feat: add FeatureFlags shim with SwiftUI domain fallback

Author: Kyle-Ye

Sources/OpenSwiftUI/Event/HitTest/ResponderBasedHitTesting.swift

@@ -9,9 +9,6 @@
 #if os(macOS)
 import Foundation
 import OpenSwiftUICore
-#if OPENSWIFTUI_LINK_FEATUREFLAGS
-import FeatureFlags
-#endif
 
 struct ResponderBasedHitTesting: Feature {
     static var isEnabled: Bool {
@@ -30,16 +27,12 @@ struct ResponderBasedHitTesting: Feature {
         Bundle.main.bundleIdentifier == "com.apple.ScreenContinuity" ? false : nil
     }()
 
-    #if OPENSWIFTUI_LINK_FEATUREFLAGS
     fileprivate struct Key: FeatureFlagsKey {
         let domain: StaticString = "OpenSwiftUI"
         let feature: StaticString = "ResponderBasedHitTesting"
     }
     private static let featureFlagValue: Bool = {
-        FeatureFlags.isFeatureEnabled(Key())
+        isFeatureEnabled(Key())
     }()
-    #else
-    private static let featureFlagValue: Bool = false
-    #endif
 }
 #endif

Sources/OpenSwiftUICore/Event/Gesture/GestureFeature.swift

@@ -5,10 +5,6 @@
 //  Audited for 6.5.4
 //  Status: Complete
 
-#if OPENSWIFTUI_LINK_FEATUREFLAGS
-import FeatureFlags
-#endif
-
 // MARK: - UnifiedHitTestingFeature
 
 package struct UnifiedHitTestingFeature: Feature {
@@ -65,15 +61,10 @@ package struct GestureContainerFeature: Feature {
     package static var isEnabledOverride: Bool?
 
     package static func _isFeatureEnabled() -> Bool {
-        #if OPENSWIFTUI_LINK_FEATUREFLAGS
-        FeatureFlags.isFeatureEnabled(Self()) && Semantics.UnifiedHitTesting.isEnabled
-        #else
-        Semantics.UnifiedHitTesting.isEnabled
-        #endif
+        isFeatureEnabled(Self()) && Semantics.UnifiedHitTesting.isEnabled
     }
 }
 
-#if OPENSWIFTUI_LINK_FEATUREFLAGS
 extension GestureContainerFeature: FeatureFlagsKey {
     package var domain: StaticString {
         "OpenSwiftUI"
@@ -83,7 +74,6 @@ extension GestureContainerFeature: FeatureFlagsKey {
         "gestureContainer"
     }
 }
-#endif
 
 // MARK: - GestureRecognizerBasedEvents
 

Sources/OpenSwiftUICore/Util/FeatureFlagsShims.swift

@@ -0,0 +1,61 @@
+//
+//  FeatureFlagsShims.swift
+//  OpenSwiftUICore
+//
+//  Status: Complete
+
+#if OPENSWIFTUI_LINK_FEATUREFLAGS
+package import FeatureFlags
+#endif
+
+#if OPENSWIFTUI_LINK_FEATUREFLAGS
+/// Identifies a feature flag by its domain and feature name.
+package protocol FeatureFlagsKey: FeatureFlags.FeatureFlagsKey {}
+#else
+/// Identifies a feature flag by its domain and feature name.
+package protocol FeatureFlagsKey {
+    /// The domain that owns the feature, such as `"OpenSwiftUI"`.
+    var domain: StaticString { get }
+
+    /// The feature name within the domain.
+    var feature: StaticString { get }
+}
+#endif
+
+/// Returns whether the feature identified by a key is enabled.
+///
+/// When `OPENSWIFTUI_LINK_FEATUREFLAGS` is enabled, queries the key's domain
+/// in the system FeatureFlags framework. If an `"OpenSwiftUI"` lookup returns
+/// `false`, also checks the same feature in `"SwiftUI"`. A `true` result in
+/// either domain enables the feature. An OpenSwiftUI flag cannot disable a
+/// feature enabled by SwiftUI.
+///
+/// The SwiftUI feature flag configuration is stored at
+/// `/System/Library/FeatureFlags/Domain/SwiftUI.plist`.
+///
+/// Returns `false` when `OPENSWIFTUI_LINK_FEATUREFLAGS` is disabled.
+package func isFeatureEnabled(_ key: any FeatureFlagsKey) -> Bool {
+    #if OPENSWIFTUI_LINK_FEATUREFLAGS
+    /// Reuses an OpenSwiftUI feature name in the SwiftUI domain.
+    struct SwiftUICompatibilityKey: FeatureFlagsKey {
+        /// Creates a compatibility key only for the `"OpenSwiftUI"` domain.
+        init?<Key>(_ key: Key) where Key: FeatureFlagsKey {
+            guard key.domain.description == "OpenSwiftUI" else { return nil }
+            self.feature = key.feature
+        }
+
+        var domain: StaticString { "SwiftUI" }
+        let feature: StaticString
+    }
+
+    if let compatibilityKey = SwiftUICompatibilityKey(key) {
+        return FeatureFlags.isFeatureEnabled(key) || FeatureFlags.isFeatureEnabled(compatibilityKey)
+    } else {
+        return FeatureFlags.isFeatureEnabled(key)
+    }
+    #else
+    // TODO: Provide a platform implementation when FeatureFlags framework is not avaiable
+    _openSwiftUIPlatformUnimplementedWarning()
+    return false
+    #endif
+}