[Fabric] Implement scrollEventThrottle for ScrollView (#14555)

anupriya13 · web-flow · commit ba877502d9af · 2025-04-21T21:37:22.000+05:30
diff --git a/change/react-native-windows-38ddfc46-37fc-42ff-90d8-d3f72a15f8a2.json b/change/react-native-windows-38ddfc46-37fc-42ff-90d8-d3f72a15f8a2.json
@@ -0,0 +1,7 @@
+{
+  "type": "prerelease",
+  "comment": "Implement scrollEventThrottle for ScrollView in Fabric",
+  "packageName": "react-native-windows",
+  "email": "54227869+anupriya13@users.noreply.github.com",
+  "dependentChangeType": "patch"
+}
diff --git a/packages/playground/Samples/scrollViewSnapSample.tsx b/packages/playground/Samples/scrollViewSnapSample.tsx
@@ -304,7 +304,8 @@ export default class Bootstrap extends React.Component<{}, any> {
             onScroll={() => {
               console.log('onScroll');
             }}
-            decelerationRate={0.95}>
+            decelerationRate={0.95}
+            scrollEventThrottle={50}>
             {this.makeItems(20, [styles.itemWrapper])}
           </ScrollView>
         </View>
diff --git a/vnext/Microsoft.ReactNative/Fabric/Composition/ScrollViewComponentView.cpp b/vnext/Microsoft.ReactNative/Fabric/Composition/ScrollViewComponentView.cpp
@@ -779,6 +779,20 @@ void ScrollViewComponentView::updateProps(
     updateDecelerationRate(newViewProps.decelerationRate);
   }
 
+  if (!oldProps || oldViewProps.scrollEventThrottle != newViewProps.scrollEventThrottle) {
+    // Zero means "send value only once per significant logical event".
+    // Prop value is in milliseconds.
+    auto throttleInSeconds = newViewProps.scrollEventThrottle / 1000.0;
+    auto msPerFrame = 1.0 / 60.0;
+    if (throttleInSeconds < 0) {
+      m_scrollEventThrottle = INFINITY;
+    } else if (throttleInSeconds <= msPerFrame) {
+      m_scrollEventThrottle = 0;
+    } else {
+      m_scrollEventThrottle = throttleInSeconds;
+    }
+  }
+
   if (oldViewProps.maximumZoomScale != newViewProps.maximumZoomScale) {
     m_scrollVisual.SetMaximumZoomScale(newViewProps.maximumZoomScale);
   }
@@ -1000,6 +1014,8 @@ bool ScrollViewComponentView::scrollToEnd(bool animate) noexcept {
 
   auto x = (m_contentSize.width - m_layoutMetrics.frame.size.width) * m_layoutMetrics.pointScaleFactor;
   auto y = (m_contentSize.height - m_layoutMetrics.frame.size.height) * m_layoutMetrics.pointScaleFactor;
+  // Ensure at least one scroll event will fire
+  m_allowNextScrollNoMatterWhat = true;
   m_scrollVisual.TryUpdatePosition({static_cast<float>(x), static_cast<float>(y), 0.0f}, animate);
   return true;
 }
@@ -1240,19 +1256,27 @@ winrt::Microsoft::ReactNative::Composition::Experimental::IVisual ScrollViewComp
       [this](
           winrt::IInspectable const & /*sender*/,
           winrt::Microsoft::ReactNative::Composition::Experimental::IScrollPositionChangedArgs const &args) {
-        updateStateWithContentOffset();
-        auto eventEmitter = GetEventEmitter();
-        if (eventEmitter) {
-          facebook::react::ScrollViewEventEmitter::Metrics scrollMetrics;
-          scrollMetrics.containerSize.height = m_layoutMetrics.frame.size.height;
-          scrollMetrics.containerSize.width = m_layoutMetrics.frame.size.width;
-          scrollMetrics.contentOffset.x = args.Position().x / m_layoutMetrics.pointScaleFactor;
-          scrollMetrics.contentOffset.y = args.Position().y / m_layoutMetrics.pointScaleFactor;
-          scrollMetrics.zoomScale = 1.0;
-          scrollMetrics.contentSize.height = std::max(m_contentSize.height, m_layoutMetrics.frame.size.height);
-          scrollMetrics.contentSize.width = std::max(m_contentSize.width, m_layoutMetrics.frame.size.width);
-          std::static_pointer_cast<facebook::react::ScrollViewEventEmitter const>(eventEmitter)
-              ->onScroll(scrollMetrics);
+        auto now = std::chrono::steady_clock::now();
+        auto elapsed = std::chrono::duration_cast<std::chrono::duration<double>>(now - m_lastScrollEventTime).count();
+
+        if (m_allowNextScrollNoMatterWhat ||
+            (m_scrollEventThrottle < std::max(std::chrono::duration<double>(0.017).count(), elapsed))) {
+          updateStateWithContentOffset();
+          auto eventEmitter = GetEventEmitter();
+          if (eventEmitter) {
+            facebook::react::ScrollViewEventEmitter::Metrics scrollMetrics;
+            scrollMetrics.containerSize.height = m_layoutMetrics.frame.size.height;
+            scrollMetrics.containerSize.width = m_layoutMetrics.frame.size.width;
+            scrollMetrics.contentOffset.x = args.Position().x / m_layoutMetrics.pointScaleFactor;
+            scrollMetrics.contentOffset.y = args.Position().y / m_layoutMetrics.pointScaleFactor;
+            scrollMetrics.zoomScale = 1.0;
+            scrollMetrics.contentSize.height = std::max(m_contentSize.height, m_layoutMetrics.frame.size.height);
+            scrollMetrics.contentSize.width = std::max(m_contentSize.width, m_layoutMetrics.frame.size.width);
+            std::static_pointer_cast<facebook::react::ScrollViewEventEmitter const>(eventEmitter)
+                ->onScroll(scrollMetrics);
+            m_lastScrollEventTime = now;
+            m_allowNextScrollNoMatterWhat = false;
+          }
         }
       });
 
@@ -1261,6 +1285,7 @@ winrt::Microsoft::ReactNative::Composition::Experimental::IVisual ScrollViewComp
       [this](
           winrt::IInspectable const & /*sender*/,
           winrt::Microsoft::ReactNative::Composition::Experimental::IScrollPositionChangedArgs const &args) {
+        m_allowNextScrollNoMatterWhat = true; // Ensure next scroll event is recorded, regardless of throttle
         updateStateWithContentOffset();
         auto eventEmitter = GetEventEmitter();
         if (eventEmitter) {
diff --git a/vnext/Microsoft.ReactNative/Fabric/Composition/ScrollViewComponentView.h b/vnext/Microsoft.ReactNative/Fabric/Composition/ScrollViewComponentView.h
@@ -147,6 +147,9 @@ struct ScrollInteractionTrackerOwner : public winrt::implements<
   bool m_isHorizontal = false;
   bool m_changeViewAfterLoaded = false;
   bool m_dismissKeyboardOnDrag = false;
+  double m_scrollEventThrottle{0.0};
+  bool m_allowNextScrollNoMatterWhat{false};
+  std::chrono::steady_clock::time_point m_lastScrollEventTime{};
   std::shared_ptr<facebook::react::ScrollViewShadowNode::ConcreteState const> m_state;
 };
 
