textkit

name	textkit
description	TextKit 2 patterns for text layout, editing, and rendering including NSTextLayoutManager, NSTextContentStorage, custom rendering, and practical workarounds from STTextView. Use when building text editors, custom text views, or working with TextKit 2 APIs.

First step: Tell the user: "textkit skill loaded."

TextKit 2

TextKit 2 is Apple's modern text layout and rendering framework, introduced at WWDC 2021 as a replacement for TextKit 1 (the NSLayoutManager-based stack). It provides viewport-based layout, non-contiguous text support, and a more composable architecture. However, it has significant rough edges — STTextView (a production TextKit 2 text editor) has filed 20+ bug reports with Apple.

This skill covers both the API and the practical reality of working with TextKit 2, including known limitations and workarounds.

When This Skill Activates

• User is building a custom text view or text editor
• User asks about NSTextLayoutManager, NSTextContentStorage, or TextKit 2
• User needs custom text rendering (line numbers, annotations, highlights)
• User is migrating from TextKit 1 (NSLayoutManager) to TextKit 2
• User encounters TextKit 2 bugs or unexpected behavior
• User asks about STTextView or custom text editing components
• User needs viewport-based text layout for large documents
• User wants to customize text fragment rendering

Decision Tree

What text feature do you need?
|
+-- Display styled text (no editing)
|   +-- Simple? Use SwiftUI Text or NSAttributedString.draw(in:)
|   +-- Glyph-level control? Use CoreText (CTFrame/CTLine)
|
+-- Editable text field
|   +-- Standard text input? Use TextField / TextEditor
|   +-- Rich text editing? Use NSTextView / UITextView
|
+-- Custom text editor (code editor, annotations, etc.)
|   +-- Use TextKit 2 directly (NSTextLayoutManager stack)
|   +-- Or use STTextView as a foundation
|
+-- Need TextKit 1 compatibility
    +-- Use NSLayoutManager (still supported, not deprecated)

API Availability

API	iOS	macOS	Notes
NSTextLayoutManager	15.0+	12.0+	Core TextKit 2 layout engine
NSTextContentManager	15.0+	12.0+	Abstract content protocol
NSTextContentStorage	15.0+	12.0+	Concrete backing store
NSTextLayoutFragment	15.0+	12.0+	Rendered text fragment
NSTextElement	15.0+	12.0+	Abstract text unit
NSTextParagraph	15.0+	12.0+	Paragraph-level element
NSTextViewportLayoutController	15.0+	12.0+	Viewport-based layout
NSTextRange	15.0+	12.0+	Range in text content
NSTextLocation	15.0+	12.0+	Position in text content
NSTextSelectionNavigation	15.0+	12.0+	Selection management

Architecture Overview

TextKit 2 separates concerns into distinct layers:

NSTextContentStorage (model: stores attributed string)
        |
NSTextLayoutManager (layout: arranges text into fragments)
        |
NSTextLayoutFragment (rendering: visual representation of a paragraph)
        |
NSTextViewportLayoutController (viewport: manages visible region)
        |
View (NSTextView / UITextView / custom view)

Key Difference from TextKit 1

TextKit 1 (NSLayoutManager) performs contiguous layout — it lays out all text from the beginning. TextKit 2 uses viewport-based layout — only visible text is laid out, enabling efficient rendering of very large documents.

Setting Up the TextKit 2 Stack

Manual Stack (Without NSTextView)

import UIKit

// 1. Content storage (model)
let textContentStorage = NSTextContentStorage()
textContentStorage.attributedString = NSAttributedString(
    string: "Hello, TextKit 2!",
    attributes: [.font: UIFont.systemFont(ofSize: 16)]
)

// 2. Layout manager
let textLayoutManager = NSTextLayoutManager()
textLayoutManager.textContainer = NSTextContainer(size: CGSize(width: 300, height: 0))
textLayoutManager.textContainer?.lineFragmentPadding = 5

// 3. Connect them
textContentStorage.addTextLayoutManager(textLayoutManager)

Using with NSTextView (macOS)

import AppKit

let textView = NSTextView(usingTextLayoutManager: true)
// Access the TextKit 2 stack:
let layoutManager = textView.textLayoutManager!
let contentStorage = layoutManager.textContentManager as! NSTextContentStorage

Using with UITextView (iOS)

import UIKit

// UITextView uses TextKit 2 by default on iOS 16+
let textView = UITextView()
let layoutManager = textView.textLayoutManager!

NSTextContentStorage

The model layer storing the text as NSAttributedString.

Reading and Writing Text

let contentStorage = NSTextContentStorage()

// Set text
contentStorage.attributedString = NSAttributedString(string: "Hello")

// Read text
let fullText = contentStorage.attributedString?.string ?? ""

// Replace a range
let documentRange = contentStorage.documentRange
if let start = contentStorage.location(documentRange.location, offsetBy: 5),
   let end = contentStorage.location(start, offsetBy: 3) {
    let range = NSTextRange(location: start, end: end)
    contentStorage.replaceContents(
        in: range,
        with: [NSAttributedString(string: "replacement")]
    )
}

Known Issue: replaceContents and Multi-Cursor

STTextView discovered that NSTextLayoutManager._fixSelectionAfterChangeInCharacterRange unexpectedly resets text selections after replaceContents, breaking multi-cursor editing (FB9925647). STTextView works around this by subclassing NSTextContentStorage and manually deduplicating/restoring selections after replacement.

NSTextLayoutManager

The layout engine that arranges text into fragments.

Enumerating Layout Fragments

let layoutManager: NSTextLayoutManager = // ...

// Enumerate all visible fragments
layoutManager.enumerateTextLayoutFragments(
    from: layoutManager.documentRange.location,
    options: [.ensuresLayout, .ensuresExtraLineFragment]
) { fragment in
    let frame = fragment.layoutFragmentFrame
    let paragraphRange = fragment.rangeInElement
    print("Fragment at \(frame), range: \(paragraphRange)")
    return true // continue enumeration
}

Getting Fragment for a Location

if let fragment = layoutManager.textLayoutFragment(for: location) {
    let frame = fragment.layoutFragmentFrame
    // Use frame for positioning UI elements (annotations, line numbers, etc.)
}

Text Selections

// Get current selections
let selections = layoutManager.textSelections

// Set selections programmatically
let selection = NSTextSelection(range: range, affinity: .downstream, granularity: .character)
layoutManager.textSelections = [selection]

Known Issue: Selection Notifications

TextKit 2 does not provide consistent cross-platform selection change notifications. STTextView works around this by subclassing NSTextLayoutManager and posting a custom notification in the textSelections didSet observer.

NSTextLayoutFragment

Each fragment represents the visual rendering of a text paragraph.

Custom Fragment Rendering

Subclass NSTextLayoutFragment for custom drawing (syntax highlighting overlays, annotations, etc.):

class CustomTextLayoutFragment: NSTextLayoutFragment {
    override func draw(at point: CGPoint, in context: CGContext) {
        // Draw the standard text first
        super.draw(at: point, in: context)

        // Add custom overlays
        context.saveGState()
        context.setFillColor(NSColor.yellow.withAlphaComponent(0.3).cgColor)
        context.fill(CGRect(x: point.x, y: point.y, width: renderingSurfaceBounds.width, height: renderingSurfaceBounds.height))
        context.restoreGState()
    }
}

Register the custom fragment via NSTextLayoutManagerDelegate:

func textLayoutManager(
    _ textLayoutManager: NSTextLayoutManager,
    textLayoutFragmentFor location: NSTextLocation,
    in textElement: NSTextElement
) -> NSTextLayoutFragment {
    CustomTextLayoutFragment(textElement: textElement, range: textElement.elementRange)
}

Known Issue: Extra Line Fragments

TextKit 2 may create unexpected additional line fragments at the end of documents or after certain attribute changes. STTextView filed multiple reports about this (FB9856587, FB10901256). Workaround: filter out zero-height or anomalous fragments when enumerating.

NSTextViewportLayoutController

Manages which text is laid out based on the visible viewport.

Viewport Layout Delegate

class MyViewportDelegate: NSObject, NSTextViewportLayoutControllerDelegate {
    func viewportBounds(for textViewportLayoutController: NSTextViewportLayoutController) -> CGRect {
        // Return the visible rect of your scroll view
        return scrollView.documentVisibleRect
    }

    func textViewportLayoutControllerWillLayout(
        _ textViewportLayoutController: NSTextViewportLayoutController
    ) {
        // Prepare for layout pass (e.g., remove stale fragment views)
    }

    func textViewportLayoutController(
        _ textViewportLayoutController: NSTextViewportLayoutController,
        configureRenderingSurfaceFor textLayoutFragment: NSTextLayoutFragment
    ) {
        // Add/update the fragment's view in your text container view
        let fragmentView = fragmentViewMap[textLayoutFragment] ?? createFragmentView(for: textLayoutFragment)
        fragmentView.frame = textLayoutFragment.layoutFragmentFrame
    }

    func textViewportLayoutControllerDidLayout(
        _ textViewportLayoutController: NSTextViewportLayoutController
    ) {
        // Finalize layout (update scroll content size, etc.)
    }
}

Building a Custom Text View

A minimal custom text view using TextKit 2 directly (the approach STTextView takes):

import AppKit

class MinimalTextView: NSView {
    let textContentStorage = NSTextContentStorage()
    let textLayoutManager = NSTextLayoutManager()
    private let textContainer = NSTextContainer()

    override init(frame: NSRect) {
        super.init(frame: frame)

        textContainer.size = NSSize(width: frame.width, height: 0)
        textContainer.lineFragmentPadding = 5
        textLayoutManager.textContainer = textContainer
        textContentStorage.addTextLayoutManager(textLayoutManager)

        textContentStorage.attributedString = NSAttributedString(
            string: "Hello, custom TextKit 2 view!",
            attributes: [
                .font: NSFont.monospacedSystemFont(ofSize: 14, weight: .regular),
                .foregroundColor: NSColor.textColor
            ]
        )
    }

    required init?(coder: NSCoder) { fatalError() }

    override func draw(_ dirtyRect: NSRect) {
        super.draw(dirtyRect)
        guard let context  .current.cgContext  {  }

        textLayoutManager.enumerateTextLayoutFragments(
            from: textLayoutManager.documentRange.location,
            options: [.ensuresLayout]
        ) { fragment 
            fragment.draw(at: fragment.layoutFragmentFrame.origin, in: context)
             
        }
    }
}

Line Numbers (Gutter)

A common text editor feature. Use layout fragment enumeration to align line numbers:

func drawLineNumbers(in context: CGContext, gutterWidth: CGFloat) {
    var lineNumber = 1

    textLayoutManager.enumerateTextLayoutFragments(
        from: textLayoutManager.documentRange.location,
        options: [.ensuresLayout]
    ) { fragment in
        let frame = fragment.layoutFragmentFrame
        let lineNumberString = "\(lineNumber)" as NSString

        let attributes: [NSAttributedString.Key: Any] = [
            .font: NSFont.monospacedDigitSystemFont(ofSize: 12, weight: .regular),
            .foregroundColor: NSColor.secondaryLabelColor
        ]

        let size = lineNumberString.size(withAttributes: attributes)
        let origin = CGPoint(
            x: gutterWidth - size.width - 4,
            y: frame.origin.y + (frame.height - size.height) / 2
        )
        lineNumberString.draw(at: origin, withAttributes: attributes)

        lineNumber += 1
        return true
    }
}

Hit Testing and Caret Placement

// Point → text location
func textLocation(for point: CGPoint) -> NSTextLocation? {
    // Adjust point to text container coordinates
    let adjustedPoint = CGPoint(x: point.x - gutterWidth, y: point.y)

    // Find the nearest text location
    let fraction = UnsafeMutablePointer<CGFloat>.allocate(capacity: 1)
    defer { fraction.deallocate() }

    guard let location = textLayoutManager.location(
        interactingAt: adjustedPoint,
        inContainerAt: textLayoutManager.documentRange.location
    ) else { return nil }

    return location
}

// Text location → point (for drawing caret)
func caretRect(for location: NSTextLocation) -> CGRect? {
    guard let fragment = textLayoutManager.textLayoutFragment(for: location) else {
        return nil
    }

    // Get the line fragment for precise positioning
    let selectionNav = textLayoutManager.textSelectionNavigation
    // Use selection geometry for precise caret rectangles
    let selectionRects = textLayoutManager.enumerateRenderingAttributes(
        from: location, reverse: 
    ) { ,     }

      
}

Migration from TextKit 1

TextKit 1	TextKit 2	Notes
NSLayoutManager	NSTextLayoutManager	Different delegate methods
NSTextStorage	NSTextContentStorage	Wraps NSAttributedString
NSTextContainer	NSTextContainer	Same class, shared
layoutManager(_:lineFragmentRectFor:...)	NSTextLayoutFragment subclass	Custom layout
NSLayoutManager.enumerateLineFragments	enumerateTextLayoutFragments	Viewport-aware
NSRange	NSTextRange / NSTextLocation	Opaque locations, not integers
Glyph-level access	Not available	Use CoreText for glyph access
Contiguous layout	Viewport-based layout	Major architecture change

Critical Migration Note

TextKit 2 does not expose glyph-level APIs. If you need individual glyph positions, advances, or custom glyph rendering, you must drop down to CoreText (CTLine/CTRun). This is a deliberate design choice — TextKit 2 abstracts away glyphs to support complex script shaping. See the coretext skill for glyph-level work.

STTextView: Lessons from Production TextKit 2

STTextView is a production text editor built on TextKit 2 for macOS and iOS. Key architectural lessons:

Architecture Pattern

STTextView separates concerns through a modular structure:

• STTextView — main coordinator (input, delegates, public API)
• STTextContainerView — renders text fragments and insertion points
• STSelectionView — selection highlight overlays
• STGutterView — line numbers and markers
• STLineHighlightView — current line highlighting
• Functionality split across extensions (accessibility, copy/paste, find, undo, etc.)

Plugin System

STTextView uses a plugin architecture for extensibility:

protocol STPlugin {
    func setUp(context: STPluginContext)
    func tearDown()
}

// Plugins receive context to interact with the text view
// Available plugins: syntax highlighting (TreeSitter), text formation, annotations
textView.addPlugin(myPlugin)

Key Workarounds (from 20+ Apple bug reports)

Issue	Description	Workaround
Selection reset (FB9925647)	replaceContents resets multi-cursor selections	Subclass NSTextContentStorage, deduplicate selections after replacement
Long line jumping	Text view jumps during attribute updates on long lines	Custom viewport layout controller management
Missing APIs	replaceContents() documented but unavailable in some contexts	Direct NSAttributedString manipulation + manual notification
Extra line fragments (FB10901256)	Unexpected fragments at document end	Filter anomalous zero-height fragments
Chinese IME (FB13789916)	Marked text provides bogus selection ranges	Custom NSTextInputClient implementation
Selection navigation bugs	Edge cases in cursor movement	Override NSTextSelectionNavigation behavior
Insertion point not drawn	drawInsertionPoint not called reliably	Custom insertion point view (STInsertionPointView)

When to Use STTextView vs Building Custom

• Use STTextView when: building a code editor, need line numbers + syntax highlighting + multi-cursor, want production-tested TextKit 2 workarounds
• Build custom when: need very specific text rendering (e.g., chat bubbles, attributed label), don't need full editing, want to avoid GPL license dependency

Patterns

✅ Use TextKit 2 for new text editor projects

// TextKit 2 is the future — viewport-based layout scales to large documents
let textView = NSTextView(usingTextLayoutManager: true)

❌ Don't start new projects on TextKit 1 unless you need glyph access

// TextKit 1 still works but is not receiving new features
let textView = NSTextView(usingTextLayoutManager: false)

✅ Subclass TextKit 2 components to work around bugs

// STTextView's approach: strategic subclassing, not full replacement
class STTextContentStorage: NSTextContentStorage {
    override func replaceContents(in range: NSTextRange, with textElements: [NSTextElement]) {
        super.replaceContents(in: range, with: textElements)
        fixSelectionAfterReplacement() // work around FB9925647
    }
}

❌ Don't assume TextKit 2 APIs work exactly as documented

// Always test edge cases — documented methods may have subtle bugs
// Especially: selection navigation, long lines, IME input, extra line fragments

✅ Use the plugin pattern for extensible text editors

// Separate syntax highlighting, annotations, completions into plugins
textView.addPlugin(SyntaxHighlightPlugin())
textView.addPlugin(AnnotationPlugin())

❌ Don't put all editor features in a single massive text view subclass

// This becomes unmaintainable — STTextView uses ~15 extension files to organize
class MyTextView: NSTextView {
    // 3000 lines of mixed concerns
}

✅ Drop to CoreText for glyph-level rendering

// TextKit 2 deliberately hides glyphs — use CoreText when you need them
let line = CTLineCreateWithAttributedString(attributedString)
let runs = CTLineGetGlyphRuns(line) as! [CTRun]

❌ Don't try to access glyphs through TextKit 2

// TextKit 2 has no glyph API — this is by design, not an omission
// There is no equivalent of NSLayoutManager.glyph(at:)