From 2ac8dc1e78ddf72fe196aa6217a93d414ea36cad Mon Sep 17 00:00:00 2001
From: Thomas Nardone <tnardone@meta.com>
Date: Wed, 8 May 2024 14:32:24 -0700
Subject: [PATCH] Coordinate Synchronous Events with Choreographer (#44491)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/44491

Changelog: [internal]

Ensure only one scroll event is fired per frame by tracking the events in FabricUIManager

Reviewed By: sammy-SC

Differential Revision: D57018741

fbshipit-source-id: c1ad59f934e359edfeb8f3e084106eebd467a0b1
---
 .../facebook/react/fabric/FabricUIManager.java | 14 +++++++++++++-
 .../facebook/react/fabric/SynchronousEvent.kt  | 18 ++++++++++++++++++
 2 files changed, 31 insertions(+), 1 deletion(-)
 create mode 100644 packages/react-native/ReactAndroid/src/main/java/com/facebook/react/fabric/SynchronousEvent.kt

diff --git a/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/fabric/FabricUIManager.java b/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/fabric/FabricUIManager.java
index d86ea31b67f..a6fd6c05875 100644
--- a/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/fabric/FabricUIManager.java
+++ b/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/fabric/FabricUIManager.java
@@ -89,9 +89,11 @@ import com.facebook.react.uimanager.events.SynchronousEventReceiver;
 import com.facebook.react.views.text.TextLayoutManager;
 import java.util.ArrayList;
 import java.util.HashMap;
+import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Queue;
+import java.util.Set;
 import java.util.concurrent.CopyOnWriteArrayList;
 
 /**
@@ -183,6 +185,9 @@ public class FabricUIManager
   @NonNull
   private final DispatchUIFrameCallback mDispatchUIFrameCallback;
 
+  /** Set of events sent synchronously during the current frame render. Cleared after each frame. */
+  private final Set<SynchronousEvent> mSynchronousEvents = new HashSet<>();
+
   /**
    * This is used to keep track of whether or not the FabricUIManager has been destroyed. Once the
    * Catalyst instance is being destroyed, we should cease all operation here.
@@ -995,7 +1000,12 @@ public class FabricUIManager
     }
 
     if (experimental_isSynchronous) {
-      eventEmitter.dispatchEventSynchronously(eventName, params);
+      // add() returns true only if there are no equivalent events already in the set
+      boolean firstEventForFrame =
+          mSynchronousEvents.add(new SynchronousEvent(surfaceId, reactTag, eventName));
+      if (firstEventForFrame) {
+        eventEmitter.dispatchEventSynchronously(eventName, params);
+      }
     } else {
       if (canCoalesceEvent) {
         eventEmitter.dispatchUnique(eventName, params);
@@ -1351,6 +1361,8 @@ public class FabricUIManager
             .postFrameCallback(
                 ReactChoreographer.CallbackType.DISPATCH_UI, mDispatchUIFrameCallback);
       }
+
+      mSynchronousEvents.clear();
     }
   }
 }
diff --git a/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/fabric/SynchronousEvent.kt b/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/fabric/SynchronousEvent.kt
new file mode 100644
index 00000000000..5d9cffcc332
--- /dev/null
+++ b/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/fabric/SynchronousEvent.kt
@@ -0,0 +1,18 @@
+/*
+ * 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.
+ */
+
+package com.facebook.react.fabric
+
+/**
+ * Represents the identifying criteria of a synchronous event that was sent directly on the main
+ * thread. Used to determine if subsequent events are duplicates and should not be emitted.
+ */
+internal data class SynchronousEvent(
+    val surfaceId: Int,
+    val viewTag: Int,
+    val eventName: String,
+)