swiftui-uikit-interop

name swiftui-uikit-interop description Bridges UIKit and SwiftUI with UIViewRepresentable, UIViewControllerRepresentable, UIHostingController, UIHostingConfiguration, coordinator delegates, and UIKit automatic observation tracking for shared @Observable state. Use when wrapping UIKit-only or third-party UIKit views/controllers in SwiftUI, embedding SwiftUI in UIKit, integrating mail/share/document/PDF/text-view surfaces, or migrating UIKit apps to SwiftUI incrementally. 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 Method When Called Purpose makeCoordinator() Before makeUIView . Once per representable lifetime. Create the delegate/datasource reference type. makeUIView(context:) Once, when the representable enters the view tree. Allocate and configure the UIKit view. updateUIView(_:context:) Immediately after makeUIView , then on every relevant state change. Push SwiftUI state into the UIKit view. dismantleUIView(_:coordinator:) When the representable is removed from the view tree. Clean up observers, timers, subscriptions. sizeThatFits(_:uiView:context:) During layout, when SwiftUI needs the view's ideal size. iOS 16+. Return a custom size proposal. 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 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. 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. 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] Option Effect .intrinsicContentSize The hosting controller's view reports its SwiftUI content size as intrinsicContentSize . Use in Auto Layout when the hosted view should size itself. .preferredContentSize Updates preferredContentSize to match SwiftUI content. Use when presenting as a popover or form sheet. 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 SwiftUI Modifier Effect on Representable No modifier SwiftUI uses intrinsicContentSize as ideal size; the view is flexible. .fixedSize() Forces the representable to its ideal (intrinsic) size in both axes. .fixedSize(horizontal: true, vertical: false) Fixes width to intrinsic; height remains flexible. .frame(width:height:) Overrides the proposed size; UIKit view receives this size. 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 Mistake Fix UIKit object or delegate recreated in update* Create once in make* ; update only changed state. Represented frame/bounds/transform mutated directly Let SwiftUI size it; use intrinsic size, sizeThatFits , modifiers, or internal layout. Coordinator retained by UIKit closures Capture it weakly and keep one lifecycle owner. Picker/controller exit path misses dismissal or completion Route every delegate exit through one coordinator cleanup path. Observers, timers, or subscriptions outlive the view Remove/cancel them in dismantle* . Hosting controller uses manual frame without containment Use Auto Layout and addChild / didMove(toParent:) . @State used inside a coordinator Use stored properties and communicate through bindings/callbacks. withObservationTracking is polled manually Use UIKit automatic observation hooks where available or explicit invalidation on older targets. 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:) )