From 32cb9ec49c801fcebe61486149134ab542d9364b Mon Sep 17 00:00:00 2001
From: Davide Mainas <davide.mainas@me.com>
Date: Fri, 1 Feb 2019 03:59:23 -0800
Subject: [PATCH] Fix Inverted Horizontal ScrollView on Android (#23233)

Summary:
ScrollView (as well as FlatList) used with horizontal={true} and inverted={true} do not scroll as expected due to a known Android P bug.

Fixes #22710. This is pretty much a clone of PR #21117, applied to a horizontal ScrollView.

[Android] [Fixed] - Fix Inverted Horizontal ScrollView on Android

1. Create a test application with a FlatList with horizontal={true} inverted={true}.
2. Fill it with data
3. Scroll and release your finger
Pull Request resolved: https://github.com/facebook/react-native/pull/23233

Differential Revision: D13915911

Pulled By: cpojer

fbshipit-source-id: d32c82e6d9076f5ffdf342fcd71bd921e9c8a97b
---
 .../scroll/ReactHorizontalScrollView.java      | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactHorizontalScrollView.java b/ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactHorizontalScrollView.java
index bf6c3bd51a1554..973421db6cbce3 100644
--- a/ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactHorizontalScrollView.java
+++ b/ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactHorizontalScrollView.java
@@ -286,8 +286,18 @@ public boolean onTouchEvent(MotionEvent ev) {
 
   @Override
   public void fling(int velocityX) {
+
+    // Workaround.
+    // On Android P if a ScrollView is inverted, we will get a wrong sign for
+    // velocityX (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 correctedVelocityX = (int)(Math.abs(velocityX) * Math.signum(mOnScrollDispatchHelper.getXFlingVelocity()));
+
     if (mPagingEnabled) {
-      flingAndSnap(velocityX);
+      flingAndSnap(correctedVelocityX);
     } else if (mScroller != null) {
       // FB SCROLLVIEW CHANGE
 
@@ -302,7 +312,7 @@ public void fling(int velocityX) {
       mScroller.fling(
         getScrollX(), // startX
         getScrollY(), // startY
-        velocityX, // velocityX
+        correctedVelocityX, // velocityX
         0, // velocityY
         0, // minX
         Integer.MAX_VALUE, // maxX
@@ -316,9 +326,9 @@ public void fling(int velocityX) {
 
       // END FB SCROLLVIEW CHANGE
     } else {
-      super.fling(velocityX);
+      super.fling(correctedVelocityX);
     }
-    handlePostTouchScrolling(velocityX, 0);
+    handlePostTouchScrolling(correctedVelocityX, 0);
   }
 
   @Override