Add View.onPreferenceChange support

Example/Modules/Platform/cocoa/SwiftUI_SPI.swiftinterface

@@ -5,6 +5,23 @@ import QuartzCore
 import Swift
 @_exported import SwiftUI
 
+@available(iOS 18.2, macOS 15.2, *)
+@frozen public struct TransactionalPreferenceActionModifier<Key> : SwiftUI.ViewModifier where Key : SwiftUI.PreferenceKey, Key.Value : Swift.Equatable {
+  public var action: (Key.Value, SwiftUI.Transaction) -> Swift.Void
+  @_alwaysEmitIntoClient public init(action: @escaping (Key.Value, SwiftUI.Transaction) -> Swift.Void) {
+    self.action = action
+  }
+  nonisolated public static func _makeView(modifier: SwiftUI._GraphValue<SwiftUI_SPI.TransactionalPreferenceActionModifier<Key>>, inputs: SwiftUI._ViewInputs, body: @escaping (SwiftUI._Graph, SwiftUI._ViewInputs) -> SwiftUI._ViewOutputs) -> SwiftUI._ViewOutputs
+  public typealias Body = Swift.Never
+}
+
+@available(iOS 18.2, macOS 15.2, *)
+extension SwiftUI.View {
+  @_alwaysEmitIntoClient nonisolated public func onPreferenceChange<Key>(_ key: Key.Type = Key.self, action: @escaping @Sendable (Key.Value, SwiftUI.Transaction) -> Swift.Void) -> some SwiftUI.View where Key : SwiftUI.PreferenceKey, Key.Value : Swift.Equatable {
+    return modifier(SwiftUI_SPI.TransactionalPreferenceActionModifier<Key>(action: action))
+  }
+}
+
 @_hasMissingDesignatedInitializers @available(iOS 18.0, macOS 15.0, tvOS 18.0, watchOS 11.0, visionOS 2.0, *)
 public class CAHostingLayer<Content> : QuartzCore.CALayer where Content : SwiftUI.View {
   public init(rootView: Content, environment: SwiftUI.EnvironmentValues = .init())

Example/OpenSwiftUIUITests/Modifier/ViewModifier/PreferenceActionModifierUITests.swift

@@ -0,0 +1,78 @@
+//
+//  PreferenceActionModifierUITests.swift
+//  OpenSwiftUIUITests
+
+import Foundation
+import Testing
+@testable import TestingHost
+
+private final class PreferenceActionRecorder: @unchecked Sendable {
+    private let lock = NSLock()
+    private var value: String?
+
+    func record(value: String) {
+        lock.lock()
+        defer { lock.unlock() }
+        self.value = value
+    }
+
+    func snapshot() -> String? {
+        lock.lock()
+        defer { lock.unlock() }
+        return value
+    }
+}
+
+private final class TransactionalPreferenceActionRecorder: @unchecked Sendable {
+    private let lock = NSLock()
+    private var value: String?
+    private var disablesAnimations = false
+
+    func record(value: String, disablesAnimations: Bool) {
+        lock.lock()
+        defer { lock.unlock() }
+        self.value = value
+        self.disablesAnimations = disablesAnimations
+    }
+
+    func snapshot() -> (value: String?, disablesAnimations: Bool) {
+        lock.lock()
+        defer { lock.unlock() }
+        return (value, disablesAnimations)
+    }
+}
+
+@MainActor
+struct PreferenceActionModifierUITests {
+    @Test
+    func onPreferenceChange() {
+        let recorder = PreferenceActionRecorder()
+        let controller = AnimationDebugController(
+            PreferenceActionModifierExample(action: { value in
+                recorder.record(value: value)
+            })
+        )
+        controller.advance(interval: .zero)
+        #expect(recorder.snapshot() == "changed")
+        withExtendedLifetime(controller) {}
+    }
+
+    @Test
+    @available(iOS 18.2, macOS 15.2, *)
+    func transactionalOnPreferenceChange() {
+        let recorder = TransactionalPreferenceActionRecorder()
+        let controller = AnimationDebugController(
+            TransactionalPreferenceActionExample(action: { value, transaction in
+                recorder.record(
+                    value: value,
+                    disablesAnimations: transaction.disablesAnimations
+                )
+            })
+        )
+        controller.advance(interval: .zero)
+        let snapshot = recorder.snapshot()
+        #expect(snapshot.value == "changed")
+        #expect(snapshot.disablesAnimations)
+        withExtendedLifetime(controller) {}
+    }
+}

Example/Project.swift

@@ -460,7 +460,8 @@ let project = Project(
     targets: targets,
     schemes: schemes,
     additionalFiles: [
-        "../Configurations/**",
+        "../Configurations/OpenSwiftUI-Info.plist",
+        "../Configurations/Shared/basic/**",
         "Modules/**",
         "ReferenceImages/**",
         "OpenSwiftUIUITests/OpenSwiftUIUITests.xctestplan",

Example/Shared/ViewModifier/PreferenceActionModifierExample.swift

@@ -0,0 +1,27 @@
+//
+//  PreferenceActionModifierExample.swift
+//  Shared
+
+#if OPENSWIFTUI
+import OpenSwiftUI
+#else
+import SwiftUI
+#endif
+
+struct PreferenceActionModifierExample: View {
+    private struct Key: PreferenceKey {
+        static let defaultValue = ""
+
+        static func reduce(value: inout String, nextValue: () -> String) {
+            value = nextValue()
+        }
+    }
+
+    var action: (String) -> Void
+
+    var body: some View {
+        Color.red
+            .preference(key: Key.self, value: "changed")
+            .onPreferenceChange(Key.self, perform: action)
+    }
+}

Example/Shared/ViewModifier/TransactionalPreferenceActionModifierExample.swift

@@ -0,0 +1,37 @@
+//
+//  TransactionalPreferenceActionModifierExample.swift
+//  Shared
+
+#if OPENSWIFTUI
+@_spi(Private) import OpenSwiftUI
+#else
+import SwiftUI_SPI
+#endif
+
+#if OPENSWIFTUI
+@available(OpenSwiftUI_v8_0, *)
+#else
+@available(iOS 18.2, macOS 15.2, *)
+#endif
+struct TransactionalPreferenceActionExample: View {
+    private struct Key: PreferenceKey {
+        static let defaultValue = ""
+
+        static func reduce(value: inout String, nextValue: () -> String) {
+            value = nextValue()
+        }
+    }
+
+    var action: @Sendable (String, Transaction) -> Void
+
+    var body: some View {
+        Color.red
+            .preference(key: Key.self, value: "changed")
+            .onPreferenceChange(Key.self) { value, transaction in
+                action(value, transaction)
+            }
+            .transaction { transaction in
+                transaction.disablesAnimations = true
+            }
+    }
+}

Sources/OpenSwiftUI/Modifier/SceneModifier/PreferenceSceneModifier.swift

@@ -9,16 +9,14 @@
 
 @available(OpenSwiftUI_v2_0, *)
 extension _PreferenceWritingModifier: _SceneModifier {
-    @MainActor
-    @preconcurrency
     public static func _makeScene(
         modifier: _GraphValue<Self>,
         inputs: _SceneInputs,
         body: @escaping (_Graph, _SceneInputs) -> _SceneOutputs
     ) -> _SceneOutputs {
-        var inputs = inputs
-        inputs.preferences.remove(Key.self)
-        var outputs = body(_Graph(), inputs)
+        var newInputs = inputs
+        newInputs.preferences.remove(Key.self)
+        var outputs = body(_Graph(), newInputs)
         outputs.preferences
             .makePreferenceWriter(
                 inputs: inputs.preferences,
@@ -32,9 +30,7 @@ extension _PreferenceWritingModifier: _SceneModifier {
 @available(OpenSwiftUI_v4_0, *)
 extension Scene {
     @inlinable
-    @MainActor
-    @preconcurrency
-    internal func preference<K>(
+    package func preference<K>(
         key: K.Type = K.self,
         value: K.Value
     ) -> some Scene where K: PreferenceKey {
@@ -46,8 +42,6 @@ extension Scene {
 
 @available(OpenSwiftUI_v2_0, *)
 extension _PreferenceTransformModifier: _SceneModifier {
-    @MainActor
-    @preconcurrency
     public static func _makeScene(
         modifier: _GraphValue<Self>,
         inputs: _SceneInputs,
@@ -66,9 +60,7 @@ extension _PreferenceTransformModifier: _SceneModifier {
 @available(OpenSwiftUI_v4_0, *)
 extension Scene {
     @inlinable
-    @MainActor
-    @preconcurrency
-    internal func transformPreference<K>(
+    func transformPreference<K>(
         _ key: K.Type = K.self,
         _ callback: @escaping (inout K.Value) -> Void
     ) -> some Scene where K: PreferenceKey {

Sources/OpenSwiftUI/Modifier/ViewModifier/PreferenceActionModifier.swift

@@ -0,0 +1,21 @@
+//
+//  PreferenceActionModifier.swift
+//  OpenSwiftUI
+//
+//  Audited for 8.0.66
+//  Status: Complete
+
+package import OpenSwiftUICore
+
+@_spi(Private)
+@available(OpenSwiftUI_v8_0, *)
+extension View {
+    /// Adds an action to perform when the specified preference key's value
+    /// changes, including the transaction that produced the change.
+    nonisolated public func onPreferenceChange<K>(
+        _ key: K.Type = K.self,
+        action: @escaping @Sendable (K.Value, Transaction) -> Void
+    ) -> some View where K: PreferenceKey, K.Value: Equatable {
+        return modifier(TransactionalPreferenceActionModifier<K>(action: action))
+    }
+}