Avoid per-child dynamic_casts during Yoga layout

packages/react-native/Libraries/Components/LayoutConformance/__tests__/LayoutConformance-itest.js

@@ -0,0 +1,72 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow strict-local
+ * @fantom_flags enableFlexboxAutoMinSizeInStrictMode:*
+ * @format
+ */
+
+import '@react-native/fantom/src/setUpDefaultReactNativeEnvironment';
+
+import type {HostInstance} from 'react-native';
+
+import * as Fantom from '@react-native/fantom';
+import nullthrows from 'nullthrows';
+import * as React from 'react';
+import {createRef} from 'react';
+import {
+  View,
+  experimental_LayoutConformance as LayoutConformance,
+} from 'react-native';
+
+type Mode = 'strict' | 'compatibility';
+
+function renderRowWidth(mode: Mode, outerMode?: Mode): number {
+  const root = Fantom.createRoot();
+  const rowRef = createRef<HostInstance>();
+
+  const content = (
+    <LayoutConformance mode={mode}>
+      <View
+        collapsable={false}
+        style={{
+          width: 60,
+          height: 60,
+          flexDirection: 'row',
+          alignItems: 'center',
+        }}>
+        {/* With YGErrataStretchFlexBasis this row grows to its parent's
+            width, even though its content takes no space. */}
+        <View collapsable={false} ref={rowRef} style={{flexDirection: 'row'}}>
+          <View collapsable={false} style={{height: 30, flexGrow: 1}} />
+        </View>
+      </View>
+    </LayoutConformance>
+  );
+
+  Fantom.runTask(() => {
+    root.render(
+      outerMode != null ? (
+        <LayoutConformance mode={outerMode}>{content}</LayoutConformance>
+      ) : (
+        content
+      ),
+    );
+  });
+
+  const width = nullthrows(rowRef.current).getBoundingClientRect().width;
+  root.destroy();
+  return width;
+}
+
+test('strict mode lays out its subtree without the stretch flex-basis errata', () => {
+  expect(renderRowWidth('strict')).toBe(0);
+});
+
+// Compatibility is also the default, so check it under a strict subtree.
+test('compatibility mode inside strict mode brings the stretch flex-basis errata back', () => {
+  expect(renderRowWidth('compatibility', 'strict')).toBe(60);
+});

packages/react-native/ReactCommon/react/renderer/components/view/LayoutConformanceShadowNode.cpp

@@ -0,0 +1,15 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#include "LayoutConformanceShadowNode.h"
+
+namespace facebook::react {
+
+// NOLINTNEXTLINE(facebook-hte-CArray,modernize-avoid-c-arrays)
+const char LayoutConformanceShadowNodeComponentName[] = "LayoutConformance";
+
+} // namespace facebook::react

packages/react-native/ReactCommon/react/renderer/components/view/LayoutConformanceShadowNode.h

@@ -15,7 +15,7 @@
 namespace facebook::react {
 
 // NOLINTNEXTLINE(facebook-hte-CArray,modernize-avoid-c-arrays)
-constexpr const char LayoutConformanceShadowNodeComponentName[] = "LayoutConformance";
+extern const char LayoutConformanceShadowNodeComponentName[];
 
 class LayoutConformanceShadowNode final : public ConcreteShadowNode<
                                               LayoutConformanceShadowNodeComponentName,

packages/react-native/ReactCommon/react/renderer/components/view/YogaLayoutableShadowNode.cpp

@@ -560,9 +560,15 @@ void YogaLayoutableShadowNode::configureYogaTree(
 }
 
 YGErrata YogaLayoutableShadowNode::resolveErrata(YGErrata defaultErrata) const {
-  if (auto layoutConformanceNode =
-          dynamic_cast<const LayoutConformanceShadowNode*>(this)) {
-    switch (layoutConformanceNode->getConcreteProps().mode) {
+  // This runs for every child on every layout, and almost no node is a
+  // LayoutConformance node, so a failing `dynamic_cast` would be the common
+  // case. The class is final and its component name is defined once, in
+  // LayoutConformanceShadowNode.cpp, so comparing component handles is
+  // equivalent.
+  if (getComponentHandle() == LayoutConformanceShadowNode::Handle()) {
+    const auto& layoutConformanceNode =
+        static_cast<const LayoutConformanceShadowNode&>(*this);
+    switch (layoutConformanceNode.getConcreteProps().mode) {
       case LayoutConformance::Strict:
         // CSS Flexbox §4.5 automatic minimum sizing is the spec-compliant
         // behaviour, so strict conformance should adopt it — but its probe
@@ -829,7 +835,12 @@ Rect YogaLayoutableShadowNode::getContentBounds() const {
 
     auto layoutMetricsWithOverflowInset = childNode.getLayoutMetrics();
     if (layoutMetricsWithOverflowInset.displayType != DisplayType::None) {
-      auto viewChildNode = dynamic_cast<const ViewShadowNode*>(&childNode);
+      // ViewShadowNode is final, so comparing component handles is equivalent
+      // to a `dynamic_cast`, without its cost for every non-View child.
+      auto viewChildNode =
+          childNode.getComponentHandle() == ViewShadowNode::Handle()
+          ? static_cast<const ViewShadowNode*>(&childNode)
+          : nullptr;
       auto hitSlop = viewChildNode != nullptr
           ? viewChildNode->getConcreteProps().hitSlop
           : EdgeInsets{};
@@ -951,8 +962,8 @@ float YogaLayoutableShadowNode::yogaNodeBaselineCallbackConnector(
 
 YogaLayoutableShadowNode& YogaLayoutableShadowNode::shadowNodeFromContext(
     YGNodeConstRef yogaNode) {
-  return dynamic_cast<YogaLayoutableShadowNode&>(
-      *static_cast<ShadowNode*>(YGNodeGetContext(yogaNode)));
+  // The context is only set by the constructors of this class, to `this`.
+  return *static_cast<YogaLayoutableShadowNode*>(YGNodeGetContext(yogaNode));
 }
 
 yoga::Config& YogaLayoutableShadowNode::initializeYogaConfig(

private/react-native-fantom/tester/src/stubs/StubComponentRegistryFactory.h

@@ -16,6 +16,7 @@
 #include <react/renderer/components/text/RawTextComponentDescriptor.h>
 #include <react/renderer/components/text/SelectableParagraphComponentDescriptor.h>
 #include <react/renderer/components/text/TextComponentDescriptor.h>
+#include <react/renderer/components/view/LayoutConformanceComponentDescriptor.h>
 #include <react/renderer/components/view/ViewComponentDescriptor.h>
 
 namespace facebook::react {
@@ -27,6 +28,7 @@ inline ComponentRegistryFactory getDefaultComponentRegistryFactory()
     static auto providerRegistry = []() {
       auto providerRegistry = std::make_shared<ComponentDescriptorProviderRegistry>();
       providerRegistry->add(concreteComponentDescriptorProvider<ImageComponentDescriptor>());
+      providerRegistry->add(concreteComponentDescriptorProvider<LayoutConformanceComponentDescriptor>());
       providerRegistry->add(concreteComponentDescriptorProvider<ParagraphComponentDescriptor>());
       providerRegistry->add(concreteComponentDescriptorProvider<ScrollViewComponentDescriptor>());
       providerRegistry->add(concreteComponentDescriptorProvider<RawTextComponentDescriptor>());

scripts/cxx-api/api-snapshots/ReactAndroidFrameworksCxx.api

@@ -376,6 +376,7 @@ struct facebook::hermes::ISetEventLoopControl : public facebook::jsi::ICast {
 const char facebook::react::AndroidTextInputComponentName[];
 const char facebook::react::ImageComponentName[];
 const char facebook::react::ImageManagerKey[];
+const char facebook::react::LayoutConformanceShadowNodeComponentName[];
 const char facebook::react::ModalHostViewComponentName[];
 const char facebook::react::ParagraphComponentName[];
 const char facebook::react::RawTextComponentName[];
@@ -386,7 +387,6 @@ const char facebook::react::TextComponentName[];
 const char facebook::react::ViewComponentName[];
 const facebook::react::EventTag facebook::react::EMPTY_EVENT_TAG;
 constexpr auto facebook::react::kSimpleThreadSafeCacheSizeCap;
-constexpr char facebook::react::LayoutConformanceShadowNodeComponentName[];
 constexpr char* const facebook::react::TextLayoutManagerKey;
 constexpr facebook::react::ReactNativeVersionType facebook::react::ReactNativeVersion;
 constexpr facebook::react::Tag facebook::react::kNoTag;

scripts/cxx-api/api-snapshots/ReactAppleFrameworksCxx.api

@@ -3221,6 +3221,7 @@ struct facebook::hermes::ISetEventLoopControl : public facebook::jsi::ICast {
 
 const char facebook::react::ImageComponentName[];
 const char facebook::react::ImageManagerKey[];
+const char facebook::react::LayoutConformanceShadowNodeComponentName[];
 const char facebook::react::ModalHostViewComponentName[];
 const char facebook::react::ParagraphComponentName[];
 const char facebook::react::RawTextComponentName[];
@@ -3232,7 +3233,6 @@ const char facebook::react::TextInputComponentName[];
 const char facebook::react::ViewComponentName[];
 const facebook::react::EventTag facebook::react::EMPTY_EVENT_TAG;
 constexpr auto facebook::react::kSimpleThreadSafeCacheSizeCap;
-constexpr char facebook::react::LayoutConformanceShadowNodeComponentName[];
 constexpr char* const facebook::react::TextLayoutManagerKey;
 constexpr facebook::react::ReactNativeVersionType facebook::react::ReactNativeVersion;
 constexpr facebook::react::Tag facebook::react::kNoTag;

scripts/cxx-api/api-snapshots/ReactCommonFrameworksCxx.api

@@ -73,6 +73,7 @@ struct facebook::hermes::ISetEventLoopControl : public facebook::jsi::ICast {
 
 const char facebook::react::ImageComponentName[];
 const char facebook::react::ImageManagerKey[];
+const char facebook::react::LayoutConformanceShadowNodeComponentName[];
 const char facebook::react::ModalHostViewComponentName[];
 const char facebook::react::ParagraphComponentName[];
 const char facebook::react::RawTextComponentName[];
@@ -83,7 +84,6 @@ const char facebook::react::TextComponentName[];
 const char facebook::react::ViewComponentName[];
 const facebook::react::EventTag facebook::react::EMPTY_EVENT_TAG;
 constexpr auto facebook::react::kSimpleThreadSafeCacheSizeCap;
-constexpr char facebook::react::LayoutConformanceShadowNodeComponentName[];
 constexpr char* const facebook::react::TextLayoutManagerKey;
 constexpr facebook::react::ReactNativeVersionType facebook::react::ReactNativeVersion;
 constexpr facebook::react::Tag facebook::react::kNoTag;