sitapix/txt-attributed-string

AttributedString and NSAttributedString

Authored against iOS 26.x / Swift 6.x / Xcode 26.x.

This skill is the picker between Swift's AttributedString (value type, type-safe attributes, Codable, Markdown-aware) and Foundation's older NSAttributedString (reference type, untyped key/value dictionary, required by every UIKit/AppKit text API and TextKit). It also covers defining custom attribute keys, building an AttributeScope, and converting cleanly across the boundary. The Foundation team has expanded AttributedString in every recent release — added scopes, refined Codable behavior, gained Markdown attributes — so before claiming a specific scope, key protocol, or conversion overload is current, fetch the canonical entry via Sosumi (sosumi.ai/documentation/foundation/attributedstring).

The model decision is rarely about preference; it is about the destination. SwiftUI Text consumes AttributedString directly. UIKit attributedText, AppKit attributedString, and every TextKit storage class consume NSAttributedString. Cross that boundary and you must convert — and conversion silently drops attributes that aren't in the scope you specify. Most "my custom highlight disappeared" bugs trace to a missing including: argument.

Contents

• Picking AttributedString vs NSAttributedString
• AttributedString basics
• Runs and views
• Index invalidation
• NSAttributedString basics
• Conversion across the boundary
• Custom attributes and scopes
• Storage backing for production editors
• Common Mistakes
• References

Picking AttributedString vs NSAttributedString

AttributedString is a Swift value type with type-safe attributes accessed via key paths (str.font, str[range].foregroundColor). It is Codable, parses Markdown, and is what SwiftUI Text expects. Mutation is in-place on a var — there is no separate "mutable" subtype.

NSAttributedString is a Foundation reference type with a [NSAttributedString.Key: Any] dictionary. It is what UIKit, AppKit, and TextKit have consumed since 10.4 / iOS 3.2, and it is the only attributed type that survives an RTF or RTFD round-trip without custom encoding. Mutation requires NSMutableAttributedString.

The destination usually picks the model:

• SwiftUI Text(_:) consumes AttributedString
• UIKit UILabel.attributedText, UITextView.attributedText consume NSAttributedString
• AppKit NSTextField.attributedStringValue, NSTextView.textStorage consume NSAttributedString
• TextKit (1 and 2) — NSTextStorage is itself an NSMutableAttributedString subclass
• Core Text — CFAttributedString, toll-free bridged to NSAttributedString

A practical rule for new code: keep an AttributedString as the in-memory model, convert to NSAttributedString at UIKit/AppKit/TextKit boundaries, and define an AttributeScope that includes the standard scopes plus any custom keys so conversions round-trip cleanly.

AttributedString basics

var str = AttributedString("Hello World")
str.font = .body
str.foregroundColor = .red

if let range = str.range(of: "World") {
    str[range].font = .body.bold()
    str[range].link = URL(string: "https://example.com")
}

Concatenation with + is value-type-safe — neither operand is mutated:

var greeting = AttributedString("Hello ")
greeting.font = .body

var name = AttributedString("World")
name.font = .body.bold()
name.foregroundColor = .blue

let combined = greeting + name

An AttributeContainer lets you build a bag of attributes once and merge it into multiple ranges:

var emphasis = AttributeContainer()
emphasis.font = .body.bold()
emphasis.foregroundColor = .accentColor

str.mergeAttributes(emphasis)            // entire string
str[range].mergeAttributes(emphasis)     // sub-range

Markdown parsing is built in. The default option parses block-level structure into presentationIntent; the inline-only option is safer for user-generated short strings:

let inline = try AttributedString(
    markdown: "Visit [Apple](https://apple.com) for **details**.",
    options: .init(interpretedSyntax: .inlineOnlyPreservingWhitespace)
)

Markdown rendering semantics — what SwiftUI actually shows, how presentationIntent maps to UITextView paragraph styles — belong in the txt-markdown skill.

Runs and views

A run is a maximal contiguous range of identical attributes. Iterating runs is the standard way to walk an attributed string:

for run in str.runs {
    let substring = str[run.range]
    let font = run.font
    let color = run.foregroundColor
}

Runs can be filtered by attribute, which is the cleanest way to find all spans with a particular value:

for (color, range) in str.runs[\.foregroundColor] {
    print("Color: \(String(describing: color)) at \(range)")
}

For character-level iteration that doesn't care about attributes, AttributedString exposes the same view types as String: characters, unicodeScalars, utf8, utf16. These are useful when bridging to UTF-16-based APIs (anything that takes NSRange).

Index invalidation

Mutating an AttributedString invalidates every existing index and range derived from it. This is the single most common pitfall and the one that catches refactors:

// WRONG — `range` is invalidated by the replaceSubrange call below
var str = AttributedString("Hello World")
let range = str.range(of: "World")!
str.replaceSubrange(str.range(of: "Hello")!, with: AttributedString("Hi"))
str[range].font = .body.bold()  // crash or silent corruption

// CORRECT — re-derive the range after each mutation
var str = AttributedString("Hello World")
str.replaceSubrange(str.range(of: "Hello")!, with: AttributedString("Hi"))
if let range = str.range(of: "World") {
    str[range].font = .body.bold()
}

This is identical to Swift String index behavior. Treat any AttributedString.Index or Range<AttributedString.Index> as scoped to the mutation that produced it.

NSAttributedString basics

let attrs: [NSAttributedString.Key: Any] = [
    .font: UIFont.systemFont(ofSize: 16),
    .foregroundColor: UIColor.label,
    .kern: 1.5,
]
let str = NSAttributedString(string: "Hello", attributes: attrs)

Mutation requires NSMutableAttributedString:

let mutable = NSMutableAttributedString(string: "Hello World")

mutable.addAttribute(.font, value: UIFont.boldSystemFont(ofSize: 16),
                     range: NSRange(location: 6, length: 5))

mutable.replaceCharacters(in: NSRange(location: 0, length: 5), with: "Hi")

Enumeration walks runs in NSRange terms:

let full = NSRange(location: 0, length: str.length)

str.enumerateAttributes(in: full) { attrs, range, _ in
    if let font = attrs[.font] as? UIFont { /* ... */ }
}

str.enumerateAttribute(.foregroundColor, in: full) { value, range, _ in
    if let color = value as? UIColor { /* ... */ }
}

NSTextStorage is itself an NSMutableAttributedString subclass, so anything that mutates an attributed string applies to text storage as well — but storage mutation has its own lifecycle (processEditing, edited(_:range:changeInLength:)) covered in txt-nstextstorage.

Conversion across the boundary

The two type initializers carry an including: parameter that selects which AttributeScope to translate. Without it, conversion uses the default Foundation scope and silently drops anything outside it — including custom attributes.

// AttributedString → NSAttributedString
let nsAS = NSAttributedString(swiftAS)                       // Foundation only
let nsAS = try NSAttributedString(swiftAS, including: \.myApp) // custom scope

// NSAttributedString → AttributedString
let swiftAS = try AttributedString(nsAS, including: \.foundation)
let swiftAS = try AttributedString(nsAS, including: \.myApp)

For round-trip code, define one scope that combines Foundation, the platform UI scope, and any app-specific keys, and use it for both directions. See Custom attributes and scopes.

Behavioral notes that catch people:

• URL and String are interchangeable for .link in NSAttributedString but AttributedString.link is typed URL?. A String link converted from NS doesn't survive cleanly — wrap it in URL(string:) first.
• NSParagraphStyle carries over as a single value; sub-paragraph splits remain stuck to whole paragraphs after fixAttributes.
• NSTextAttachment round-trips only if both ends understand the same attachment type. Image-only attachments are fine; view-provider attachments lose the live view (the attachment data survives).

Custom attributes and scopes

A custom attribute key is a type that conforms to AttributedStringKey, plus optional protocols for Codable storage and Markdown parsing. The name is the string key under which the value is stored in NSAttributedString representations.

enum HighlightAttribute: AttributedStringKey {
    typealias Value = Bool
    static let name = "com.example.highlight"
}

// For Codable serialization round-tripping
enum HighlightAttribute2: CodableAttributedStringKey {
    typealias Value = Bool
    static let name = "com.example.highlight"
}

// For ^[text](key: value) Markdown syntax
enum HighlightAttribute3: CodableAttributedStringKey, MarkdownDecodableAttributedStringKey {
    typealias Value = Bool
    static let name = "com.example.highlight"
}

To make the attribute usable via key path (str.highlight = true), wrap it in an AttributeScope and extend AttributeDynamicLookup:

extension AttributeScopes {
    struct MyAppAttributes: AttributeScope {
        let highlight: HighlightAttribute
        // include the standard scopes you want round-tripped:
        let foundation: FoundationAttributes
        let swiftUI: SwiftUIAttributes
    }

    var myApp: MyAppAttributes.Type { MyAppAttributes.self }
}

extension AttributeDynamicLookup {
    subscript<T: AttributedStringKey>(
        dynamicMember keyPath: KeyPath<AttributeScopes.MyAppAttributes, T>
    ) -> T { self[T.self] }
}

Now str.highlight = true and str[range].highlight = true work, and conversions that pass including: \.myApp carry custom values cleanly:

var str = AttributedString("Read this")
if let range = str.range(of: "this") {
    str[range].highlight = true
}

let nsAS = try NSAttributedString(str, including: \.myApp)
// ...UIKit code path...
let roundTripped = try AttributedString(nsAS, including: \.myApp)

A scope that doesn't include FoundationAttributes will strip font, color, and link on conversion. The standard scopes worth including by default: FoundationAttributes, SwiftUIAttributes (when targeting SwiftUI), and UIKitAttributes or AppKitAttributes (for the platform UI). Availability differs by platform: UIKitAttributes is iOS, tvOS, watchOS, visionOS, and Mac Catalyst — not native macOS. AppKitAttributes is macOS-only. A cross-platform editor that picks the wrong scope per build target will silently strip the platform's UI attributes. Use #if canImport(UIKit) / #if canImport(AppKit) to gate scope composition.

Storage backing for production editors

The on-disk representation of a document and the in-memory model are different decisions. Shipping editors that handle real document workloads — Bear, iA Writer, Drafts, Pretext, Runestone — keep the document on disk in plain UTF-8 (or Markdown) and recompute attributes on load. They do not store an NSAttributedString, an AttributedString, or a Codable archive of either as the document. Ulysses uses Markdown XL serialization for the same reason. The persistent format is plain text; the attributed representation is a derived view of it.

The reason is that attributed-string persistence loses information. RTF and RTFD carry a Cocoa-specific subset of attributes — anything outside RTF's vocabulary is dropped (the per-key survival list is in txt-attribute-keys). HTML import requires a hidden WebKit parser that drags in main-thread cost and brittle CSS interpretation. NSKeyedArchiver of an NSAttributedString (or Codable encoding of an AttributedString with platform scopes) ties the document to whatever the local app and OS understand today; cross-version reads degrade silently when an attribute moves, a key is renamed, or a custom scope changes. Diff and merge tools have no insight into the binary blob, so cross-device sync, version control, and document recovery all become format-specific tooling problems.

Plain text plus a reproducible attribute pipeline avoids those costs. The document is git-diffable, search-indexable, and trivially mergeable. The attribute layer is regenerated from the source on each load — Markdown parsed into presentationIntent, syntax highlighting applied from a tree-sitter parse, link detection run from a single pass. If the attribute pipeline changes, every existing document picks up the new behavior automatically; no migration runs.

Use AttributedString (or NSTextStorage's NSMutableAttributedString content) as the in-memory model and the editor view's source of truth for live editing. Persist the document as plain text or Markdown. The boundary at save/load is where the attributed model is rebuilt — typically AttributedString(markdown:) or a custom highlighter — not where it is serialized verbatim. Code editors take this further by treating NSTextStorage as a view-side cache backed by a separate document data structure (see txt-textkit-choice for the storage architecture variants).

NSAttributedString archives still have a place — pasteboard payloads, undo snapshots, user-visible "rich-text export" features. They do not have a place as the canonical document.

Common Mistakes

1. Conversion without including: drops custom attributes. Both NSAttributedString(swiftAS) and AttributedString(nsAS) use the default Foundation scope. Custom keys (and platform UI keys) silently disappear. The fix is always to specify the scope: including: \.myApp.

2. Reusing an index across a mutation. Once replaceSubrange, insert, or any structural change runs, every previously captured index/range is invalid. Re-derive ranges from str.range(of:) after the mutation, or perform mutations inside an iteration that captures fresh ranges.

3. Trying to mutate an NSParagraphStyle after assignment. It's immutable. Configure NSMutableParagraphStyle first, then set it as the attribute value. After it's stored in an attributed string, treat it as frozen.

4. AttributedString.link accepts only URL. NSAttributedString accepts URL or String for .link; AttributedString.link is typed URL?. Strings round-tripped from NS need to be wrapped.

   // WRONG — Swift compiler will reject; in dynamic Any-based code, won't render
   str.link = "https://apple.com"
   
   // CORRECT
   str.link = URL(string: "https://apple.com")
   

5. Forgetting that NSTextStorage is NSMutableAttributedString. No conversion is needed to apply attributes to a TextKit storage — it already supports all NSMutableAttributedString APIs. The attributedText property on UITextView is a copy of the storage's contents at access time; mutating it doesn't mutate storage.

6. Custom scope without standard scopes. A scope that only includes the app's custom attributes will strip .font, .foregroundColor, .link, etc. on conversion. Compose with FoundationAttributes plus the platform UI scope.

7. Using String ranges with NSAttributedString. Bridging requires NSRange in UTF-16 units. NSRange(swiftRange, in: text) is the correct converter; text.distance(from: ...) is wrong.

References

• references/latest-apis.md — current Apple API surface refreshed against Sosumi (signatures, since-annotations, scope membership)
• references/advanced-patterns.md — custom-attribute walkthrough, paragraph-style catalog, RTF/HTML/Codable persistence matrix
• txt-attribute-keys — full catalog of NSAttributedString.Key values, value types, and view compatibility
• txt-markdown — Markdown parsing in AttributedString, presentationIntent, custom Markdown attributes
• txt-swiftui-interop — SwiftUI/TextKit boundary rules for which attributes survive
• txt-nstextstorage — NSTextStorage subclassing and the editing lifecycle
• AttributedString
• NSAttributedString
• AttributeScope
• AttributeContainer
• MarkdownDecodableAttributedStringKey