Android ScrollView: Add scrollBy method

Libraries/Components/ScrollResponder.js

@@ -438,6 +438,21 @@ var ScrollResponderMixin = {
     );
   },
 
+  /**
+   * A helper function to scroll by a specific offset in the ScrollView.
+   * This is useful when you want to change the size of the content view and scroll
+   * position at the same time. Syntax:
+   *
+   * scrollResponderScrollBy(options: {deltaX: number = 0; deltaY: number = 0; animated: boolean = true})
+   */
+  scrollResponderScrollBy: function(options : {deltaX?: number, deltaY?: number, animated?: boolean}) {
+    UIManager.dispatchViewManagerCommand(
+      this.scrollResponderGetScrollableNode(),
+      UIManager.RCTScrollView.Commands.scrollBy,
+      [options.deltaX || 0, options.deltaY || 0, options.animated !== false],
+    );
+  },
+
   /**
    * Deprecated, do not use.
    */

Libraries/Components/ScrollView/ScrollView.js

@@ -530,6 +530,19 @@ const ScrollView = createReactClass({
     });
   },
 
+  /**
+   * Scrolls by a given offset, either immediately or with a smooth animation.
+   *
+   * Syntax:
+   *
+   * `scrollBy(options: {deltaX: number = 0; deltaY: number = 0; animated: boolean = true})`
+   */
+  scrollBy: function(options: { deltaX?: number, deltaY?: number, animated?: boolean } ) {
+    const data = {deltaX: options.deltaX || 0, deltaY: options.deltaY || 0,
+      animated: options.animated !== false};
+    this.getScrollResponder().scrollResponderScrollBy(data);
+  },
+
   /**
    * Deprecated, use `scrollTo` instead.
    */

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

@@ -150,6 +150,19 @@ public void scrollToEnd(
     }
   }
 
+  @Override
+  public void scrollBy(
+      ReactHorizontalScrollView scrollView,
+      ReactScrollViewCommandHelper.ScrollByCommandData data) {
+    int x = scrollView.getScrollX() + data.mDeltaX;
+    int y = scrollView.getScrollY() + data.mDeltaY;
+    if (data.mAnimated) {
+      scrollView.smoothScrollTo(x, y);
+    } else {
+      scrollView.scrollTo(x, y);
+    }
+  }
+
   /**
    * When set, fills the rest of the scrollview with a color to avoid setting a background and
    * creating unnecessary overdraw.

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

@@ -25,11 +25,13 @@ public class ReactScrollViewCommandHelper {
   public static final int COMMAND_SCROLL_TO = 1;
   public static final int COMMAND_SCROLL_TO_END = 2;
   public static final int COMMAND_FLASH_SCROLL_INDICATORS = 3;
+  public static final int COMMAND_SCROLL_BY = 4;
 
   public interface ScrollCommandHandler<T> {
     void scrollTo(T scrollView, ScrollToCommandData data);
     void scrollToEnd(T scrollView, ScrollToEndCommandData data);
     void flashScrollIndicators(T scrollView);
+    void scrollBy(T scrollView, ScrollByCommandData data);
   }
 
   public static class ScrollToCommandData {
@@ -53,14 +55,28 @@ public static class ScrollToEndCommandData {
     }
   }
 
+  public static class ScrollByCommandData {
+
+    public final int mDeltaX, mDeltaY;
+    public final boolean mAnimated;
+
+    ScrollByCommandData(int deltaX, int deltaY, boolean animated) {
+      mDeltaX = deltaX;
+      mDeltaY = deltaY;
+      mAnimated = animated;
+    }
+  }
+
   public static Map<String,Integer> getCommandsMap() {
     return MapBuilder.of(
         "scrollTo",
         COMMAND_SCROLL_TO,
         "scrollToEnd",
         COMMAND_SCROLL_TO_END,
         "flashScrollIndicators",
-        COMMAND_FLASH_SCROLL_INDICATORS);
+        COMMAND_FLASH_SCROLL_INDICATORS,
+        "scrollBy",
+        COMMAND_SCROLL_BY);
   }
 
   public static <T> void receiveCommand(
@@ -88,6 +104,14 @@ public static <T> void receiveCommand(
         viewManager.flashScrollIndicators(scrollView);
         return;
 
+      case COMMAND_SCROLL_BY: {
+        int deltaX = Math.round(PixelUtil.toPixelFromDIP(args.getDouble(0)));
+        int deltaY = Math.round(PixelUtil.toPixelFromDIP(args.getDouble(1)));
+        boolean animated = args.getBoolean(2);
+
+        viewManager.scrollBy(scrollView, new ScrollByCommandData(deltaX, deltaY, animated));
+        return;
+      }
       default:
         throw new IllegalArgumentException(String.format(
             "Unsupported command %d received by %s.",

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

@@ -210,6 +210,19 @@ public void scrollToEnd(
     }
   }
 
+  @Override
+  public void scrollBy(
+      ReactScrollView scrollView,
+      ReactScrollViewCommandHelper.ScrollByCommandData data) {
+    int x = scrollView.getScrollX() + data.mDeltaX;
+    int y = scrollView.getScrollY() + data.mDeltaY;
+    if (data.mAnimated) {
+      scrollView.smoothScrollTo(x, y);
+    } else {
+      scrollView.scrollTo(x, y);
+    }
+  }
+
   @Override
   public @Nullable Map getExportedCustomDirectEventTypeConstants() {
     return createExportedCustomDirectEventTypeConstants();