Kyle-Ye

Using CAHostingLayer in SwiftUI

Background

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.

Accessing CAHostingLayer without Apple Internal SDK

Kyle-Ye

command script import --relative-to-command-file swiftui_displaylist_minimal_description.py

Kyle-Ye

// 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

Kyle-Ye

Inspired by https://infosec.exchange/@jjtech/112612685494089718

Method 1: LLDB

Step 0

Reboot into Recovery OS + Disable SIP

csrutil disable

Kyle-Ye

Disabling CALayerSystem Behavior in SwiftUI on iOS 26

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.

Option 1: Environment Variable

Set the environment variable SWIFTUI_DISABLE_MIXED_VIEW_HIERARCHY=1 in your scheme's Run configuration (Edit Scheme → Run → Arguments → Environment Variables).

Kyle-Ye

// 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()

Kyle-Ye

Warning

The following guide need to disable SIP to work.

Please confirm the risk of disabling the SIP by yourself.

Another solution which does not require disabling SIP is currently under investigation.

Step 0

Reboot into Recovery OS + Disable SIP

Kyle-Ye

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.

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<?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>

Kyle-Ye

#!/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