From 3d5204cd34e7a0d292d80b13cec69fcfc6ed9dc7 Mon Sep 17 00:00:00 2001
From: Nick Gerleman <ngerlem@meta.com>
Date: Fri, 4 Aug 2023 11:23:45 -0700
Subject: [PATCH] Add MC to disable `onScroll` requiring `scrollEventThrottle`
 on iOS. (#38742)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/38742

If you try to add an `onScroll` event listener on iOS, you will get a warning that you must additionally add a `scrollEventThrottle`, otherwise you will only receive a single event throughout the duration of the gesturing.

Values under 17ms are unthrottled, so it is common to see components passing magic values `16` and `0.0001` to ask for full events.

This behavior is inconsistent with Android, Web, and Windows.

This change runs an experiment on iOS to continuously fire scroll events to users of `onScroll` without `scrollEventThrottle`. Because there are warnings already, I think this case will be rare, and any code written for Android will already have to be written to be compatible with it.

I intentionally left the warning in for now, so that folks don't add new code relying on this QE while it is being tested.

Changelog: [Internal]

Reviewed By: yungsters

Differential Revision: D47975260

fbshipit-source-id: 8f182c5514cc557e1804586e8cbc68fc54502d83
---
 .../ComponentViews/ScrollView/RCTScrollViewComponentView.mm   | 3 ++-
 packages/react-native/React/Fabric/RCTSurfacePresenter.mm     | 4 ++++
 .../react-native/ReactCommon/react/utils/CoreFeatures.cpp     | 1 +
 packages/react-native/ReactCommon/react/utils/CoreFeatures.h  | 4 ++++
 4 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/packages/react-native/React/Fabric/Mounting/ComponentViews/ScrollView/RCTScrollViewComponentView.mm b/packages/react-native/React/Fabric/Mounting/ComponentViews/ScrollView/RCTScrollViewComponentView.mm
index 94c82ad08ad..5c3f18d1c16 100644
--- a/packages/react-native/React/Fabric/Mounting/ComponentViews/ScrollView/RCTScrollViewComponentView.mm
+++ b/packages/react-native/React/Fabric/Mounting/ComponentViews/ScrollView/RCTScrollViewComponentView.mm
@@ -442,7 +442,8 @@ static void RCTSendScrollEventForNativeAnimations_DEPRECATED(UIScrollView *scrol
   }
 
   NSTimeInterval now = CACurrentMediaTime();
-  if ((_lastScrollEventDispatchTime == 0) || (now - _lastScrollEventDispatchTime > _scrollEventThrottle)) {
+  if (CoreFeatures::disableScrollEventThrottleRequirement || (_lastScrollEventDispatchTime == 0) ||
+      (now - _lastScrollEventDispatchTime > _scrollEventThrottle)) {
     _lastScrollEventDispatchTime = now;
     if (_eventEmitter) {
       static_cast<ScrollViewEventEmitter const &>(*_eventEmitter).onScroll([self _scrollViewMetrics]);
diff --git a/packages/react-native/React/Fabric/RCTSurfacePresenter.mm b/packages/react-native/React/Fabric/RCTSurfacePresenter.mm
index d17a646fb19..6c2e6936774 100644
--- a/packages/react-native/React/Fabric/RCTSurfacePresenter.mm
+++ b/packages/react-native/React/Fabric/RCTSurfacePresenter.mm
@@ -280,6 +280,10 @@ static BackgroundExecutor RCTGetBackgroundExecutor()
     CoreFeatures::enableMountHooks = true;
   }
 
+  if (reactNativeConfig && reactNativeConfig->getBool("react_fabric:disable_scroll_event_throttle_requirement")) {
+    CoreFeatures::disableScrollEventThrottleRequirement = true;
+  }
+
   auto componentRegistryFactory =
       [factory = wrapManagedObject(_mountingManager.componentViewRegistry.componentViewFactory)](
           EventDispatcher::Weak const &eventDispatcher, ContextContainer::Shared const &contextContainer) {
diff --git a/packages/react-native/ReactCommon/react/utils/CoreFeatures.cpp b/packages/react-native/ReactCommon/react/utils/CoreFeatures.cpp
index e44b0190501..0554485f543 100644
--- a/packages/react-native/ReactCommon/react/utils/CoreFeatures.cpp
+++ b/packages/react-native/ReactCommon/react/utils/CoreFeatures.cpp
@@ -19,5 +19,6 @@ bool CoreFeatures::enableGranularScrollViewStateUpdatesIOS = false;
 bool CoreFeatures::enableMountHooks = false;
 bool CoreFeatures::doNotSwapLeftAndRightOnAndroidInLTR = false;
 bool CoreFeatures::enableCleanParagraphYogaNode = false;
+bool CoreFeatures::disableScrollEventThrottleRequirement = false;
 
 } // namespace facebook::react
diff --git a/packages/react-native/ReactCommon/react/utils/CoreFeatures.h b/packages/react-native/ReactCommon/react/utils/CoreFeatures.h
index 2d693ffc68b..3c9af96e609 100644
--- a/packages/react-native/ReactCommon/react/utils/CoreFeatures.h
+++ b/packages/react-native/ReactCommon/react/utils/CoreFeatures.h
@@ -56,6 +56,10 @@ class CoreFeatures {
 
   // Clean yoga node when <Text /> does not change.
   static bool enableCleanParagraphYogaNode;
+
+  // Fire `onScroll` events continuously on iOS without a `scrollEventThrottle`
+  // props, and provide continuous `onScroll` upates like other platforms.
+  static bool disableScrollEventThrottleRequirement;
 };
 
 } // namespace facebook::react