From 9b2f1e943eb453d1685b9e29c4338ba641531fb5 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Leo=20Nikkil=C3=A4?= <hello@lnikki.la>
Date: Tue, 25 Sep 2018 14:08:01 +0300
Subject: [PATCH] Fix Android momentum scroll differences to iOS

- Momentum scroll events should not be emitted when there's no momentum
- Momentum scroll should be interrupted by a drag, which should not emit
  an onMomentumScrollEnd event. Instead a new momentum scroll should
  begin once dragging ends.
---
 .../views/scroll/ReactHorizontalScrollView.java      | 12 ++++++++++++
 .../facebook/react/views/scroll/ReactScrollView.java | 12 ++++++++++++
 2 files changed, 24 insertions(+)

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 566b6018b02e1f..b71a654ecd9d0a 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
@@ -246,6 +246,12 @@ public boolean onInterceptTouchEvent(MotionEvent ev) {
         ReactScrollViewHelper.emitScrollBeginDragEvent(this);
         mDragging = true;
         enableFpsListener();
+        // The user might be interrupting a fling/snap and we need to cancel any ongoing animations.
+        // Mirrors behaviour on iOS by starting a new momentum scroll event series after this drag.
+        if (mPostTouchRunnable != null) {
+          removeCallbacks(mPostTouchRunnable);
+          mPostTouchRunnable = null;
+        }
         return true;
       }
     } catch (IllegalArgumentException e) {
@@ -445,6 +451,12 @@ private void handlePostTouchScrolling(int velocityX, int velocityY) {
       return;
     }
 
+    // Don't emit momentum events if there's not going to be a momentum scroll, mirrors iOS momentum
+    // scroll behaviour.
+    if (!mPagingEnabled && velocityX == 0 && velocityY == 0) {
+      return;
+    }
+
     // Check if we are already handling this which may occur if this is called by both the touch up
     // and a fling call
     if (mPostTouchRunnable != null) {
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 d99d878de4ce99..12b8015cfda8db 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
@@ -235,6 +235,12 @@ public boolean onInterceptTouchEvent(MotionEvent ev) {
         ReactScrollViewHelper.emitScrollBeginDragEvent(this);
         mDragging = true;
         enableFpsListener();
+        // The user might be interrupting a fling/snap and we need to cancel any ongoing animations.
+        // Mirrors behaviour on iOS by starting a new momentum scroll event series after this drag.
+        if (mPostTouchRunnable != null) {
+          removeCallbacks(mPostTouchRunnable);
+          mPostTouchRunnable = null;
+        }
         return true;
       }
     } catch (IllegalArgumentException e) {
@@ -412,6 +418,12 @@ private void handlePostTouchScrolling(int velocityX, int velocityY) {
       return;
     }
 
+    // Don't emit momentum events if there's not going to be a momentum scroll, mirrors iOS momentum
+    // scroll behaviour.
+    if (!mPagingEnabled && velocityX == 0 && velocityY == 0) {
+      return;
+    }
+
     // Check if we are already handling this which may occur if this is called by both the touch up
     // and a fling call
     if (mPostTouchRunnable != null) {