Add snapToInterval support for Android ScrollView

Author: sigvef

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,
     /**

ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactHorizontalScrollView.java

@@ -25,9 +25,10 @@
 
 import com.facebook.infer.annotation.Assertions;
 import com.facebook.react.uimanager.MeasureSpecAssertions;
-import com.facebook.react.uimanager.events.NativeGestureUtil;
+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;
 import com.facebook.react.views.view.ReactViewBackgroundDrawable;
 
 /**
@@ -43,6 +44,7 @@ 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;
@@ -92,6 +94,10 @@ public void setPagingEnabled(boolean pagingEnabled) {
     mPagingEnabled = pagingEnabled;
   }
 
+  public void setSnapToInterval(double snapToInterval) {
+    mSnapToInterval = snapToInterval;
+  }
+
   @Override
   protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
     MeasureSpecAssertions.assertExplicitMeasureSpec(widthMeasureSpec, heightMeasureSpec);
@@ -233,6 +239,13 @@ 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();
   }
@@ -310,7 +323,7 @@ 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;

ReactAndroid/src/main/java/com/facebook/react/views/scroll/ReactHorizontalScrollViewManager.java

@@ -108,6 +108,11 @@ 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
    */