feat(ios): add markdown theme system

packages/ios-enriched-markdown/Sources/EnrichedMarkdown/Theme/DefaultMarkdownTheme.swift

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+import SwiftUI
+
+private typealias Semantic = ThemeColorSpec.SemanticColor
+
+enum DefaultMarkdownTheme {
+    static func make() -> MarkdownTheme {
+        MarkdownTheme {
+            Paragraph()
+                .font(.body)
+                .foregroundStyle(Semantic.primary)
+                .lineHeight(26)
+                .marginBottom(16)
+
+            Strong()
+            Emphasis()
+        }
+    }
+}

packages/ios-enriched-markdown/Sources/EnrichedMarkdown/Theme/Elements/Emphasis.swift

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+import SwiftUI
+
+public struct Emphasis: MarkdownThemeElement {
+    public var fontSpec: ThemeFontSpec?
+    public var fontWeight: Font.Weight?
+    public var fontDesign: Font.Design?
+    public var foregroundColorSpec: ThemeColorSpec?
+    public var marginTop: CGFloat?
+    public var marginBottom: CGFloat?
+    public var lineHeight: CGFloat?
+    public var textAlignment: TextAlignment?
+
+    public init() {}
+
+    public func apply(to config: inout MarkdownStyleConfig, traitCollection: UITraitCollection) {
+        applyElementStyle(to: &config.emphasis, traitCollection: traitCollection)
+    }
+}

packages/ios-enriched-markdown/Sources/EnrichedMarkdown/Theme/Elements/Paragraph.swift

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+import SwiftUI
+
+public struct Paragraph: MarkdownThemeElement {
+    public var fontSpec: ThemeFontSpec?
+    public var fontWeight: Font.Weight?
+    public var fontDesign: Font.Design?
+    public var foregroundColorSpec: ThemeColorSpec?
+    public var marginTop: CGFloat?
+    public var marginBottom: CGFloat?
+    public var lineHeight: CGFloat?
+    public var textAlignment: TextAlignment?
+
+    public init() {}
+
+    public func apply(to config: inout MarkdownStyleConfig, traitCollection: UITraitCollection) {
+        applyElementStyle(to: &config.paragraph, traitCollection: traitCollection)
+    }
+}

packages/ios-enriched-markdown/Sources/EnrichedMarkdown/Theme/Elements/Strong.swift

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+import SwiftUI
+
+public struct Strong: MarkdownThemeElement {
+    public var fontSpec: ThemeFontSpec?
+    public var fontWeight: Font.Weight?
+    public var fontDesign: Font.Design?
+    public var foregroundColorSpec: ThemeColorSpec?
+    public var marginTop: CGFloat?
+    public var marginBottom: CGFloat?
+    public var lineHeight: CGFloat?
+    public var textAlignment: TextAlignment?
+
+    public init() {}
+
+    public func apply(to config: inout MarkdownStyleConfig, traitCollection: UITraitCollection) {
+        applyElementStyle(to: &config.strong, traitCollection: traitCollection)
+    }
+}

packages/ios-enriched-markdown/Sources/EnrichedMarkdown/Theme/Environment+MarkdownTheme.swift

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+import SwiftUI
+
+private struct MarkdownThemeLayersKey: EnvironmentKey {
+    static let defaultValue: [MarkdownTheme] = [.default]
+}
+
+public extension EnvironmentValues {
+    var markdownThemeLayers: [MarkdownTheme] {
+        get { self[MarkdownThemeLayersKey.self] }
+        set { self[MarkdownThemeLayersKey.self] = newValue }
+    }
+}
+
+private struct MarkdownThemeLayerModifier: ViewModifier {
+    @Environment(\.markdownThemeLayers) private var parentLayers
+    let theme: MarkdownTheme
+
+    func body(content: Content) -> some View {
+        content.environment(\.markdownThemeLayers, parentLayers + [theme])
+    }
+}
+
+public extension View {
+    func markdownTheme(_ theme: MarkdownTheme) -> some View {
+        modifier(MarkdownThemeLayerModifier(theme: theme))
+    }
+
+    func markdownTheme(@MarkdownThemeBuilder _ content: () -> MarkdownThemeGroup) -> some View {
+        markdownTheme(MarkdownTheme(content: content()))
+    }
+}

packages/ios-enriched-markdown/Sources/EnrichedMarkdown/Theme/MarkdownStyleConfig+Resolve.swift

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+import UIKit
+
+extension MarkdownStyleConfig {
+    public static func resolve(
+        layers: [MarkdownTheme],
+        traitCollection: UITraitCollection
+    ) -> MarkdownStyleConfig {
+        var config = MarkdownStyleConfig()
+        for layer in layers {
+            layer.apply(to: &config, traitCollection: traitCollection)
+        }
+        return config
+    }
+
+    public static func baseline(traitCollection: UITraitCollection = .current) -> MarkdownStyleConfig {
+        resolve(layers: [.default], traitCollection: traitCollection)
+    }
+}

packages/ios-enriched-markdown/Sources/EnrichedMarkdown/Theme/MarkdownStyleConfig.swift

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+import UIKit
+
+public struct ElementStyle: Equatable, Sendable {
+    public var font: UIFont?
+    public var foregroundColor: UIColor?
+    public var marginTop: CGFloat?
+    public var marginBottom: CGFloat?
+    public var lineHeight: CGFloat?
+    public var textAlignment: NSTextAlignment?
+
+    public init(
+        font: UIFont? = nil,
+        foregroundColor: UIColor? = nil,
+        marginTop: CGFloat? = nil,
+        marginBottom: CGFloat? = nil,
+        lineHeight: CGFloat? = nil,
+        textAlignment: NSTextAlignment? = nil
+    ) {
+        self.font = font
+        self.foregroundColor = foregroundColor
+        self.marginTop = marginTop
+        self.marginBottom = marginBottom
+        self.lineHeight = lineHeight
+        self.textAlignment = textAlignment
+    }
+
+    public mutating func merge(_ other: ElementStyle) {
+        if let font = other.font { self.font = font }
+        if let foregroundColor = other.foregroundColor { self.foregroundColor = foregroundColor }
+        if let marginTop = other.marginTop { self.marginTop = marginTop }
+        if let marginBottom = other.marginBottom { self.marginBottom = marginBottom }
+        if let lineHeight = other.lineHeight { self.lineHeight = lineHeight }
+        if let textAlignment = other.textAlignment { self.textAlignment = textAlignment }
+    }
+}
+
+public struct MarkdownStyleConfig: Equatable, Sendable {
+    public var paragraph: ElementStyle
+    public var strong: ElementStyle
+    public var emphasis: ElementStyle
+
+    public init(
+        paragraph: ElementStyle = ElementStyle(),
+        strong: ElementStyle = ElementStyle(),
+        emphasis: ElementStyle = ElementStyle()
+    ) {
+        self.paragraph = paragraph
+        self.strong = strong
+        self.emphasis = emphasis
+    }
+
+    public mutating func merge(_ other: MarkdownStyleConfig) {
+        paragraph.merge(other.paragraph)
+        strong.merge(other.strong)
+        emphasis.merge(other.emphasis)
+    }
+
+}

packages/ios-enriched-markdown/Sources/EnrichedMarkdown/Theme/MarkdownTheme.swift

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+import SwiftUI
+import UIKit
+
+public struct MarkdownTheme: Sendable {
+    private let content: any MarkdownThemeContent
+
+    public init(@MarkdownThemeBuilder _ content: () -> MarkdownThemeGroup) {
+        self.content = content()
+    }
+
+    init(content: any MarkdownThemeContent) {
+        self.content = content
+    }
+
+    func apply(to config: inout MarkdownStyleConfig, traitCollection: UITraitCollection) {
+        content.apply(to: &config, traitCollection: traitCollection)
+    }
+
+    public static let `default` = DefaultMarkdownTheme.make()
+}

packages/ios-enriched-markdown/Sources/EnrichedMarkdown/Theme/MarkdownThemeBuilder.swift

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+@resultBuilder
+public enum MarkdownThemeBuilder {
+    public static func buildBlock(_ components: any MarkdownThemeContent...) -> MarkdownThemeGroup {
+        MarkdownThemeGroup(contents: components)
+    }
+
+    public static func buildOptional(_ component: (any MarkdownThemeContent)?) -> MarkdownThemeGroup {
+        if let component {
+            return MarkdownThemeGroup(contents: [component])
+        }
+        return MarkdownThemeGroup(contents: [])
+    }
+
+    public static func buildEither(first component: any MarkdownThemeContent) -> MarkdownThemeGroup {
+        MarkdownThemeGroup(contents: [component])
+    }
+
+    public static func buildEither(second component: any MarkdownThemeContent) -> MarkdownThemeGroup {
+        MarkdownThemeGroup(contents: [component])
+    }
+
+    public static func buildArray(_ components: [any MarkdownThemeContent]) -> MarkdownThemeGroup {
+        MarkdownThemeGroup(contents: components)
+    }
+}

packages/ios-enriched-markdown/Sources/EnrichedMarkdown/Theme/MarkdownThemeContent.swift

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+import UIKit
+
+public protocol MarkdownThemeContent {
+    func apply(to config: inout MarkdownStyleConfig, traitCollection: UITraitCollection)
+}
+
+public struct MarkdownThemeGroup: MarkdownThemeContent, Sendable {
+    let contents: [any MarkdownThemeContent]
+
+    public init(contents: [any MarkdownThemeContent]) {
+        self.contents = contents
+    }
+
+    public func apply(to config: inout MarkdownStyleConfig, traitCollection: UITraitCollection) {
+        for content in contents {
+            content.apply(to: &config, traitCollection: traitCollection)
+        }
+    }
+}

packages/ios-enriched-markdown/Sources/EnrichedMarkdown/Theme/MarkdownThemeElement.swift

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+import SwiftUI
+import UIKit
+
+public protocol MarkdownThemeElement: MarkdownThemeContent {
+    var fontSpec: ThemeFontSpec? { get set }
+    var fontWeight: Font.Weight? { get set }
+    var fontDesign: Font.Design? { get set }
+    var foregroundColorSpec: ThemeColorSpec? { get set }
+    var marginTop: CGFloat? { get set }
+    var marginBottom: CGFloat? { get set }
+    var lineHeight: CGFloat? { get set }
+    var textAlignment: TextAlignment? { get set }
+}
+
+public extension MarkdownThemeElement {
+    func font(_ font: Font) -> Self {
+        var copy = self
+        copy.fontSpec = ThemeResolver.font(from: font, traitCollection: .current)
+        return copy
+    }
+
+    func bold() -> Self {
+        var copy = self
+        copy.fontWeight = .bold
+        return copy
+    }
+
+    func foregroundStyle(_ color: Color) -> Self {
+        var copy = self
+        copy.foregroundColorSpec = ThemeResolver.color(from: color, traitCollection: .current)
+        return copy
+    }
+
+    func foregroundStyle(_ semantic: ThemeColorSpec.SemanticColor) -> Self {
+        var copy = self
+        copy.foregroundColorSpec = .semantic(semantic)
+        return copy
+    }
+
+    func marginTop(_ value: CGFloat) -> Self {
+        var copy = self
+        copy.marginTop = value
+        return copy
+    }
+
+    func marginBottom(_ value: CGFloat) -> Self {
+        var copy = self
+        copy.marginBottom = value
+        return copy
+    }
+
+    func lineHeight(_ value: CGFloat) -> Self {
+        var copy = self
+        copy.lineHeight = value
+        return copy
+    }
+
+    func textAlignment(_ alignment: TextAlignment) -> Self {
+        var copy = self
+        copy.textAlignment = alignment
+        return copy
+    }
+
+    func applyElementStyle(
+        to style: inout ElementStyle,
+        traitCollection: UITraitCollection
+    ) {
+        if fontSpec != nil || fontWeight != nil || fontDesign != nil {
+            style.font = ThemeResolver.applyFont(
+                spec: fontSpec,
+                weight: fontWeight,
+                design: fontDesign,
+                to: style.font,
+                traitCollection: traitCollection
+            )
+        }
+        if let foregroundColorSpec {
+            style.foregroundColor = foregroundColorSpec.resolve(traitCollection: traitCollection)
+        }
+        if let marginTop { style.marginTop = marginTop }
+        if let marginBottom { style.marginBottom = marginBottom }
+        if let lineHeight { style.lineHeight = lineHeight }
+        if let textAlignment { style.textAlignment = nsTextAlignment(from: textAlignment) }
+    }
+
+    private func nsTextAlignment(from alignment: TextAlignment) -> NSTextAlignment {
+        switch alignment {
+        case .leading: return .left
+        case .center: return .center
+        case .trailing: return .right
+        }
+    }
+}

packages/ios-enriched-markdown/Sources/EnrichedMarkdown/Theme/RememberMarkdownTheme.swift

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+import SwiftUI
+
+/// Re-create a `MarkdownTheme` when `colorScheme` or `dynamicTypeSize` changes.
+///
+/// Call from a `View.body` after reading those values from the environment:
+///
+/// ```swift
+/// @Environment(\.colorScheme) private var colorScheme
+/// @Environment(\.dynamicTypeSize) private var dynamicTypeSize
+///
+/// var body: some View {
+///     let theme = rememberMarkdownTheme(colorScheme: colorScheme, dynamicTypeSize: dynamicTypeSize) {
+///         Paragraph().foregroundStyle(.primary)
+///     }
+///     RootView()
+///         .markdownTheme(theme)
+/// }
+/// ```
+@MainActor
+public func rememberMarkdownTheme(
+    colorScheme: ColorScheme,
+    dynamicTypeSize: DynamicTypeSize,
+    @MarkdownThemeBuilder _ content: () -> MarkdownThemeGroup
+) -> MarkdownTheme {
+    _ = colorScheme
+    _ = dynamicTypeSize
+    return MarkdownTheme(content: content())
+}

packages/ios-enriched-markdown/Sources/EnrichedMarkdown/Theme/ThemeColorModifiers.swift

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+import SwiftUI
+
+enum ThemeColorModifiers {
+    static func spec(from color: Color) -> ThemeColorSpec {
+        ThemeResolver.color(from: color, traitCollection: .current)
+    }
+
+    static func spec(from semantic: ThemeColorSpec.SemanticColor) -> ThemeColorSpec {
+        .semantic(semantic)
+    }
+}