From 1ae9b9adb6ac0a3a1d4d0de4659b30db7416cb87 Mon Sep 17 00:00:00 2001
From: Igor Mandrigin <i@mandrigin.ru>
Date: Fri, 14 Sep 2018 12:53:25 +0200
Subject: [PATCH] Workaround a wrong fling direction if FlatList or
 VirtualizedList is inverted.

---
 .../react/views/scroll/ReactScrollView.java    | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactScrollView.java b/ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactScrollView.java
index f10bd47ebbebb1..d39159a8b9de9b 100644
--- a/ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactScrollView.java
+++ b/ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactScrollView.java
@@ -309,8 +309,18 @@ public void getClippingRect(Rect outClippingRect) {
 
   @Override
   public void fling(int velocityY) {
+    // Workaround.
+    // On Android P if a ScrollView is inverted, we will get a wrong sign for
+    // velocityY (see https://issuetracker.google.com/issues/112385925). 
+    // At the same time, mOnScrollDispatchHelper tracks the correct velocity direction. 
+    //
+    // Hence, we can use the absolute value from whatever the OS gives
+    // us and use the sign of what mOnScrollDispatchHelper has tracked.
+    final int correctedVelocityY = (int)(Math.abs(velocityY) * Math.signum(mOnScrollDispatchHelper.getYFlingVelocity()));
+
+
     if (mPagingEnabled) {
-      flingAndSnap(velocityY);
+      flingAndSnap(correctedVelocityY);
     } else if (mScroller != null) {
       // FB SCROLLVIEW CHANGE
 
@@ -326,7 +336,7 @@ public void fling(int velocityY) {
         getScrollX(), // startX
         getScrollY(), // startY
         0, // velocityX
-        velocityY, // velocityY
+        correctedVelocityY, // velocityY
         0, // minX
         0, // maxX
         0, // minY
@@ -339,9 +349,9 @@ public void fling(int velocityY) {
 
       // END FB SCROLLVIEW CHANGE
     } else {
-      super.fling(velocityY);
+      super.fling(correctedVelocityY);
     }
-    handlePostTouchScrolling(0, velocityY);
+    handlePostTouchScrolling(0, correctedVelocityY);
   }
 
   private void enableFpsListener() {