Unleash the UIKit trait system

// Build a new trait collection instance from scratch
let myTraits = UITraitCollection { mutableTraits in
    mutableTraits.userInterfaceIdiom = .phone
    mutableTraits.horizontalSizeClass = .regular
}

// Get a new instance by modifying traits of an existing one
let otherTraits = myTraits.modifyingTraits { mutableTraits in
    mutableTraits.horizontalSizeClass = .compact
    mutableTraits.userInterfaceStyle = .dark
}

struct ContainedInSettingsTrait: UITraitDefinition {
    static let defaultValue = false
}

let traitCollection = UITraitCollection { mutableTraits in
    mutableTraits[ContainedInSettingsTrait.self] = true
}

let value = traitCollection[ContainedInSettingsTrait.self]
// true

struct ContainedInSettingsTrait: UITraitDefinition {
    static let defaultValue = false
}

extension UITraitCollection {
    var isContainedInSettings: Bool { self[ContainedInSettingsTrait.self] }
}

extension UIMutableTraits {
    var isContainedInSettings: Bool {
        get { self[ContainedInSettingsTrait.self] }
        set { self[ContainedInSettingsTrait.self] = newValue }
    }
}

let traitCollection = UITraitCollection { mutableTraits in
    mutableTraits.isContainedInSettings = true
}

let value = traitCollection.isContainedInSettings
// true

enum MyAppTheme: Int {
    case standard, pastel, bold, monochrome
}

struct MyAppThemeTrait: UITraitDefinition {
    static let defaultValue = MyAppTheme.standard
    static let affectsColorAppearance = true
    static let name = "Theme"
    static let identifier = "com.myapp.theme"
}

extension UITraitCollection {
    var myAppTheme: MyAppTheme { self[MyAppThemeTrait.self] }
}

extension UIMutableTraits {
    var myAppTheme: MyAppTheme {
        get { self[MyAppThemeTrait.self] }
        set { self[MyAppThemeTrait.self] = newValue }
    }
}

let customBackgroundColor = UIColor { traitCollection in
    switch traitCollection.myAppTheme {
    case .standard:    return UIColor(named: "StandardBackground")!
    case .pastel:      return UIColor(named: "PastelBackground")!
    case .bold:        return UIColor(named: "BoldBackground")!
    case .monochrome:  return UIColor(named: "MonochromeBackground")!
    }
}

let view = UIView()
view.backgroundColor = customBackgroundColor

func toggleThemeOverride(_ overrideTheme: MyAppTheme) {
    if view.traitOverrides.contains(MyAppThemeTrait.self) {
        // There's an existing theme override; remove it
        view.traitOverrides.remove(MyAppThemeTrait.self)
    } else {
        // There's no existing theme override; apply one
        view.traitOverrides.myAppTheme = overrideTheme
    }
}

// Efficient implementation that only updates when necessary
override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
    if traitCollection.horizontalSizeClass != previousTraitCollection?.horizontalSizeClass {
        updateViews(sizeClass: traitCollection.horizontalSizeClass)
    }
}

func updateViews(sizeClass: UIUserInterfaceSizeClass) {
    // Update views for the new size class...
}

// Register for horizontal size class changes on self
registerForTraitChanges(
    [UITraitHorizontalSizeClass.self]
) { (self: Self, previousTraitCollection: UITraitCollection) in
    self.updateViews(sizeClass: self.traitCollection.horizontalSizeClass)
}

// Register for changes to multiple traits on another view
let anotherView: MyView
anotherView.registerForTraitChanges(
    [UITraitHorizontalSizeClass.self, ContainedInSettingsTrait.self]
) { (view: MyView, previousTraitCollection: UITraitCollection) in
    // Handle the trait change for this view...
}

// Register for horizontal size class changes on self
registerForTraitChanges(
    [UITraitHorizontalSizeClass.self],
    action: #selector(UIView.setNeedsLayout)
)

// Register for changes to multiple traits on another view
let anotherView: MyView
anotherView.registerForTraitChanges(
    [UITraitHorizontalSizeClass.self, ContainedInSettingsTrait.self],
    target: self,
    action: #selector(handleTraitChange(view:previousTraitCollection:))
)

@objc func handleTraitChange(view: MyView, previousTraitCollection: UITraitCollection) {
    // Handle the trait change for this view...
}

registerForTraitChanges(
    UITraitCollection.systemTraitsAffectingColorAppearance,
    action: #selector(handleColorAppearanceChange)
)

@objc func handleColorAppearanceChange() {
    // Handle the color appearance trait changes...
}

// Store the returned registration token
let registration = registerForTraitChanges([UITraitHorizontalSizeClass.self], action: #selector(handleTraitChange))

// Later, use the stored registration token to manually unregister
unregisterForTraitChanges(registration)

@objc func handleTraitChange() {
    // Handle the trait change...
}

enum MyAppTheme: Int {
    case standard, pastel, bold, monochrome
}

// Custom UIKit trait
struct MyAppThemeTrait: UITraitDefinition {
    static let defaultValue = MyAppTheme.standard
    static let affectsColorAppearance = true
}

extension UITraitCollection {
    var myAppTheme: MyAppTheme { self[MyAppThemeTrait.self] }
}

extension UIMutableTraits {
    var myAppTheme: MyAppTheme {
        get { self[MyAppThemeTrait.self] }
        set { self[MyAppThemeTrait.self] = newValue }
    }
}

// Custom SwiftUI environment key
struct MyAppThemeKey: EnvironmentKey {
    static let defaultValue = MyAppTheme.standard
}

extension EnvironmentValues {
    var myAppTheme: MyAppTheme {
        get { self[MyAppThemeKey.self] }
        set { self[MyAppThemeKey.self] = newValue }
    }
}

// Bridge SwiftUI environment key with UIKit trait
extension MyAppThemeKey: UITraitBridgedEnvironmentKey {
    static func read(from traitCollection: UITraitCollection) -> MyAppTheme {
        traitCollection.myAppTheme
    }

    static func write(to mutableTraits: inout UIMutableTraits, value: MyAppTheme) {
        mutableTraits.myAppTheme = value
    }
}

// UIKit trait override applied to the window scene
let windowScene: UIWindowScene
windowScene.traitOverrides.myAppTheme = .monochrome

// Cell in a UICollectionView configured to display a SwiftUI view
let cell: UICollectionViewCell
cell.contentConfiguration = UIHostingConfiguration {
    CellView()
}

// SwiftUI view displayed in the cell, which reads the bridged value from the environment
struct CellView: View {
    @Environment(\.myAppTheme) var theme: MyAppTheme

    var body: some View {
        Text("Settings")
            .foregroundStyle(theme == .monochrome ? .gray : .blue)
    }
}

// SwiftUI environment value applied to a UIViewControllerRepresentable
struct SettingsView: View {
    var body: some View {
        SettingsControllerRepresentable()
            .environment(\.myAppTheme, .standard)
    }
}

final class SettingsControllerRepresentable: UIViewControllerRepresentable {
    func makeUIViewController(context: Context) -> SettingsViewController {
        SettingsViewController()
    }
    
    func updateUIViewController(_ uiViewController: SettingsViewController, context: Context) {
        // Update the view controller...
    }
}

// UIKit view controller contained in the SettingsControllerRepresentable
class SettingsViewController: UIViewController {
    override func viewWillLayoutSubviews() {
        super.viewWillLayoutSubviews()
        title = settingsTitle(for: traitCollection.myAppTheme)
    }
    
    func settingsTitle(for theme: MyAppTheme) -> String {
        switch theme {
        case .standard:   return "Standard"
        case .pastel:     return "Pastel"
        case .bold:       return "Bold"
        case .monochrome: return "Monochrome"
        }
    }
}