Add onChange modifier support (#413)

* Add ValueActionModifier implementation

* Add ValueActionModifierExample

* Fix onChange implementation issue

* Add ValueActionModifierCompatibilityTests

* Fix DeprecatedDeclaration werror

Author: Kyle-Ye

Example/HostingExample/ViewController.swift

@@ -66,6 +66,6 @@ class ViewController: NSViewController {
 
 struct ContentView: View {
     var body: some View {
-        ColorAnimationExample()
+        ValueActionModifierExample()
     }
 }

Example/SharedExample/Animation/ColorAnimationExample.swift

@@ -1,9 +1,7 @@
 //
 //  ColorAnimationExample.swift
 //  SharedExample
-//
-//  Created by Kyle on 2025/7/20.
-//
+
 #if OPENSWIFTUI
 import OpenSwiftUI
 #else

…ew/AppearanceActionModifierExample.swift …er/AppearanceActionModifierExample.swiftExample/SharedExample/View/AppearanceActionModifierExample.swift renamed to Example/SharedExample/ViewModifier/AppearanceActionModifierExample.swift Example/SharedExample/View/AppearanceActionModifierExample.swift renamed to Example/SharedExample/ViewModifier/AppearanceActionModifierExample.swift

@@ -7,7 +7,6 @@ import OpenSwiftUI
 #else
 import SwiftUI
 #endif
-import Foundation
 
 struct AppearanceActionModifierExample: View {
     @State private var first = true

Example/SharedExample/ViewModifier/ValueActionModifierExample.swift

@@ -0,0 +1,25 @@
+//
+//  ValueActionModifier.swift
+//  SharedExample
+
+#if OPENSWIFTUI
+import OpenSwiftUI
+#else
+import SwiftUI
+#endif
+
+struct ValueActionModifierExample: View {
+    @State private var value = 0
+    var body: some View {
+        Color.red
+            .onAppear {
+                value = 1
+                DispatchQueue.main.async {
+                    value = 2
+                }
+            }
+            .onChange(of: value) { newValue in
+                print(newValue)
+            }
+    }
+}

Package.swift

@@ -58,6 +58,7 @@ var sharedSwiftSettings: [SwiftSetting] = [
     .enableUpcomingFeature("BareSlashRegexLiterals"),
     .enableUpcomingFeature("InternalImportsByDefault"),
     .enableUpcomingFeature("InferSendableFromCaptures"),
+    .unsafeFlags(["-Wwarning", "DeprecatedDeclaration"]), // We want to use deprecated APIs in test targets
     .define("OPENSWIFTUI_SUPPRESS_DEPRECATED_WARNINGS"),
     // FIXME: -unavailable-decl-optimization=stub is not working somehow (eg. Color.vibrancy). Dig into this later
     .unsafeFlags(["-unavailable-decl-optimization=stub"]),
@@ -136,7 +137,6 @@ if warningsAsErrorsCondition {
     // items we want to ignore:
     // [error_from_clang] [error_in_future_swift_version]
     // sharedSwiftSettings.append(.unsafeFlags(["-warnings-as-errors"]))
-    sharedSwiftSettings.append(.unsafeFlags(["-Werror", "DeprecatedDeclaration"]))
     sharedSwiftSettings.append(.unsafeFlags(["-Werror", "Unsafe"]))
     sharedSwiftSettings.append(.unsafeFlags(["-Werror", "UnknownWarningGroup"]))
     sharedSwiftSettings.append(.unsafeFlags(["-Werror", "ExistentialAny"]))

Sources/OpenSwiftUI/Modifier/SceneModifier/ValueActionSceneModifier.swift

@@ -0,0 +1,53 @@
+//
+//  ValueActionSceneModifier.swift
+//  OpenSwiftUI
+//
+//  Audited for 6.5.4
+//  Status: WIP
+
+package import OpenSwiftUICore
+
+@available(OpenSwiftUI_v2_0, *)
+extension Scene {
+    @available(*, deprecated, message: "Use `onChange` with a two or zero parameter action closure instead.")
+    @inlinable
+    nonisolated public func onChange<V>(
+        of value: V,
+        perform action: @escaping (_ newValue: V) -> Void
+    ) -> some Scene where V: Equatable {
+        // modifier(_ValueActionModifier(value: value, action: action))
+        return self
+    }
+}
+
+@available(OpenSwiftUI_v2_0, *)
+extension _ValueActionModifier: _SceneModifier {
+    @MainActor
+    @preconcurrency
+    public static func _makeScene(
+        modifier: _GraphValue<_ValueActionModifier<Value>>,
+        inputs: _SceneInputs,
+        body: @escaping (_Graph, _SceneInputs) -> _SceneOutputs
+    ) -> _SceneOutputs {
+        _openSwiftUIUnimplementedFailure()
+    }
+}
+
+@available(OpenSwiftUI_v5_0, *)
+extension Scene {
+    nonisolated public func onChange<V>(
+        of value: V,
+        initial: Bool = false,
+        _ action: @escaping (_ oldValue: V, _ newValue: V) -> Void
+    ) -> some Scene where V: Equatable {
+        _openSwiftUIUnimplementedFailure()
+    }
+
+    nonisolated public func onChange<V>(
+        of value: V,
+        initial: Bool = false,
+        _ action: @escaping () -> Void
+    ) -> some Scene where V: Equatable {
+        _openSwiftUIUnimplementedFailure()
+    }
+}