swiftui-uikit-interop

Imported from dpearson2699/swift-ios-skills (skills/swiftui-uikit-interop/SKILL.md) via skills.sh. Install upstream with npx skills add dpearson2699/swift-ios-skills --skill swiftui-uikit-interop. Copyright stays with the author.

SwiftUI-UIKit Interop

Bridge UIKit and SwiftUI in both directions: wrap UIKit views and controllers, embed SwiftUI in UIKit screens, and synchronize state without duplicating lifecycle ownership.

See references/representable-recipes.md for complete wrapping recipes and references/hosting-migration.md for UIKit-to-SwiftUI migration patterns.

Contents

• UIViewRepresentable Protocol
• UIViewControllerRepresentable Protocol
• The Coordinator Pattern
• UIHostingController
• Sizing and Layout
• State Synchronization Patterns
• UIKit Automatic Observation Tracking
• Sendable Considerations
• Common Mistakes
• Review Checklist
• References

UIViewRepresentable Protocol

Use UIViewRepresentable to wrap any UIView subclass for use in SwiftUI.

Required Methods

struct WrappedTextView: UIViewRepresentable {
    @Binding var text: String

    func makeUIView(context: Context) -> UITextView {
        // Called ONCE when SwiftUI inserts this view into the hierarchy.
        // Create and return the UIKit view. One-time setup goes here.
        let textView = UITextView()
        textView.delegate = context.coordinator
        textView.font = .preferredFont(forTextStyle: .body)
        return textView
    }

    func updateUIView(_ uiView: UITextView, context: Context) {
        // Called on EVERY SwiftUI state change that affects this view.
        // Synchronize SwiftUI state into the UIKit view.
        // Guard against redundant updates to avoid loops.
        if uiView.text != text {
            uiView.text = text
        }
    }
}

Lifecycle Timing

Why updateUIView is the most important method: SwiftUI calls it every time any @Binding, @State, @Environment, or @Observable property read by the representable changes. All state synchronization from SwiftUI to UIKit happens here. If you skip a property, the UIKit view will fall out of sync.

Optional: dismantleUIView

static func dismantleUIView(_ uiView: UITextView, coordinator: Coordinator) {
    // Remove observers, invalidate timers, cancel subscriptions.
    // The coordinator is passed in so you can access state stored on it.
    coordinator.cancellables.removeAll()
}

Optional: sizeThatFits (iOS 16+)

@available(iOS 16.0, *)
func sizeThatFits(
    _ proposal: ProposedViewSize,
    uiView: UITextView,
    context: Context
) -> CGSize? {
    // Return nil to fall back to UIKit's intrinsicContentSize.
    // Return a CGSize to override SwiftUI's sizing for this view.
    let width = proposal.width ?? UIView.layoutFittingExpandedSize.width
    let size = uiView.sizeThatFits(CGSize(width: width, height: .greatestFiniteMagnitude))
    return size
}

UIViewControllerRepresentable Protocol

Use UIViewControllerRepresentable to wrap a UIViewController subclass -- typically for system pickers, document scanners, mail compose, or any controller that presents modally.

struct DocumentScannerView: UIViewControllerRepresentable {
    @Binding var scannedImages: [UIImage]
    @Environment(\.dismiss) private var dismiss

    func makeUIViewController(context: Context) -> VNDocumentCameraViewController {
        let scanner = VNDocumentCameraViewController()
        scanner.delegate = context.coordinator
        return scanner
    }

    func updateUIViewController(_ uiViewController: VNDocumentCameraViewController, context: Context) {
        // Usually empty for modal controllers -- nothing to push from SwiftUI.
    }

    func makeCoordinator() -> Coordinator { Coordinator(self) }
}

Handling Results from Presented Controllers

The coordinator captures delegate callbacks and routes results back to SwiftUI through the parent's @Binding or closures:

extension DocumentScannerView {
    final class Coordinator: NSObject, VNDocumentCameraViewControllerDelegate {
        let parent: DocumentScannerView

        init(_ parent: DocumentScannerView) { self.parent = parent }

        func documentCameraViewController(
            _ controller: VNDocumentCameraViewController,
            didFinishWith scan: VNDocumentCameraScan
        ) {
            parent.scannedImages = (0..<scan.pageCount).map { scan.imageOfPage(at: $0) }
            parent.dismiss()
        }

        func documentCameraViewControllerDidCancel(_ controller: VNDocumentCameraViewController) {
            parent.dismiss()
        }

        func documentCameraViewController(
            _ controller: VNDocumentCameraViewController,
            didFailWithError error: Error
        ) {
            parent.dismiss()
        }
    }
}

The Coordinator Pattern

Why Coordinators Exist

UIKit delegates, data sources, and target-action patterns require a reference type (class). SwiftUI representable structs are value types and cannot serve as delegates. The Coordinator is a class instance that SwiftUI creates and manages for you -- it lives as long as the representable view.

Structure

Always nest the Coordinator inside the representable or in an extension. Store a reference to parent (the representable struct) so the coordinator can write back to @Binding properties.

struct SearchBarView: UIViewRepresentable {
    @Binding var text: String
    var onSearch: (String) -> Void

    func makeCoordinator() -> Coordinator { Coordinator(self) }

    func makeUIView(context: Context) -> UISearchBar {
        let bar = UISearchBar()
        bar.delegate = context.coordinator  // Set delegate HERE, not in updateUIView
        return bar
    }

    func updateUIView(_ uiView: UISearchBar, context: Context) {
        context.coordinator.parent = self

        if uiView.text != text {
            uiView.text = text
        }
    }

    final class Coordinator: NSObject, UISearchBarDelegate {
        var parent: SearchBarView

        init(_ parent: SearchBarView) { self.parent = parent }

        func searchBar(_ searchBar: UISearchBar, textDidChange searchText: String) {
            parent.text = searchText
        }

        func searchBarSearchButtonClicked(_ searchBar: UISearchBar) {
            parent.onSearch(parent.text)
            searchBar.resignFirstResponder()
        }
    }
}

Key Rules

1. Set the delegate in makeUIView/makeUIViewController, never in updateUIView. The update method can run many times for state changes affecting the represented view -- setting the delegate there causes redundant assignment and can trigger unexpected side effects.

2. Refresh copied parent state yourself. If the coordinator stores the representable in a var parent, assign context.coordinator.parent = self at the start of updateUIView or updateUIViewController. Bindings still point at their source of truth, but closures and non-binding values are copied into the coordinator.

3. Use [weak coordinator] in closures to avoid retain cycles between the coordinator and UIKit objects that capture it.

UIHostingController

Embed SwiftUI views inside UIKit view controllers using UIHostingController.

Basic Embedding

final class ProfileViewController: UIViewController {
    private let hostingController = UIHostingController(rootView: ProfileView())

    override func viewDidLoad() {
        super.viewDidLoad()

        // 1. Add as child
        addChild(hostingController)

        // 2. Add and constrain the view
        hostingController.view.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(hostingController.view)
        NSLayoutConstraint.activate([
            hostingController.view.topAnchor.constraint(equalTo: view.topAnchor),
            hostingController.view.leadingAnchor.constraint(equalTo: view.leadingAnchor),
            hostingController.view.trailingAnchor.constraint(equalTo: view.trailingAnchor),
            hostingController.view.bottomAnchor.constraint(equalTo: view.bottomAnchor),
        ])

        // 3. Notify the child
        hostingController.didMove(toParent: self)
    }
}

The three-step sequence (addChild, add view, didMove) is mandatory. Skipping any step causes containment callbacks to misfire, which breaks appearance transitions and trait propagation.

Sizing Options (iOS 16+)

@available(iOS 16.0, *)
hostingController.sizingOptions = [.intrinsicContentSize]

Updating the Root View

When data changes in UIKit, push new state into the hosted SwiftUI view:

func updateProfile(_ profile: Profile) {
    hostingController.rootView = ProfileView(profile: profile)
}

For observable models, pass an @Observable object and SwiftUI tracks changes automatically -- no need to reassign rootView.

UIHostingConfiguration (iOS 16+)

Render SwiftUI content directly inside UICollectionViewCell or UITableViewCell without managing a child hosting controller:

@available(iOS 16.0, *)
func collectionView(
    _ collectionView: UICollectionView,
    cellForItemAt indexPath: IndexPath
) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cell", for: indexPath)
    cell.contentConfiguration = UIHostingConfiguration {
        ItemRow(item: items[indexPath.item])
    }
    return cell
}

Sizing and Layout

intrinsicContentSize Bridging

UIKit views wrapped in UIViewRepresentable communicate their natural size to SwiftUI through intrinsicContentSize. SwiftUI respects this during layout unless overridden by frame() or fixedSize().

SwiftUI-Owned Geometry

SwiftUI owns the represented view's center, bounds, frame, and transform. Do not set those properties directly on the uiView in makeUIView or updateUIView. Use sizeThatFits, intrinsic content size, SwiftUI layout modifiers, or layout code inside a custom UIKit subview for internal sublayers.

fixedSize() and frame() Interactions

Auto Layout with UIHostingController

When embedding UIHostingController as a child, pin its view with constraints. Use .sizingOptions = [.intrinsicContentSize] so Auto Layout can query the SwiftUI content's natural size for self-sizing cells or variable-height sections.

State Synchronization Patterns

@Binding: Two-Way Sync (SwiftUI <-> UIKit)

Use @Binding when both sides read and write the same value. The coordinator writes to parent.bindingProperty in delegate callbacks; updateUIView reads the binding and pushes it into the UIKit view.

// SwiftUI -> UIKit: in updateUIView
if uiView.text != text { uiView.text = text }

// UIKit -> SwiftUI: in Coordinator delegate method
func textViewDidChange(_ textView: UITextView) {
    parent.text = textView.text
}

Closures: One-Way Events (UIKit -> SwiftUI)

For fire-and-forget events (button tapped, search submitted, scan completed), pass a closure instead of a binding:

struct WebViewWrapper: UIViewRepresentable {
    let url: URL
    var onNavigationFinished: ((URL) -> Void)?
}

Environment Values

Access SwiftUI environment values inside representable methods via context.environment:

func updateUIView(_ uiView: UITextView, context: Context) {
    let isEnabled = context.environment.isEnabled
    uiView.isEditable = isEnabled

    // Respond to color scheme changes
    let colorScheme = context.environment.colorScheme
    uiView.backgroundColor = colorScheme == .dark ? .systemGray6 : .white
}

Avoiding Update Loops

updateUIView is called when SwiftUI has new state for the represented view -- including changes triggered by the coordinator writing to a @Binding. Guard against redundant updates to prevent infinite loops:

func updateUIView(_ uiView: UITextView, context: Context) {
    // GUARD: Only update if values actually differ
    if uiView.text != text {
        uiView.text = text
    }
}

Without the guard, setting uiView.text may trigger the delegate's textViewDidChange, which writes to parent.text, which triggers updateUIView again.

UIKit Automatic Observation Tracking

For UIKit screens that share an @Observable model with SwiftUI, keep the screen UIKit and read observed state from UIKit's tracked update hooks:

• iOS 26+: use updateProperties() for labels, colors, visibility, enabled state, and other non-layout UI; use layout hooks for geometry; use cell configuration update handlers for cells.
• iOS 18: automatic UIKit tracking requires UIObservationTrackingEnabled in Info.plist.
• iOS 17: @Observable exists, but UIKit automatic observation tracking is not available. Manual withObservationTracking is one-shot; do not build polling loops around it.
• iOS 15-16 or existing ObservableObject: use Combine objectWillChange, delegates, notifications, or explicit callbacks.

See references/hosting-migration.md for migration patterns.

Sendable Considerations

UIKit delegate protocols are not Sendable. When the coordinator conforms to a UIKit delegate, it inherits main-actor isolation from UIKit. Mark coordinators @MainActor or use nonisolated only for methods that truly do not touch UIKit state. In Swift 6 strict concurrency:

@MainActor
final class Coordinator: NSObject, UISearchBarDelegate {
    var parent: SearchBarView
    init(_ parent: SearchBarView) { self.parent = parent }
    // Delegate methods are main-actor-isolated -- safe to access UIKit and @Binding.
}

If passing closures across isolation boundaries, ensure they are @Sendable or captured on the correct actor.

Common Mistakes

Review Checklist

• View/controller created in make*, not update*
• Coordinator set as delegate in make*, not update*
• @Binding used for two-way state sync
• updateUIView handles all SwiftUI state changes with redundancy guards
• dismantleUIView cleans up observers/timers if needed
• No retain cycles between coordinator and closures ([weak coordinator])
• UIHostingController properly added as child (addChild + didMove(toParent:))
• Sizing strategy chosen (intrinsicContentSize vs fixed frame vs sizeThatFits)
• Represented view geometry left to SwiftUI (frame, bounds, center, transform not mutated directly)
• Environment values read in updateUIView via context.environment where needed
• UIKit @Observable reads use automatic tracking hooks on iOS 18+/26+, not polling; iOS 17 is manual one-shot only
• Coordinator marked @MainActor for strict concurrency
• Modal controllers dismiss in all delegate exit paths (success, cancel, error)
• UIHostingConfiguration used for collection/table view cells instead of manual hosting (iOS 16+)

References

• Wrapping recipes: references/representable-recipes.md
• Migration patterns: references/hosting-migration.md
• Apple docs: UIViewRepresentable
• Apple docs: UIViewControllerRepresentable
• Apple docs: UIHostingController