Add View.onPreferenceChange

Example/Shared/ViewModifier/PreferenceActionModifierExample.swift

@@ -0,0 +1,29 @@
+//
+//  PreferenceActionModifierExample.swift
+//  Shared
+
+#if OPENSWIFTUI
+import OpenSwiftUI
+#else
+import SwiftUI
+#endif
+
+struct MyKey: PreferenceKey {
+    static let defaultValue = ""
+
+    static func reduce(value: inout String, nextValue: () -> String) {
+        value = nextValue()
+    }
+}
+
+struct PreferenceActionModifierExample: View {
+    var body: some View {
+        VStack {
+            Color.red
+                .preference(key: MyKey.self, value: "changed")
+        }
+        .onPreferenceChange(MyKey.self) {
+            print("onPreferenceChange: \($0)")
+        }
+    }
+}

Sources/OpenSwiftUI/Modifier/SceneModifier/PreferenceSceneModifier.swift

@@ -16,9 +16,9 @@ extension _PreferenceWritingModifier: _SceneModifier {
         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,

Sources/OpenSwiftUICore/Data/Preference/PreferenceActionModifier.swift

@@ -0,0 +1,124 @@
+//
+//  PreferenceActionModifier.swift
+//  OpenSwiftUICore
+//
+//  Audited for 6.5.4
+//  Status: Complete
+//  ID: 264234112339315C9A664F0B7F8B50C1 (SwiftUICore)
+
+import OpenAttributeGraphShims
+
+extension View {
+    /// Adds an action to perform when the specified preference key's value
+    /// changes.
+    ///
+    /// - Parameters:
+    ///   - key: The key to monitor for value changes.
+    ///   - action: The action to perform when the value for `key` changes. The
+    ///     `action` closure passes the new value as its parameter.
+    ///
+    /// - Returns: A view that triggers `action` when the value for `key`
+    ///   changes.
+    @inlinable
+    nonisolated public func onPreferenceChange<K>(
+        _ key: K.Type = K.self,
+        perform action: @escaping (K.Value) -> Void
+    ) -> some View where K: PreferenceKey, K.Value: Equatable {
+        return modifier(_PreferenceActionModifier<K>(action: action))
+    }
+}
+
+// MARK: - PreferenceActionModifier
+
+@frozen
+public struct _PreferenceActionModifier<K>: MultiViewModifier, PrimitiveViewModifier where K: PreferenceKey, K.Value: Equatable {
+    public var action: (_ value: K.Value) -> Void
+
+    @inlinable
+    public init(action: @escaping (_ value: K.Value) -> Void) {
+        self.action = action
+    }
+
+    nonisolated public static func _makeView(
+        modifier: _GraphValue<Self>,
+        inputs: _ViewInputs,
+        body: @escaping (_Graph, _ViewInputs) -> _ViewOutputs
+    ) -> _ViewOutputs {
+        var inputs = inputs
+        inputs.preferences.add(K.self)
+        let outputs = body(_Graph(), inputs)
+        guard let keyValue = outputs[K.self] else {
+            return outputs
+        }
+        let binder = Attribute(PreferenceBinder<K>(
+            modifier: modifier.value,
+            keyValue: keyValue,
+            phase: inputs.viewPhase,
+            lastResetSeed: .zero,
+            lastValue: nil
+        ))
+        binder.flags = .transactional
+        return outputs
+    }
+}
+
+@available(*, unavailable)
+extension _PreferenceActionModifier: Sendable {}
+
+// MARK: - PreferenceBinder
+
+private struct PreferenceBinder<K>: StatefulRule, AsyncAttribute where K: PreferenceKey, K.Value: Equatable {
+    @Attribute var modifier: _PreferenceActionModifier<K>
+    @Attribute var keyValue: K.Value
+    @Attribute var phase: _GraphInputs.Phase
+    var cycleDetector: UpdateCycleDetector
+    var lastResetSeed: UInt32
+    var lastValue: K.Value?
+
+    init(
+        modifier: Attribute<_PreferenceActionModifier<K>>,
+        keyValue: Attribute<K.Value>,
+        phase: Attribute<_GraphInputs.Phase>,
+        cycleDetector: UpdateCycleDetector = .init(),
+        lastResetSeed: UInt32,
+        lastValue: K.Value?
+    ) {
+        self._modifier = modifier
+        self._keyValue = keyValue
+        self._phase = phase
+        self.cycleDetector = cycleDetector
+        self.lastResetSeed = lastResetSeed
+        self.lastValue = lastValue
+    }
+
+    typealias Value = Void
+
+    mutating func updateValue() {
+        if lastResetSeed != phase.resetSeed {
+            lastResetSeed = phase.resetSeed
+            cycleDetector.reset()
+            lastValue = nil
+        }
+        let (newValue, changed) = $keyValue.changedValue()
+        guard changed || (lastValue == nil && _SemanticFeature_v6_1.isEnabled) else {
+            return
+        }
+        guard lastValue != newValue else {
+            return
+        }
+        lastValue = newValue
+
+        guard cycleDetector.dispatch(
+            label: "Bound preference \(K.self)",
+            isDebug: true
+        ) else {
+            return
+        }
+        let action = Graph.withoutUpdate {
+            modifier.action
+        }
+        Update.enqueueAction(reason: nil) {
+            action(newValue)
+        }
+    }
+}

Sources/OpenSwiftUICore/Data/Preference/PreferenceWritingModifier.swift

@@ -30,9 +30,9 @@ public struct _PreferenceWritingModifier<Key>: ViewModifier, MultiViewModifier,
         inputs: _ViewInputs,
         body: @escaping (_Graph, _ViewInputs) -> _ViewOutputs
     ) -> _ViewOutputs {
-        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,