From 2e41b46dfeac30cc5435281e5d0dbb0b1d3b2ce8 Mon Sep 17 00:00:00 2001
From: Sigve Sebastian Farstad <sigvefarstad@gmail.com>
Date: Sat, 26 Aug 2017 15:08:32 +0200
Subject: [PATCH] Add snapToInterval support for Android ScrollView

---
 Libraries/Components/ScrollView/ScrollView.js     |  2 --
 .../views/scroll/ReactHorizontalScrollView.java   | 15 ++++++++++++++-
 .../scroll/ReactHorizontalScrollViewManager.java  |  5 +++++
 3 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/Libraries/Components/ScrollView/ScrollView.js b/Libraries/Components/ScrollView/ScrollView.js
index 980d5d734e0ede..6cb9977b252600 100644
--- a/Libraries/Components/ScrollView/ScrollView.js
+++ b/Libraries/Components/ScrollView/ScrollView.js
@@ -335,8 +335,6 @@ const ScrollView = createReactClass({
      * that have lengths smaller than the scroll view. Typically used in
      * combination with `snapToAlignment` and `decelerationRate="fast"`.
      * Overrides less configurable `pagingEnabled` prop.
-     *
-     * @platform ios
      */
     snapToInterval: PropTypes.number,
     /**
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 3acb61e9c9e407..ab90ce7664f360 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
@@ -22,6 +22,7 @@
 import android.widget.HorizontalScrollView;
 import com.facebook.infer.annotation.Assertions;
 import com.facebook.react.uimanager.MeasureSpecAssertions;
+import com.facebook.react.uimanager.PixelUtil;
 import com.facebook.react.uimanager.ReactClippingViewGroup;
 import com.facebook.react.uimanager.ReactClippingViewGroupHelper;
 import com.facebook.react.uimanager.events.NativeGestureUtil;
@@ -41,6 +42,7 @@ public class ReactHorizontalScrollView extends HorizontalScrollView implements
   private @Nullable Rect mClippingRect;
   private boolean mDragging;
   private boolean mPagingEnabled = false;
+  private double mSnapToInterval;
   private @Nullable Runnable mPostTouchRunnable;
   private boolean mRemoveClippedSubviews;
   private boolean mScrollEnabled = true;
@@ -94,6 +96,10 @@ public void flashScrollIndicators() {
     awakenScrollBars();
   }
 
+  public void setSnapToInterval(double snapToInterval) {
+    mSnapToInterval = snapToInterval;
+  }
+
   @Override
   protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
     MeasureSpecAssertions.assertExplicitMeasureSpec(widthMeasureSpec, heightMeasureSpec);
@@ -235,6 +241,13 @@ private void disableFpsListener() {
     }
   }
 
+  private int getPageWidth() {
+    if(mSnapToInterval != 0) {
+      return (int) (PixelUtil.toPixelFromDIP(mSnapToInterval) + 0.5);
+    }
+    return getWidth();
+  }
+
   private boolean isScrollPerfLoggingEnabled() {
     return mFpsListener != null && mScrollPerfTag != null && !mScrollPerfTag.isEmpty();
   }
@@ -312,7 +325,7 @@ public void run() {
    * scrolling.
    */
   private void smoothScrollToPage(int velocity) {
-    int width = getWidth();
+    int width = getPageWidth();
     int currentX = getScrollX();
     // TODO (t11123799) - Should we do anything beyond linear accounting of the velocity
     int predictedX = currentX + velocity;
diff --git a/ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactHorizontalScrollViewManager.java b/ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactHorizontalScrollViewManager.java
index 9d40757919493c..539250493a8912 100644
--- a/ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactHorizontalScrollViewManager.java
+++ b/ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactHorizontalScrollViewManager.java
@@ -105,6 +105,11 @@ public void setPagingEnabled(ReactHorizontalScrollView view, boolean pagingEnabl
     view.setPagingEnabled(pagingEnabled);
   }
 
+  @ReactProp(name = "snapToInterval")
+  public void setSnapToInterval(ReactHorizontalScrollView view, double snapToInterval) {
+    view.setSnapToInterval(snapToInterval);
+  }
+
   /**
    * Controls overScroll behaviour
    */