Generate nested scroll view (#55239)

Summary:

This diff adds the ability to experiment with using `NestedScrollView` instead of `ScrollView` as the parent class for `ReactScrollView` on Android.

Since Java doesn't support multiple inheritance or conditional parent class selection, this is implemented using code generation:
- A Python script (`generate-nested-scroll-view.py`) generates `ReactNestedScrollView.java` and `ReactNestedScrollViewManager.kt` from their respective source files
- The generated files are identical to the originals except they extend `NestedScrollView` instead of `ScrollView`
- A Buck genrule verifies the generated files stay in sync with source files at build time
- The `useNestedScrollViewAndroid` feature flag controls which implementation is used at runtime

This approach allows us to safely A/B test the NestedScrollView implementation without requiring JS changes, since both managers register with the same `REACT_CLASS` name (`"RCTScrollView"`).

Changelog: [Internal]

Differential Revision: D90902079

Author: Abbondanzo

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/internal/featureflags/ReactNativeFeatureFlags.kt

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<27dfa9832869aa05987fa7cb9740f7ba>>
+ * @generated SignedSource<<1d13409e4db7a5a48e5d2caf59a5fbcf>>
  */
 
 /**
@@ -456,6 +456,12 @@ public object ReactNativeFeatureFlags {
   @JvmStatic
   public fun useNativeViewConfigsInBridgelessMode(): Boolean = accessor.useNativeViewConfigsInBridgelessMode()
 
+  /**
+   * When enabled, ReactScrollView will extend NestedScrollView instead of ScrollView on Android for improved nested scrolling support.
+   */
+  @JvmStatic
+  public fun useNestedScrollViewAndroid(): Boolean = accessor.useNestedScrollViewAndroid()
+
   /**
    * Use shared animation backend in C++ Animated
    */

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/internal/featureflags/ReactNativeFeatureFlagsCxxAccessor.kt

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<6fb19235ba5049e07952cfa1b564e9e2>>
+ * @generated SignedSource<<58a693fe003a9a19311fc0134d071154>>
  */
 
 /**
@@ -91,6 +91,7 @@ internal class ReactNativeFeatureFlagsCxxAccessor : ReactNativeFeatureFlagsAcces
   private var useAlwaysAvailableJSErrorHandlingCache: Boolean? = null
   private var useFabricInteropCache: Boolean? = null
   private var useNativeViewConfigsInBridgelessModeCache: Boolean? = null
+  private var useNestedScrollViewAndroidCache: Boolean? = null
   private var useSharedAnimatedBackendCache: Boolean? = null
   private var useTraitHiddenOnAndroidCache: Boolean? = null
   private var useTurboModuleInteropCache: Boolean? = null
@@ -737,6 +738,15 @@ internal class ReactNativeFeatureFlagsCxxAccessor : ReactNativeFeatureFlagsAcces
     return cached
   }
 
+  override fun useNestedScrollViewAndroid(): Boolean {
+    var cached = useNestedScrollViewAndroidCache
+    if (cached == null) {
+      cached = ReactNativeFeatureFlagsCxxInterop.useNestedScrollViewAndroid()
+      useNestedScrollViewAndroidCache = cached
+    }
+    return cached
+  }
+
   override fun useSharedAnimatedBackend(): Boolean {
     var cached = useSharedAnimatedBackendCache
     if (cached == null) {

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/internal/featureflags/ReactNativeFeatureFlagsCxxInterop.kt

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<6410500cbf5a05b1f34efd12d1e83bdf>>
+ * @generated SignedSource<<698641fe5c1a9f4933321d5b155467d1>>
  */
 
 /**
@@ -170,6 +170,8 @@ public object ReactNativeFeatureFlagsCxxInterop {
 
   @DoNotStrip @JvmStatic public external fun useNativeViewConfigsInBridgelessMode(): Boolean
 
+  @DoNotStrip @JvmStatic public external fun useNestedScrollViewAndroid(): Boolean
+
   @DoNotStrip @JvmStatic public external fun useSharedAnimatedBackend(): Boolean
 
   @DoNotStrip @JvmStatic public external fun useTraitHiddenOnAndroid(): Boolean

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/internal/featureflags/ReactNativeFeatureFlagsDefaults.kt

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<6c664176cf41ffb9f2bf820f15ed7463>>
+ * @generated SignedSource<<6ab1616102fb0807ad936f4333cb44c8>>
  */
 
 /**
@@ -165,6 +165,8 @@ public open class ReactNativeFeatureFlagsDefaults : ReactNativeFeatureFlagsProvi
 
   override fun useNativeViewConfigsInBridgelessMode(): Boolean = false
 
+  override fun useNestedScrollViewAndroid(): Boolean = false
+
   override fun useSharedAnimatedBackend(): Boolean = false
 
   override fun useTraitHiddenOnAndroid(): Boolean = false

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/internal/featureflags/ReactNativeFeatureFlagsLocalAccessor.kt

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<1403b30f4dc3d18f39303d91d6824bda>>
+ * @generated SignedSource<<f83c3d5107d900273c09061d6822f83d>>
  */
 
 /**
@@ -95,6 +95,7 @@ internal class ReactNativeFeatureFlagsLocalAccessor : ReactNativeFeatureFlagsAcc
   private var useAlwaysAvailableJSErrorHandlingCache: Boolean? = null
   private var useFabricInteropCache: Boolean? = null
   private var useNativeViewConfigsInBridgelessModeCache: Boolean? = null
+  private var useNestedScrollViewAndroidCache: Boolean? = null
   private var useSharedAnimatedBackendCache: Boolean? = null
   private var useTraitHiddenOnAndroidCache: Boolean? = null
   private var useTurboModuleInteropCache: Boolean? = null
@@ -812,6 +813,16 @@ internal class ReactNativeFeatureFlagsLocalAccessor : ReactNativeFeatureFlagsAcc
     return cached
   }
 
+  override fun useNestedScrollViewAndroid(): Boolean {
+    var cached = useNestedScrollViewAndroidCache
+    if (cached == null) {
+      cached = currentProvider.useNestedScrollViewAndroid()
+      accessedFeatureFlags.add("useNestedScrollViewAndroid")
+      useNestedScrollViewAndroidCache = cached
+    }
+    return cached
+  }
+
   override fun useSharedAnimatedBackend(): Boolean {
     var cached = useSharedAnimatedBackendCache
     if (cached == null) {

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/internal/featureflags/ReactNativeFeatureFlagsProvider.kt

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<ad478388cb9a64690c71fe8fa06c1c79>>
+ * @generated SignedSource<<c2e44ee4f139a99b8179f750b416ed2f>>
  */
 
 /**
@@ -165,6 +165,8 @@ public interface ReactNativeFeatureFlagsProvider {
 
   @DoNotStrip public fun useNativeViewConfigsInBridgelessMode(): Boolean
 
+  @DoNotStrip public fun useNestedScrollViewAndroid(): Boolean
+
   @DoNotStrip public fun useSharedAnimatedBackend(): Boolean
 
   @DoNotStrip public fun useTraitHiddenOnAndroid(): Boolean

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/shell/MainReactPackage.kt

@@ -52,6 +52,7 @@ import com.facebook.react.views.progressbar.ReactProgressBarViewManager
 import com.facebook.react.views.safeareaview.ReactSafeAreaViewManager
 import com.facebook.react.views.scroll.ReactHorizontalScrollContainerViewManager
 import com.facebook.react.views.scroll.ReactHorizontalScrollViewManager
+import com.facebook.react.views.scroll.ReactNestedScrollViewManager
 import com.facebook.react.views.scroll.ReactScrollViewManager
 import com.facebook.react.views.swiperefresh.SwipeRefreshLayoutManager
 import com.facebook.react.views.switchview.ReactSwitchManager
@@ -139,7 +140,8 @@ constructor(private val config: MainPackageConfig? = null) :
           ReactHorizontalScrollViewManager(),
           ReactHorizontalScrollContainerViewManager(),
           ReactProgressBarViewManager(),
-          ReactScrollViewManager(),
+          if (ReactNativeFeatureFlags.useNestedScrollViewAndroid()) ReactNestedScrollViewManager()
+          else ReactScrollViewManager(),
           ReactSwitchManager(),
           ReactSafeAreaViewManager(),
           SwipeRefreshLayoutManager(),
@@ -173,7 +175,11 @@ constructor(private val config: MainPackageConfig? = null) :
           ReactSafeAreaViewManager.REACT_CLASS to
               ModuleSpec.viewManagerSpec { ReactSafeAreaViewManager() },
           ReactScrollViewManager.REACT_CLASS to
-              ModuleSpec.viewManagerSpec { ReactScrollViewManager() },
+              ModuleSpec.viewManagerSpec {
+                if (ReactNativeFeatureFlags.useNestedScrollViewAndroid())
+                    ReactNestedScrollViewManager()
+                else ReactScrollViewManager()
+              },
           ReactSwitchManager.REACT_CLASS to ModuleSpec.viewManagerSpec { ReactSwitchManager() },
           SwipeRefreshLayoutManager.REACT_CLASS to
               ModuleSpec.viewManagerSpec { SwipeRefreshLayoutManager() },