SwiftUI provides more than just UIHostingController / _UIHostingView (iOS) and NSHostingController / NSHostingView (macOS) for embedding SwiftUI views into UIKit/AppKit.
Starting from iOS 18.0 / macOS 15.0, SwiftUI also offers CAHostingLayer<Content: View> — a CALayer subclass that can host SwiftUI content directly at the layer level. This API is currently marked as SPI (@_spi(ForUIKitOnly) / @_spi(ForAppKitOnly)) and is used internally by UIKit, AppKit, and WebKit.
Starting with iOS 26, SwiftUI's UIHostingView may use CALayer-based rendering (caLayerSystem) instead of the traditional UIKit-based rendering (uikitSystem) when certain conditions are met. This can cause unexpected changes in the view hierarchy — for example, _UIGraphicsView and CGDrawingView subviews may be replaced by plain CALayer and CGDrawingLayer sublayers.
Here are two approaches to temporarily disable this behavior in your local debug environment.
Set the environment variable SWIFTUI_DISABLE_MIXED_VIEW_HIERARCHY=1 in your scheme's Run configuration (Edit Scheme → Run → Arguments → Environment Variables).
| // Usage: ViewGraph.current.append(feature: HStackImplicitFeature()) | |
| struct HStackImplicitFeature: ViewGraphFeature { | |
| func modifyViewInputs(inputs: inout _ViewInputs, graph: ViewGraph) { | |
| inputs.implicitRootType = _HStackLayout.self | |
| } | |
| } | |
| struct ContentView: View { | |
| var body: some View { | |
| ChildView1() |
| // kdebug_interpose.c | |
| #include <stdbool.h> | |
| #include <stdint.h> | |
| #include <dlfcn.h> | |
| // Forward declare the original | |
| extern bool kdebug_is_enabled(uint32_t debugid); | |
| // Our replacement |
| import Testing | |
| import SwiftUI | |
| @testable import Demo | |
| typealias PlatformViewController = UIViewController | |
| typealias PlatformWindow = UIWindow | |
| typealias PlatformHostingController = UIHostingController | |
| extension PlatformViewController { | |
| // NOTE: Remember to withExtendedLifetime for window to ensure it is not deallocated duration animation or update. |
| <?xml version="1.0" encoding="UTF-8"?> | |
| <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd"> | |
| <plist version="1.0"> | |
| <dict> | |
| <key>Label</key> | |
| <string>com.github.actions-runner</string> | |
| <key>ProgramArguments</key> | |
| <array> | |
| <string>/bin/bash</string> |
| #!/bin/bash | |
| # A temporay workaround for https://github.com/swiftlang/swift-corelibs-foundation/issues/5211 | |
| set -e | |
| # Find the path to swift binary | |
| SWIFT_PATH=$(which swift) | |
| echo "Swift binary found at: $SWIFT_PATH" | |
| # Extract the toolchain path from swift binary path | |
| # Remove /usr/bin/swift from the path to get the toolchain root |
| // Ref: https://x.com/KyleSwifter/status/1899663558914572653 | |
| // The iOS 18 SwiftUI SDK has an API to change the semantic - _TestApp().setSemantics(xx). | |
| // Remember to make it public in your SDK. | |
| // For lower OS version, the API does not exist. | |
| // But `_TestApp.run()` will read and set it from the `CommandLine.arguments` after `--semantics`. | |
| @main | |
| struct SemanticsTestsApp: App { | |
| init() { | |
| let app = _TestApp() |
| // Set up swift repo | |
| mkdir -p ~/tmp/build/swift-project | |
| cd ~/tmp/build/swift-project | |
| git clone git@github.com:swiftlang/swift.git | |
| // Update the swift-project | |
| cd swift | |
| utils/update-checkout --clone-with-ssh | |
| // Check out to the 5.10 release |
