feat: implement LayoutEventEmitter for onLayout events

Author: paradowstack

packages/react-native/ReactCommon/react/renderer/mounting/ShadowTree.cpp

@@ -11,7 +11,6 @@
 #include <jsinspector-modern/tracing/PerformanceTracerSection.h>
 #include <react/debug/react_native_assert.h>
 #include <react/renderer/components/root/RootComponentDescriptor.h>
-#include <react/renderer/components/view/ViewShadowNode.h>
 #include <react/renderer/core/LayoutContext.h>
 #include <react/renderer/core/LayoutPrimitives.h>
 #include <react/renderer/mounting/ShadowTreeRevision.h>
@@ -463,8 +462,6 @@ CommitStatus ShadowTree::tryCommit(
   delegate_.shadowTreeDidCommit(
       *this, newRevision.rootShadowNode, affectedLayoutableNodes);
 
-  emitLayoutEvents(affectedLayoutableNodes);
-
   if (isReactBranch) {
     scheduleReactRevisionPromotion();
   } else if (commitMode == CommitMode::Normal) {
@@ -630,25 +627,6 @@ void ShadowTree::commitEmptyTree() const {
       {/* default commit options */});
 }
 
-void ShadowTree::emitLayoutEvents(
-    std::vector<const LayoutableShadowNode*>& affectedLayoutableNodes) const {
-  TraceSection s(
-      "ShadowTree::emitLayoutEvents",
-      "affectedLayoutableNodes",
-      affectedLayoutableNodes.size());
-
-  for (const auto* layoutableNode : affectedLayoutableNodes) {
-    if (auto viewProps =
-            dynamic_cast<const ViewProps*>(layoutableNode->getProps().get())) {
-      if (viewProps->onLayout) {
-        static_cast<const BaseViewEventEmitter&>(
-            *layoutableNode->getEventEmitter())
-            .onLayout(layoutableNode->getLayoutMetrics());
-      }
-    }
-  }
-}
-
 void ShadowTree::notifyDelegatesOfUpdates() const {
   delegate_.shadowTreeDidFinishTransaction(mountingCoordinator_, true);
 }

packages/react-native/ReactCommon/react/renderer/mounting/ShadowTree.h

@@ -161,8 +161,6 @@ class ShadowTree final {
 
   void mount(ShadowTreeRevision revision, bool mountSynchronously) const;
 
-  void emitLayoutEvents(std::vector<const LayoutableShadowNode *> &affectedLayoutableNodes) const;
-
   void scheduleReactRevisionPromotion() const;
 
   const SurfaceId surfaceId_;

packages/react-native/ReactCommon/react/renderer/scheduler/Scheduler.cpp

@@ -20,6 +20,7 @@
 #include <react/renderer/mounting/MountingOverrideDelegate.h>
 #include <react/renderer/mounting/ShadowViewMutation.h>
 #include <react/renderer/runtimescheduler/RuntimeScheduler.h>
+#include <react/renderer/uimanager/LayoutEventEmitter.h>
 #include <react/renderer/uimanager/UIManager.h>
 #include <react/renderer/uimanager/UIManagerBinding.h>
 #include <mutex>
@@ -148,6 +149,11 @@ Scheduler::Scheduler(
 
   delegate_ = delegate;
   commitHooks_ = schedulerToolbox.commitHooks;
+
+  // Layout events (`onLayout`) are emitted as a standalone consumer of the
+  // `shadowTreeDidCommit` commit hook.
+  commitHooks_.push_back(std::make_shared<LayoutEventEmitter>());
+
   uiManager_ = uiManager;
 
   for (auto& commitHook : commitHooks_) {

packages/react-native/ReactCommon/react/renderer/uimanager/LayoutEventEmitter.cpp

@@ -0,0 +1,38 @@
+/*
+ * 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 "LayoutEventEmitter.h"
+
+#include <cxxreact/TraceSection.h>
+#include <react/renderer/components/view/ViewShadowNode.h>
+#include <react/renderer/core/LayoutableShadowNode.h>
+
+namespace facebook::react {
+
+void LayoutEventEmitter::shadowTreeDidCommit(
+    const ShadowTree& /*shadowTree*/,
+    const RootShadowNode::Shared& /*rootShadowNode*/,
+    const std::vector<const LayoutableShadowNode*>&
+        affectedLayoutableNodes) noexcept {
+  TraceSection s(
+      "LayoutEventEmitter::shadowTreeDidCommit",
+      "affectedLayoutableNodes",
+      affectedLayoutableNodes.size());
+
+  for (const auto* layoutableNode : affectedLayoutableNodes) {
+    if (auto viewProps =
+            dynamic_cast<const ViewProps*>(layoutableNode->getProps().get())) {
+      if (viewProps->onLayout) {
+        static_cast<const BaseViewEventEmitter&>(
+            *layoutableNode->getEventEmitter())
+            .onLayout(layoutableNode->getLayoutMetrics());
+      }
+    }
+  }
+}
+
+} // namespace facebook::react

packages/react-native/ReactCommon/react/renderer/uimanager/LayoutEventEmitter.h

@@ -0,0 +1,32 @@
+/*
+ * 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.
+ */
+
+#pragma once
+
+#include <react/renderer/uimanager/UIManagerCommitHook.h>
+
+namespace facebook::react {
+
+/*
+ * Emits layout events (`onLayout`) for the nodes whose layout changed in
+ * each commit, using the `shadowTreeDidCommit` commit hook as the source of
+ * layout changes (this logic used to be implemented directly in
+ * `ShadowTree`).
+ */
+class LayoutEventEmitter final : public UIManagerCommitHook {
+ public:
+  void commitHookWasRegistered(const UIManager& uiManager) noexcept override {}
+  void commitHookWasUnregistered(const UIManager& uiManager) noexcept override {}
+
+  void shadowTreeDidCommit(
+      const ShadowTree& shadowTree,
+      const RootShadowNode::Shared& rootShadowNode,
+      const std::vector<const LayoutableShadowNode*>&
+          affectedLayoutableNodes) noexcept override;
+};
+
+} // namespace facebook::react