swiftui-uikit-interop（Swiftui uikit 互操作）

SwiftUI-UIKit Interop

Bridge UIKit and SwiftUI in both directions. Wrap UIKit views and view controllers for use in SwiftUI, embed SwiftUI views inside UIKit screens, and synchronize state across the boundary. Targets iOS 26+ with Swift 6.3 patterns; notes backward-compatible to iOS 16 unless stated otherwise.

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
• 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) {
        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 runs on every state change -- setting the delegate there causes redundant assignment and can trigger unexpected side effects.
2. The coordinator's parent property is updated automatically. SwiftUI updates the coordinator's reference to the latest representable struct value before each call to updateUIView. This means the coordinator always sees current @Binding values through parent.
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().

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 whenever SwiftUI state changes -- 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.

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

DO / DON'T

DON'T: Create the UIKit view in updateUIView. DO: Create the view once in makeUIView; only configure/update it in updateUIView. *Why:* updateUIView runs on every state change. Creating a new view each time destroys all UIKit state (selection, scroll position, first responder) and leaks memory.

DON'T: Set delegates in updateUIView. DO: Set delegates in makeUIView/makeUIViewController only. *Why:* Redundant delegate assignment on every update can reset internal delegate state in UIKit views like WKWebView or MKMapView.

DON'T: Hold strong references to the Coordinator from closures. DO: Use [weak coordinator] in closures. *Why:* UIKit objects often store closures (completion handlers, action blocks). A strong reference to the coordinator that holds a reference to the UIKit view creates a retain cycle.

DON'T: Forget to call parent.dismiss() or completion handlers. DO: Use the coordinator to track dismissal and invoke parent.dismiss() in all delegate exit paths. *Why:* Modal controllers presented by SwiftUI (via .sheet) need their dismiss binding toggled, or the sheet state becomes inconsistent.

DON'T: Ignore dismantleUIView for views that hold observers or timers. DO: Clean up NotificationCenter observers, Combine subscriptions, and Timer instances in dismantleUIView. *Why:* Without cleanup, observers and timers continue firing after the view is removed, causing crashes or stale state updates.

DON'T: Force UIHostingController's view to fill the parent without proper constraints. DO: Use Auto Layout constraints or sizingOptions for proper embedding. *Why:* Setting frame manually breaks adaptive layout, trait propagation, and safe area handling.

DON'T: Try to use @State in the Coordinator -- it is not a View. DO: Use regular stored properties on the Coordinator and communicate to SwiftUI via parent's @Binding properties. *Why:* @State only works inside View conformances. Using it on a class has no effect.

DON'T: Skip the addChild/didMove(toParent:) dance when embedding UIHostingController. DO: Always call addChild(_:), add the view to the hierarchy, then call didMove(toParent:). *Why:* Skipping containment causes viewWillAppear/viewDidAppear to never fire, breaks trait collection propagation, and causes visual glitches.

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)
• Environment values read in updateUIView via context.environment where needed
• 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