Android: preserve NestedScrollView fling nested-scroll lifecycle

Author: AmatoGiulio

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactNestedScrollView.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<<497c1f334f6b3325e032e09947f6fce4>>
+ * @generated SignedSource<<a38f03b5fd0c32076a46efce475b5598>>
  */
 
 /**
@@ -706,21 +706,6 @@ constructor(context: Context, private val fpsListener: FpsListener? = null) :
 
     if (pagingEnabled) {
       flingAndSnap(correctedVelocityY)
-    } else if (scroller != null) {
-      val scrollWindowHeight = height - paddingBottom - paddingTop
-      scroller.fling(
-          scrollX, // startX
-          scrollY, // startY
-          0, // velocityX
-          correctedVelocityY, // velocityY
-          0, // minX
-          0, // maxX
-          0, // minY
-          Int.MAX_VALUE, // maxY
-          0, // overX
-          scrollWindowHeight / 2, // overY
-      )
-      postInvalidateOnAnimation()
     } else {
       super.fling(correctedVelocityY)
     }

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/views/scroll/generate-nested-scroll-view.js

@@ -100,6 +100,46 @@ function replaceCopyrightHeader(content, sourceFile) {
   return content.replace(COPYRIGHT_HEADER_PATTERN, generatedHeader(sourceFile));
 }
 
+/**
+ * Keep NestedScrollView in charge of the non-paging fling lifecycle.
+ *
+ * ReactScrollView intentionally drives its reflected OverScroller directly. For
+ * the generated AndroidX variant, doing the same bypasses NestedScrollView.fling()
+ * and therefore its TYPE_NON_TOUCH nested-scroll lifecycle.
+ */
+function replaceNestedScrollViewFling(content) {
+  const scrollerFlingBranch = `    } else if (scroller != null) {
+      val scrollWindowHeight = height - paddingBottom - paddingTop
+      scroller.fling(
+          scrollX, // startX
+          scrollY, // startY
+          0, // velocityX
+          correctedVelocityY, // velocityY
+          0, // minX
+          0, // maxX
+          0, // minY
+          Int.MAX_VALUE, // maxY
+          0, // overX
+          scrollWindowHeight / 2, // overY
+      )
+      postInvalidateOnAnimation()
+    } else {
+      super.fling(correctedVelocityY)
+    }`;
+  const nestedScrollFlingBranch = `    } else {
+      super.fling(correctedVelocityY)
+    }`;
+  const occurrenceCount = content.split(scrollerFlingBranch).length - 1;
+
+  if (occurrenceCount !== 1) {
+    throw new Error(
+      `Expected exactly one ReactScrollView fling scroller branch; found ${occurrenceCount}.`,
+    );
+  }
+
+  return content.replace(scrollerFlingBranch, nestedScrollFlingBranch);
+}
+
 /**
  * Transform ReactScrollView.kt to ReactNestedScrollView.kt
  */
@@ -121,6 +161,10 @@ function transformScrollView(content) {
   // Replace ReactScrollView with ReactNestedScrollView
   content = replaceClassNames(content);
 
+  // Unlike android.widget.ScrollView, AndroidX NestedScrollView owns the nested-scroll
+  // lifecycle of a fling. Preserve that lifecycle in the generated variant.
+  content = replaceNestedScrollViewFling(content);
+
   // Make the class internal to keep it out of the public API
   content = content.replace(
     'public open class ReactNestedScrollView',