From 62794774b91c2b9915f21c333165179e2bbb6549 Mon Sep 17 00:00:00 2001
From: Mark Verlingieri <mkv@meta.com>
Date: Tue, 14 Apr 2026 11:16:01 -0700
Subject: [PATCH] Process event-driven animations synchronously on every event
 (#56439)
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Summary:

The C++ NativeAnimatedNodesManager only processes the first scroll event in a gesture synchronously via trigger()/onRender(). Subsequent events just store the updated value and defer graph traversal + prop commit to the next choreographer frame, introducing ~16ms (1 frame) latency.

This differs from the Java NativeAnimatedNodesManager which calls updateNodes() → updateView() → synchronouslyUpdateViewOnUIThread() for every event with no gating.

The issue was the `!isEventAnimationInProgress_` guard at line 502, which prevented trigger()/onRender() from running on the 2nd+ events. `isEventAnimationInProgress_` stays true for the entire scroll gesture (only resets when a choreographer frame produces zero changes), so every scroll event after the first was deferred.

This change moves the trigger()/onRender() call outside the `!isEventAnimationInProgress_` gate so every scroll event gets immediate synchronous processing, matching the Java implementation. The startRenderCallbackIfNeeded + isEventAnimationInProgress_ setup still only happens once (first event).

## Changelog:
[General] [Fixed] - Fix 1-frame latency in C++ NativeAnimatedNodesManager for event-driven animations by processing the animation graph synchronously on every scroll event, matching the Java implementation behavior

Reviewed By: zeyap

Differential Revision: D100719854
---
 .../animated/NativeAnimatedNodesManager.cpp   | 26 ++++++++++++-------
 1 file changed, 17 insertions(+), 9 deletions(-)

diff --git a/packages/react-native/ReactCommon/react/renderer/animated/NativeAnimatedNodesManager.cpp b/packages/react-native/ReactCommon/react/renderer/animated/NativeAnimatedNodesManager.cpp
index 8cac7589ca0c..991ec92327a1 100644
--- a/packages/react-native/ReactCommon/react/renderer/animated/NativeAnimatedNodesManager.cpp
+++ b/packages/react-native/ReactCommon/react/renderer/animated/NativeAnimatedNodesManager.cpp
@@ -499,15 +499,23 @@ void NativeAnimatedNodesManager::handleAnimatedEvent(
     }
   }
 
-  if (foundAtLeastOneDriver && !isEventAnimationInProgress_) {
-    // There is an animation driver handling this event and
-    // event driven animation has not been started yet.
-    isEventAnimationInProgress_ = true;
-    // Some platforms (e.g. iOS) have UI tick listener disable
-    // when there are no active animations. Calling
-    // `startRenderCallbackIfNeeded` will call platform specific code to
-    // register UI tick listener.
-    startRenderCallbackIfNeeded(false);
+  if (foundAtLeastOneDriver) {
+    // Process event-driven animation updates synchronously, matching the
+    // behavior of the Java NativeAnimatedNodesManager which calls
+    // updateNodes() for every event. Without this, only the first event
+    // in a scroll sequence is processed synchronously — subsequent events
+    // just store the updated value and defer graph traversal + prop commit
+    // to the next choreographer frame, introducing 1-frame latency.
+    if (!isEventAnimationInProgress_) {
+      // There is an animation driver handling this event and
+      // event driven animation has not been started yet.
+      isEventAnimationInProgress_ = true;
+      // Some platforms (e.g. iOS) have UI tick listener disable
+      // when there are no active animations. Calling
+      // `startRenderCallbackIfNeeded` will call platform specific code to
+      // register UI tick listener.
+      startRenderCallbackIfNeeded(false);
+    }
     // Calling startOnRenderCallback_ will register a UI tick listener.
     // The UI ticker listener will not be called until the next frame.
     // That's why, in case this is called from the UI thread, we need to