same attributes:

let runs = CTLineGetGlyphRuns(line) as! [CTRun]


for run in runs {
    let glyphCount = CTRunGetGlyphCount(run)


    // Get all glyphs
    var glyphs = [CGGlyph](repeating: 0, count: glyphCount)
    CTRunGetGlyphs(run, CFRange(location: 0, length: glyphCount), &glyphs)


    // Get positions (relative to line origin)
    var positions = [CGPoint](repeating: .zero, count: glyphCount)
    CTRunGetPositions(run, CFRange(location: 0, length: glyphCount), &positions)


    // Get advances (width of each glyph)
    var advances = [CGSize](repeating: .zero, count: glyphCount)
    CTRunGetAdvances(run, CFRange(location: 0, length: glyphCount), &advances)


    // Map glyph indices back to string indices (UTF-16)
    var stringIndices = [CFIndex](repeating: 0, count: glyphCount)
    CTRunGetStringIndices(run, CFRange(location: 0, length: glyphCount), &stringIndices)


    // Get the run's attributes
    let attrs = CTRunGetAttributes(run) as! [NSAttributedString.Key: Any]
    let font = attrs[.font] as! CTFont
}

CTFramesetter / CTFrame — Multi-Line Layout

Section titled “CTFramesetter / CTFrame — Multi-Line Layout”

For laying out text into a rectangular (or custom-shaped) region:

let framesetter = CTFramesetterCreateWithAttributedString(attributedString)


// Suggest frame size for constrained width
let constraint = CGSize(width: 300, height: .greatestFiniteMagnitude)
let suggestedSize = CTFramesetterSuggestFrameSizeWithConstraints(
    framesetter, CFRange(location: 0, length: 0), nil, constraint, nil)


// Create frame in a path
let path = CGPath(rect: CGRect(origin: .zero, size: suggestedSize), transform: nil)
let frame = CTFramesetterCreateFrame(framesetter, CFRange(location: 0, length: 0), path, nil)


// Get lines and origins
let lines = CTFrameGetLines(frame) as! [CTLine]
var origins = [CGPoint](repeating: .zero, count: lines.count)
CTFrameGetLineOrigins(frame, CFRange(location: 0, length: lines.count), &origins)


// Draw the entire frame
CTFrameDraw(frame, context)

CTTypesetter — Custom Line Breaking

Section titled “CTTypesetter — Custom Line Breaking”

For control over where lines break:

let typesetter = CTTypesetterCreateWithAttributedString(attributedString)


var start: CFIndex = 0
let stringLength = CFAttributedStringGetLength(attributedString)


while start < stringLength {
    // Suggest line break for a given width
    let count = CTTypesetterSuggestLineBreak(typesetter, start, 300.0)


    // Or suggest with cluster breaking (for CJK)
    // let count = CTTypesetterSuggestClusterBreak(typesetter, start, 300.0)


    let line = CTTypesetterCreateLine(typesetter, CFRange(location: start, length: count))
    // Position and draw the line
    start += count
}

CTFont — Font Metrics and Features

Section titled “CTFont — Font Metrics and Features”
// Create from UIFont
let uiFont = UIFont.systemFont(ofSize: 16)
let ctFont = CTFontCreateWithName(uiFont.fontName as CFString, uiFont.pointSize, nil)


// Metrics
let ascent = CTFontGetAscent(ctFont)
let descent = CTFontGetDescent(ctFont)
let leading = CTFontGetLeading(ctFont)
let unitsPerEm = CTFontGetUnitsPerEm(ctFont)


// Get glyphs for characters
var characters: [UniChar] = Array("A".utf16)
var glyphs = [CGGlyph](repeating: 0, count: characters.count)
CTFontGetGlyphsForCharacters(ctFont, &characters, &glyphs, characters.count)


// Glyph bounding boxes
var boundingRects = [CGRect](repeating: .zero, count: glyphs.count)
CTFontGetBoundingRectsForGlyphs(ctFont, .default, glyphs, &boundingRects, glyphs.count)


// Glyph path (for custom rendering)
if let path = CTFontCreatePathForGlyph(ctFont, glyphs[0], nil) {
    // Draw the glyph outline
    context.addPath(path)
    context.fillPath()
}


// OpenType features
let features = CTFontCopyFeatures(ctFont) as? [[String: Any]] ?? []

CTRunDelegate — Inline Custom Elements

Section titled “CTRunDelegate — Inline Custom Elements”

Reserve space in a line for non-text content (images, custom views):

var callbacks = CTRunDelegateCallbacks(version: kCTRunDelegateCurrentVersion,
    dealloc: { _ in },
    getAscent: { _ in 20 },   // Height above baseline
    getDescent: { _ in 5 },    // Depth below baseline
    getWidth: { _ in 30 }      // Width of the space
)


let delegate = CTRunDelegateCreate(&callbacks, nil)!


let attrs: [NSAttributedString.Key: Any] = [
    kCTRunDelegateAttributeName as NSAttributedString.Key: delegate
]
let placeholder = NSAttributedString(string: "\u{FFFC}", attributes: attrs)


// Insert into your attributed string
// Then draw your custom content at the run's position after layout

Critical: Coordinate System

Section titled “Critical: Coordinate System”

Core Text uses bottom-left origin (Core Graphics). UIKit uses top-left origin.

Drawing Core Text in UIKit

Section titled “Drawing Core Text in UIKit”
override func draw(_ rect: CGRect) {
    guard let context = UIGraphicsGetCurrentContext() else { return }


    // REQUIRED: Flip coordinate system for Core Text
    context.textMatrix = .identity
    context.translateBy(x: 0, y: bounds.height)
    context.scaleBy(x: 1, y: -1)


    // Now draw
    CTLineDraw(line, context)
    // or CTFrameDraw(frame, context)
}

Forgetting the flip is the #1 Core Text mistake. Text renders upside-down or at the wrong position.

Converting Line Origins from CTFrame

Section titled “Converting Line Origins from CTFrame”

CTFrameGetLineOrigins returns origins in Core Text coordinates (bottom-left). To use in UIKit:

let lines = CTFrameGetLines(frame) as! [CTLine]
var origins = [CGPoint](repeating: .zero, count: lines.count)
CTFrameGetLineOrigins(frame, CFRange(location: 0, length: lines.count), &origins)


for (i, line) in lines.enumerated() {
    // Flip y: UIKit y = frameHeight - CoreText y
    let uikitY = frameRect.height - origins[i].y
    // uikitY is now the BASELINE position in UIKit coordinates
}

Bridging TextKit ↔ Core Text

Section titled “Bridging TextKit ↔ Core Text”

TextKit 2: Getting Glyph Info from a Layout Fragment

Section titled “TextKit 2: Getting Glyph Info from a Layout Fragment”
// In a custom NSTextLayoutFragment or delegate callback:
let attributedString = textElement.attributedString
let line = CTLineCreateWithAttributedString(attributedString as CFAttributedString)
let runs = CTLineGetGlyphRuns(line) as! [CTRun]


for run in runs {
    let glyphCount = CTRunGetGlyphCount(run)
    var positions = [CGPoint](repeating: .zero, count: glyphCount)
    CTRunGetPositions(run, CFRange(location: 0, length: glyphCount), &positions)


    // positions are relative to the line origin
    // Add layoutFragmentFrame.origin to get document coordinates
    // Remember to handle the coordinate flip if drawing in UIKit
}

Font Bridging

Section titled “Font Bridging”
// macOS: NSFont ↔ CTFont is toll-free bridged
let ctFont = nsFont as CTFont
let nsFont = ctFont as NSFont


// iOS: UIFont ↔ CTFont is NOT toll-free bridged
// UIFont → CTFont
let ctFont = CTFontCreateWithName(uiFont.fontName as CFString, uiFont.pointSize, nil)


// CTFont → UIFont
let uiFont = UIFont(name: CTFontCopyPostScriptName(ctFont) as String,
                     size: CTFontGetSize(ctFont))!


// NSAttributedString ↔ CFAttributedString IS toll-free bridged (both platforms)
let cfAttrStr = attributedString as CFAttributedString

Attribute Key Differences

Section titled “Attribute Key Differences”

Core Text uses its own attribute keys that differ from UIKit/AppKit:

PurposeCore Text KeyUIKit/AppKit Key
FontkCTFontAttributeName (CTFont).font (UIFont/NSFont)
Foreground colorkCTForegroundColorAttributeName (CGColor).foregroundColor (UIColor/NSColor)
Paragraph stylekCTParagraphStyleAttributeName (CTParagraphStyle).paragraphStyle (NSParagraphStyle)
KernkCTKernAttributeName.kern

When creating attributed strings for Core Text directly, use the kCT* keys. Mixing Core Text keys and UIKit keys in the same attributed string can cause subtle rendering differences.

In practice, UIKit’s .font attribute (UIFont) works with Core Text because UIFont wraps a CTFont internally. But .foregroundColor (UIColor) does NOT — Core Text needs CGColor.

Common Pitfalls

Section titled “Common Pitfalls”
  1. Forgetting to flip coordinates — Core Text is bottom-left origin. UIKit is top-left. Text appears upside-down or at wrong position. Always set context.textMatrix = .identity and flip.
  2. Not resetting text matrix — CGContext.textMatrix persists between drawing calls. If a previous operation set a non-identity matrix, your Core Text drawing will be transformed unexpectedly.
  3. String indices are UTF-16 — CTRunGetStringIndices returns UTF-16 code unit indices (matching NSString), not Swift Character indices. A single emoji can span 2-4 UTF-16 units.
  4. CTFont ≠ UIFont on iOS — They are NOT toll-free bridged on iOS. Create CTFont explicitly.
  5. CTFrameGetLines returns non-retained array — In Swift this is usually managed automatically, but be careful with the CFArray if you bridge to C.
  6. Attribute key mismatch — kCTForegroundColorAttributeName expects CGColor, not UIColor. Passing UIColor silently fails (no color rendered).
  7. Character-glyph mapping is not 1:1 — Ligatures produce fewer glyphs than characters. Complex scripts (Arabic, Devanagari) can produce more glyphs than characters. Always use CTRunGetStringIndices for the mapping.
  8. CTParagraphStyle is not NSParagraphStyle — They are related but not interchangeable. CTParagraphStyle uses a C struct API; NSParagraphStyle has Objective-C properties. NSParagraphStyle internally wraps CTParagraphStyle.

Documentation Scope

Section titled “Documentation Scope”

This page documents the apple-text-core-text reference skill. Use it when the subsystem is already known and you need mechanics, behavior, or API detail.

Section titled “Related”
  • apple-text-textkit1-ref: Use when working with TextKit 1 and you need NSLayoutManager, NSTextStorage, or NSTextContainer APIs — glyphs, temporary attributes, multi-container.
  • apple-text-textkit2-ref: Use when working with TextKit 2 and you need NSTextLayoutManager or NSTextContentManager APIs — viewport layout, fragments, rendering attributes.
  • apple-text-viewport-rendering: Use when working with viewport layout, line-fragment geometry, rendering attributes, font substitution, or scroll-driven layout.
Full SKILL.md source
SKILL.md
---
name: apple-text-core-text
description: Use when working with Core Text for glyph-level access, custom typesetting, hit testing, font tables, or when TextKit 2 lacks the glyph APIs you need
license: MIT
---


# Core Text for TextKit Developers


Use this skill when you need glyph-level control — either because TextKit 2 has no glyph APIs, or because your use case (custom typesetting, font tables, per-glyph rendering) requires the Core Text layer directly.


## When to Use


- You need glyph positions, advances, or bounding boxes that TextKit 2 hides
- You are doing custom typesetting or non-standard line breaking
- You need font table access or OpenType feature control
- You are rendering text with custom Core Graphics effects per glyph
- You need hit testing or caret positioning outside of a TextKit text container


## Quick Decision


```
Need glyph-level access?
    TextKit 1 available? → Use NSLayoutManager glyph APIs
    TextKit 2 only? → Drop to Core Text (this skill)


Need custom line breaking?
    → CTTypesetter


Need to draw text into a CGContext directly?
    → CTLine or CTFrame


Need font metrics, tables, or OpenType features?
    → CTFont


Need inline non-text elements with custom metrics?
    → CTRunDelegate
```


## Core Guidance


## Architecture


```
CTFramesetter (factory)
    → CTTypesetter (line breaking)
        → CTFrame (laid-out region)
            → CTLine (one visual line)
                → CTRun (contiguous glyphs, same attributes)
                    → CGGlyph[] (actual glyph IDs)
                    → CGPoint[] (positions)
                    → CGSize[]  (advances)
```


Core Text sits directly above Core Graphics. It is a C API using Core Foundation types. Available since iOS 3.2 / macOS 10.5. Both TextKit 1 and TextKit 2 render through Core Text internally — it is the foundation under both, not part of either.


**Thread safety:** Font objects (CTFont, CTFontDescriptor, CTFontCollection) are thread-safe and can be shared across threads. Layout objects (CTTypesetter, CTFramesetter, CTRun, CTLine, CTFrame) are **NOT thread-safe** — use them on a single thread only.


## CTLine — The Most Common Escape Hatch


For most TextKit developers, `CTLine` is the entry point. Create one from an attributed string to get glyph information:


```swift
let attributedString = NSAttributedString(string: "Hello 👋🏽",
    attributes: [.font: UIFont.systemFont(ofSize: 16)])
let line = CTLineCreateWithAttributedString(attributedString)


// Typographic bounds
var ascent: CGFloat = 0, descent: CGFloat = 0, leading: CGFloat = 0
let width = CTLineGetTypographicBounds(line, &ascent, &descent, &leading)
let height = ascent + descent + leading


// Hit testing: point → string index
let index = CTLineGetStringIndexForPosition(line, CGPoint(x: 50, y: 0))


// Caret positioning: string index → x offset
let offset = CTLineGetOffsetForStringIndex(line, 3, nil)


// Image bounds (actual rendered pixels, not typographic)
let ctx = UIGraphicsGetCurrentContext()!
let imageBounds = CTLineGetImageBounds(line, ctx)
```


## CTRun — Glyph-Level Access


Each `CTRun` is a contiguous sequence of glyphs sharing the same attributes:


```swift
let runs = CTLineGetGlyphRuns(line) as! [CTRun]


for run in runs {
    let glyphCount = CTRunGetGlyphCount(run)


    // Get all glyphs
    var glyphs = [CGGlyph](repeating: 0, count: glyphCount)
    CTRunGetGlyphs(run, CFRange(location: 0, length: glyphCount), &glyphs)


    // Get positions (relative to line origin)
    var positions = [CGPoint](repeating: .zero, count: glyphCount)
    CTRunGetPositions(run, CFRange(location: 0, length: glyphCount), &positions)


    // Get advances (width of each glyph)
    var advances = [CGSize](repeating: .zero, count: glyphCount)
    CTRunGetAdvances(run, CFRange(location: 0, length: glyphCount), &advances)


    // Map glyph indices back to string indices (UTF-16)
    var stringIndices = [CFIndex](repeating: 0, count: glyphCount)
    CTRunGetStringIndices(run, CFRange(location: 0, length: glyphCount), &stringIndices)


    // Get the run's attributes
    let attrs = CTRunGetAttributes(run) as! [NSAttributedString.Key: Any]
    let font = attrs[.font] as! CTFont
}
```


## CTFramesetter / CTFrame — Multi-Line Layout


For laying out text into a rectangular (or custom-shaped) region:


```swift
let framesetter = CTFramesetterCreateWithAttributedString(attributedString)


// Suggest frame size for constrained width
let constraint = CGSize(width: 300, height: .greatestFiniteMagnitude)
let suggestedSize = CTFramesetterSuggestFrameSizeWithConstraints(
    framesetter, CFRange(location: 0, length: 0), nil, constraint, nil)


// Create frame in a path
let path = CGPath(rect: CGRect(origin: .zero, size: suggestedSize), transform: nil)
let frame = CTFramesetterCreateFrame(framesetter, CFRange(location: 0, length: 0), path, nil)


// Get lines and origins
let lines = CTFrameGetLines(frame) as! [CTLine]
var origins = [CGPoint](repeating: .zero, count: lines.count)
CTFrameGetLineOrigins(frame, CFRange(location: 0, length: lines.count), &origins)


// Draw the entire frame
CTFrameDraw(frame, context)
```


## CTTypesetter — Custom Line Breaking


For control over where lines break:


```swift
let typesetter = CTTypesetterCreateWithAttributedString(attributedString)


var start: CFIndex = 0
let stringLength = CFAttributedStringGetLength(attributedString)


while start < stringLength {
    // Suggest line break for a given width
    let count = CTTypesetterSuggestLineBreak(typesetter, start, 300.0)


    // Or suggest with cluster breaking (for CJK)
    // let count = CTTypesetterSuggestClusterBreak(typesetter, start, 300.0)


    let line = CTTypesetterCreateLine(typesetter, CFRange(location: start, length: count))
    // Position and draw the line
    start += count
}
```


## CTFont — Font Metrics and Features


```swift
// Create from UIFont
let uiFont = UIFont.systemFont(ofSize: 16)
let ctFont = CTFontCreateWithName(uiFont.fontName as CFString, uiFont.pointSize, nil)


// Metrics
let ascent = CTFontGetAscent(ctFont)
let descent = CTFontGetDescent(ctFont)
let leading = CTFontGetLeading(ctFont)
let unitsPerEm = CTFontGetUnitsPerEm(ctFont)


// Get glyphs for characters
var characters: [UniChar] = Array("A".utf16)
var glyphs = [CGGlyph](repeating: 0, count: characters.count)
CTFontGetGlyphsForCharacters(ctFont, &characters, &glyphs, characters.count)


// Glyph bounding boxes
var boundingRects = [CGRect](repeating: .zero, count: glyphs.count)
CTFontGetBoundingRectsForGlyphs(ctFont, .default, glyphs, &boundingRects, glyphs.count)


// Glyph path (for custom rendering)
if let path = CTFontCreatePathForGlyph(ctFont, glyphs[0], nil) {
    // Draw the glyph outline
    context.addPath(path)
    context.fillPath()
}


// OpenType features
let features = CTFontCopyFeatures(ctFont) as? [[String: Any]] ?? []
```


## CTRunDelegate — Inline Custom Elements


Reserve space in a line for non-text content (images, custom views):


```swift
var callbacks = CTRunDelegateCallbacks(version: kCTRunDelegateCurrentVersion,
    dealloc: { _ in },
    getAscent: { _ in 20 },   // Height above baseline
    getDescent: { _ in 5 },    // Depth below baseline
    getWidth: { _ in 30 }      // Width of the space
)


let delegate = CTRunDelegateCreate(&callbacks, nil)!


let attrs: [NSAttributedString.Key: Any] = [
    kCTRunDelegateAttributeName as NSAttributedString.Key: delegate
]
let placeholder = NSAttributedString(string: "\u{FFFC}", attributes: attrs)


// Insert into your attributed string
// Then draw your custom content at the run's position after layout
```


## Critical: Coordinate System


**Core Text uses bottom-left origin (Core Graphics). UIKit uses top-left origin.**


### Drawing Core Text in UIKit


```swift
override func draw(_ rect: CGRect) {
    guard let context = UIGraphicsGetCurrentContext() else { return }


    // REQUIRED: Flip coordinate system for Core Text
    context.textMatrix = .identity
    context.translateBy(x: 0, y: bounds.height)
    context.scaleBy(x: 1, y: -1)


    // Now draw
    CTLineDraw(line, context)
    // or CTFrameDraw(frame, context)
}
```


**Forgetting the flip is the #1 Core Text mistake.** Text renders upside-down or at the wrong position.


### Converting Line Origins from CTFrame


`CTFrameGetLineOrigins` returns origins in Core Text coordinates (bottom-left). To use in UIKit:


```swift
let lines = CTFrameGetLines(frame) as! [CTLine]
var origins = [CGPoint](repeating: .zero, count: lines.count)
CTFrameGetLineOrigins(frame, CFRange(location: 0, length: lines.count), &origins)


for (i, line) in lines.enumerated() {
    // Flip y: UIKit y = frameHeight - CoreText y
    let uikitY = frameRect.height - origins[i].y
    // uikitY is now the BASELINE position in UIKit coordinates
}
```


## Bridging TextKit ↔ Core Text


### TextKit 2: Getting Glyph Info from a Layout Fragment


```swift
// In a custom NSTextLayoutFragment or delegate callback:
let attributedString = textElement.attributedString
let line = CTLineCreateWithAttributedString(attributedString as CFAttributedString)
let runs = CTLineGetGlyphRuns(line) as! [CTRun]


for run in runs {
    let glyphCount = CTRunGetGlyphCount(run)
    var positions = [CGPoint](repeating: .zero, count: glyphCount)
    CTRunGetPositions(run, CFRange(location: 0, length: glyphCount), &positions)


    // positions are relative to the line origin
    // Add layoutFragmentFrame.origin to get document coordinates
    // Remember to handle the coordinate flip if drawing in UIKit
}
```


### Font Bridging


```swift
// macOS: NSFont ↔ CTFont is toll-free bridged
let ctFont = nsFont as CTFont
let nsFont = ctFont as NSFont


// iOS: UIFont ↔ CTFont is NOT toll-free bridged
// UIFont → CTFont
let ctFont = CTFontCreateWithName(uiFont.fontName as CFString, uiFont.pointSize, nil)


// CTFont → UIFont
let uiFont = UIFont(name: CTFontCopyPostScriptName(ctFont) as String,
                     size: CTFontGetSize(ctFont))!


// NSAttributedString ↔ CFAttributedString IS toll-free bridged (both platforms)
let cfAttrStr = attributedString as CFAttributedString
```


### Attribute Key Differences


Core Text uses its own attribute keys that differ from UIKit/AppKit:


| Purpose | Core Text Key | UIKit/AppKit Key |
|---------|--------------|-----------------|
| Font | `kCTFontAttributeName` (CTFont) | `.font` (UIFont/NSFont) |
| Foreground color | `kCTForegroundColorAttributeName` (CGColor) | `.foregroundColor` (UIColor/NSColor) |
| Paragraph style | `kCTParagraphStyleAttributeName` (CTParagraphStyle) | `.paragraphStyle` (NSParagraphStyle) |
| Kern | `kCTKernAttributeName` | `.kern` |


**When creating attributed strings for Core Text directly, use the `kCT*` keys.** Mixing Core Text keys and UIKit keys in the same attributed string can cause subtle rendering differences.


In practice, UIKit's `.font` attribute (UIFont) works with Core Text because UIFont wraps a CTFont internally. But `.foregroundColor` (UIColor) does NOT — Core Text needs CGColor.


## Common Pitfalls


1. **Forgetting to flip coordinates** — Core Text is bottom-left origin. UIKit is top-left. Text appears upside-down or at wrong position. Always set `context.textMatrix = .identity` and flip.
2. **Not resetting text matrix** — `CGContext.textMatrix` persists between drawing calls. If a previous operation set a non-identity matrix, your Core Text drawing will be transformed unexpectedly.
3. **String indices are UTF-16** — `CTRunGetStringIndices` returns UTF-16 code unit indices (matching NSString), not Swift Character indices. A single emoji can span 2-4 UTF-16 units.
4. **CTFont ≠ UIFont on iOS** — They are NOT toll-free bridged on iOS. Create CTFont explicitly.
5. **CTFrameGetLines returns non-retained array** — In Swift this is usually managed automatically, but be careful with the CFArray if you bridge to C.
6. **Attribute key mismatch** — `kCTForegroundColorAttributeName` expects CGColor, not UIColor. Passing UIColor silently fails (no color rendered).
7. **Character-glyph mapping is not 1:1** — Ligatures produce fewer glyphs than characters. Complex scripts (Arabic, Devanagari) can produce more glyphs than characters. Always use `CTRunGetStringIndices` for the mapping.
8. **CTParagraphStyle is not NSParagraphStyle** — They are related but not interchangeable. CTParagraphStyle uses a C struct API; NSParagraphStyle has Objective-C properties. NSParagraphStyle internally wraps CTParagraphStyle.


## Related Skills


- Use `/skill apple-text-textkit2-ref` for the TextKit 2 APIs that sit above Core Text.
- Use `/skill apple-text-textkit1-ref` when NSLayoutManager's glyph APIs are sufficient (no need to drop lower).
- Use `/skill apple-text-viewport-rendering` for how Core Text fits into the rendering pipeline.
- Use `/skill apple-text-attachments-ref` when CTRunDelegate is used for inline non-text content.
Apple Text docs for TextKit, Apple text views, and Writing Tools. Source on GitHub.
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