Implement snapToAlignment property for ScrollView in Fabric architecture

change/react-native-windows-ae44eda2-ce08-4405-916b-db6f08ee6786.json

@@ -0,0 +1,7 @@
+{
+  "type": "prerelease",
+  "comment": "Implement snapToAlignment support for Fabric ScrollView - interface and prop handling",
+  "packageName": "react-native-windows",
+  "email": "[email protected]",
+  "dependentChangeType": "patch"
+}

vnext/Microsoft.ReactNative/CompositionSwitcher.idl

@@ -31,6 +31,13 @@ namespace Microsoft.ReactNative.Composition.Experimental
     SwitchThumb,
   };
 
+  enum SnapAlignment
+  {
+    Start,
+    Center,
+    End,
+  };
+
   [webhosthidden]
   [uuid("172def51-9e1a-4e3c-841a-e5a470065acc")] // uuid needed for empty interfaces
   [version(0)]
@@ -120,7 +127,7 @@ namespace Microsoft.ReactNative.Composition.Experimental
     void SetMaximumZoomScale(Single maximumZoomScale);
     void SetMinimumZoomScale(Single minimumZoomScale);
     Boolean Horizontal;
-    void SetSnapPoints(Boolean snapToStart, Boolean snapToEnd, Windows.Foundation.Collections.IVectorView<Single> offsets);
+    void SetSnapPoints(Boolean snapToStart, Boolean snapToEnd, Windows.Foundation.Collections.IVectorView<Single> offsets, SnapAlignment snapToAlignment);
   }
 
   [webhosthidden]

vnext/Microsoft.ReactNative/Fabric/Composition/CompositionContextHelper.cpp

@@ -871,9 +871,11 @@ struct CompScrollerVisual : winrt::implements<
   void SetSnapPoints(
       bool snapToStart,
       bool snapToEnd,
-      winrt::Windows::Foundation::Collections::IVectorView<float> const &offsets) noexcept {
+      winrt::Windows::Foundation::Collections::IVectorView<float> const &offsets,
+      winrt::Microsoft::ReactNative::Composition::Experimental::SnapAlignment snapToAlignment) noexcept {
     m_snapToStart = snapToStart;
     m_snapToEnd = snapToEnd;
+    m_snapToAlignment = snapToAlignment;
     m_snapToOffsets.clear();
     if (offsets) {
       for (auto const &offset : offsets) {
@@ -1100,6 +1102,22 @@ struct CompScrollerVisual : winrt::implements<
     }
 
     snapPositions.insert(snapPositions.end(), m_snapToOffsets.begin(), m_snapToOffsets.end());
+
+    // Adjust snap positions based on alignment
+    const float viewportSize = m_horizontal ? visualSize.x : visualSize.y;
+    if (m_snapToAlignment == winrt::Microsoft::ReactNative::Composition::Experimental::SnapAlignment::Center) {
+      // For center alignment, offset snap positions by half the viewport size
+      for (auto &position : snapPositions) {
+        position = std::max(0.0f, position - viewportSize / 2.0f);
+      }
+    } else if (m_snapToAlignment == winrt::Microsoft::ReactNative::Composition::Experimental::SnapAlignment::End) {
+      // For end alignment, offset snap positions by the full viewport size
+      for (auto &position : snapPositions) {
+        position = std::max(0.0f, position - viewportSize);
+      }
+    }
+    // For Start alignment, no adjustment needed
+
     std::sort(snapPositions.begin(), snapPositions.end());
     snapPositions.erase(std::unique(snapPositions.begin(), snapPositions.end()), snapPositions.end());
 
@@ -1227,6 +1245,8 @@ struct CompScrollerVisual : winrt::implements<
   bool m_snapToStart{true};
   bool m_snapToEnd{true};
   std::vector<float> m_snapToOffsets;
+  winrt::Microsoft::ReactNative::Composition::Experimental::SnapAlignment m_snapToAlignment{
+      winrt::Microsoft::ReactNative::Composition::Experimental::SnapAlignment::Start};
   bool m_inertia{false};
   bool m_custom{false};
   winrt::Windows::Foundation::Numerics::float3 m_targetPosition;

vnext/Microsoft.ReactNative/Fabric/Composition/ScrollViewComponentView.cpp

@@ -807,12 +807,17 @@ void ScrollViewComponentView::updateProps(
   }
 
   if (oldViewProps.snapToStart != newViewProps.snapToStart || oldViewProps.snapToEnd != newViewProps.snapToEnd ||
-      oldViewProps.snapToOffsets != newViewProps.snapToOffsets) {
+      oldViewProps.snapToOffsets != newViewProps.snapToOffsets ||
+      oldViewProps.snapToAlignment != newViewProps.snapToAlignment) {
     const auto snapToOffsets = winrt::single_threaded_vector<float>();
     for (const auto &offset : newViewProps.snapToOffsets) {
       snapToOffsets.Append(static_cast<float>(offset));
     }
-    m_scrollVisual.SetSnapPoints(newViewProps.snapToStart, newViewProps.snapToEnd, snapToOffsets.GetView());
+
+    auto snapAlignment = convertSnapToAlignment(newViewProps.snapToAlignment);
+
+    m_scrollVisual.SetSnapPoints(
+        newViewProps.snapToStart, newViewProps.snapToEnd, snapToOffsets.GetView(), snapAlignment);
   }
 }
 
@@ -1435,4 +1440,17 @@ void ScrollViewComponentView::updateShowsVerticalScrollIndicator(bool value) noe
 void ScrollViewComponentView::updateDecelerationRate(float value) noexcept {
   m_scrollVisual.SetDecelerationRate({value, value, value});
 }
+
+winrt::Microsoft::ReactNative::Composition::Experimental::SnapAlignment ScrollViewComponentView::convertSnapToAlignment(
+    facebook::react::ScrollViewSnapToAlignment alignment) noexcept {
+  switch (alignment) {
+    case facebook::react::ScrollViewSnapToAlignment::Center:
+      return winrt::Microsoft::ReactNative::Composition::Experimental::SnapAlignment::Center;
+    case facebook::react::ScrollViewSnapToAlignment::End:
+      return winrt::Microsoft::ReactNative::Composition::Experimental::SnapAlignment::End;
+    case facebook::react::ScrollViewSnapToAlignment::Start:
+    default:
+      return winrt::Microsoft::ReactNative::Composition::Experimental::SnapAlignment::Start;
+  }
+}
 } // namespace winrt::Microsoft::ReactNative::Composition::implementation

vnext/Microsoft.ReactNative/Fabric/Composition/ScrollViewComponentView.h

@@ -134,6 +134,8 @@ struct ScrollInteractionTrackerOwner : public winrt::implements<
       winrt::Microsoft::ReactNative::Composition::Experimental::IScrollPositionChangedArgs const &args) noexcept;
   void updateShowsHorizontalScrollIndicator(bool value) noexcept;
   void updateShowsVerticalScrollIndicator(bool value) noexcept;
+  winrt::Microsoft::ReactNative::Composition::Experimental::SnapAlignment convertSnapToAlignment(
+      facebook::react::ScrollViewSnapToAlignment alignment) noexcept;
 
   facebook::react::Size m_contentSize;
   winrt::Microsoft::ReactNative::Composition::Experimental::IScrollVisual m_scrollVisual{nullptr};