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

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](references/representable recipes.md) for complete wrapping recipes and [references/hosting migration.md](references/hosting migration.md) for UIKit to SwiftUI migration patterns. Contents [UIViewRepresentable Protocol]( uiviewrepresentable protocol) [UIViewControllerRepresentable Protocol]( uiviewcontrollerrepresentable protocol) [The Coordinator Pattern]( the coordinator pattern) [UIHostingController]( uihostingcontroller) [Sizing and Layout]( sizing and layout) [State Synchronization Patterns]( state synchronization patterns) [UIKit Automatic Observation Tracking]( uikit automatic observation tracking) [Sendable Considerations]( sendable considerations) [Common Mistakes]( common mistakes) [Review Checklist]( review checklist) [References]( references) UIViewRepresentable Protocol Use UIViewRepresentable to wrap any UIView subclass for use in SwiftUI. Required Methods 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 Optional: sizeThatFits (iOS 16+) UIViewControllerRepresentable Protocol Use UIViewControllerRepresentable to wrap a UIViewController subclass typically for system pickers, document scanners, mail compose, or any controller that presents modally. Handling Results from Presented Controllers The coordinator captures delegate callbacks and routes results back to SwiftUI through the parent's @Binding or closures: 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. 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 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+) 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: 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: 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. Closures: One Way Events (UIKit SwiftUI) For fire and forget events (button tapped, search submitted, scan completed), pass a closure instead of a binding: Environment Values Access SwiftUI environment values inside representable methods via context.environment : 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: 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](references/hosting migration.md automatic observation tracking in uikit) 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: 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:) ) [ ] 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](references/representable recipes.md) Migration patterns: [references/hosting migration.md](references/hosting migration.md) Apple docs: [UIViewRepresentable](https://sosumi.ai/documentation/swiftui/UIViewRepresentable) Apple docs: [UIViewControllerRepresentable](https://sosumi.ai/documentation/swiftui/UIViewControllerRepresentable) Apple docs: [UIHostingController](https://sosumi.ai/documentation/swiftui/UIHostingController)

swiftui-uikit-interop