feat(focus): implement focus event proxy

Sources/OpenSwiftUI/Event/Focus/FocusEventProxy.swift

@@ -3,12 +3,58 @@
 //  OpenSwiftUI
 //
 //  Audited for 6.5.4
-//  Status: WIP
+//  Status: Complete
 //  ID: 220887A402CE7023D01D5BCC1E080716 (SwiftUI)
 
+import OpenAttributeGraphShims
 @_spi(ForOpenSwiftUIOnly)
 import OpenSwiftUICore
 
-// MARK: - FocusEventProxyResponder [TBA]
+// MARK: - FocusEventProxyResponder
 
-class FocusEventProxyResponder: DefaultLayoutViewResponder {}
+private class FocusEventProxyResponder: DefaultLayoutViewResponder {}
+
+@inline(__always)
+func asFocusEventProxyResponder(_ responder: ResponderNode) -> ViewResponder? {
+    responder as? FocusEventProxyResponder
+}
+
+#if os(iOS) || os(visionOS)
+// MARK: - FocusEventProxyModifier
+
+struct FocusEventProxyModifier: MultiViewModifier, PrimitiveViewModifier {
+    static func _makeView(
+        modifier: _GraphValue<Self>,
+        inputs: _ViewInputs,
+        body: @escaping (_Graph, _ViewInputs) -> _ViewOutputs
+    ) -> _ViewOutputs {
+        var outputs = body(_Graph(), inputs)
+        guard IOSFocusEnabledFlag.evaluate(inputs: inputs.base),
+              inputs.preferences.requiresViewResponders else {
+            return outputs
+        }
+        outputs.preferences.viewResponders = Attribute(FocusEventProxyResponderFilter(
+            children: outputs.viewResponders(),
+            responder: FocusEventProxyResponder(inputs: inputs)
+        ))
+        return outputs
+    }
+}
+
+// MARK: - FocusEventProxyResponderFilter
+
+private struct FocusEventProxyResponderFilter: StatefulRule {
+    @Attribute var children: [ViewResponder]
+
+    let responder: FocusEventProxyResponder
+
+    typealias Value = [ViewResponder]
+
+    mutating func updateValue() {
+        responder.updateChildren($children.changedValue())
+        if !hasValue {
+            value = [responder]
+        }
+    }
+}
+#endif

Sources/OpenSwiftUI/Event/Focus/Focusable.swift

@@ -403,7 +403,7 @@ private class FocusableViewResponder: DefaultLayoutViewResponder, FocusResponder
     var focusProxyResponder: ViewResponder? {
         var result: ViewResponder?
         visit { responder in
-            guard let responder = responder as? FocusEventProxyResponder else { return .next }
+            guard let responder = asFocusEventProxyResponder(responder) else { return .next }
             result = responder
             return .cancel
         }