Allow Java classes to hook into ScrollView scroll and layout events

Summary:
For advanced interop cases.

Changelog: [Internal]

Reviewed By: sammy-SC

Differential Revision: D23984345

fbshipit-source-id: f5c2a43a1bf5937f9974bcc5c66c36ec35e679c5

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

@@ -266,6 +266,7 @@ public class ReactHorizontalScrollView extends HorizontalScrollView
     int scrollToY =
         pendingContentOffsetY != UNSET_CONTENT_OFFSET ? pendingContentOffsetY : getScrollY();
     reactScrollTo(scrollToX, scrollToY);
+    ReactScrollViewHelper.emitLayoutEvent(this);
   }
 
   /**

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

@@ -223,6 +223,7 @@ public class ReactScrollView extends ScrollView
     int scrollToY =
         pendingContentOffsetY != UNSET_CONTENT_OFFSET ? pendingContentOffsetY : getScrollY();
     reactScrollTo(scrollToX, scrollToY);
+    ReactScrollViewHelper.emitLayoutEvent(this);
   }
 
   @Override

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

@@ -14,6 +14,8 @@ import android.widget.OverScroller;
 import com.facebook.react.bridge.JSApplicationIllegalArgumentException;
 import com.facebook.react.bridge.ReactContext;
 import com.facebook.react.uimanager.UIManagerHelper;
+import java.util.ArrayList;
+import java.util.List;
 
 /** Helper class that deals with emitting Scroll Events. */
 public class ReactScrollViewHelper {
@@ -23,6 +25,16 @@ public class ReactScrollViewHelper {
   public static final String AUTO = "auto";
   public static final String OVER_SCROLL_NEVER = "never";
 
+  public interface ScrollListener {
+    void onScroll(
+        ViewGroup scrollView, ScrollEventType scrollEventType, float xVelocity, float yVelocity);
+
+    void onLayout(ViewGroup scrollView);
+  }
+
+  // Support global native listeners for scroll events
+  private static List<ScrollListener> sScrollListeners = new ArrayList<>();
+
   // If all else fails, this is the hardcoded value in OverScroller.java, in AOSP.
   // The default is defined here (as of this diff):
   // https://android.googlesource.com/platform/frameworks/base/+/ae5bcf23b5f0875e455790d6af387184dbd009c1/core/java/android/widget/OverScroller.java#44
@@ -64,6 +76,10 @@ public class ReactScrollViewHelper {
       return;
     }
 
+    for (ScrollListener scrollListener : sScrollListeners) {
+      scrollListener.onScroll(scrollView, scrollEventType, xVelocity, yVelocity);
+    }
+
     ReactContext reactContext = (ReactContext) scrollView.getContext();
     UIManagerHelper.getEventDispatcherForReactTag(reactContext, scrollView.getId())
         .dispatchEvent(
@@ -80,6 +96,13 @@ public class ReactScrollViewHelper {
                 scrollView.getHeight()));
   }
 
+  /** This is only for Java listeners. onLayout events emitted to JS are handled elsewhere. */
+  public static void emitLayoutEvent(ViewGroup scrollView) {
+    for (ScrollListener scrollListener : sScrollListeners) {
+      scrollListener.onLayout(scrollView);
+    }
+  }
+
   public static int parseOverScrollMode(String jsOverScrollMode) {
     if (jsOverScrollMode == null || jsOverScrollMode.equals(AUTO)) {
       return View.OVER_SCROLL_IF_CONTENT_SCROLLS;
@@ -130,4 +153,12 @@ public class ReactScrollViewHelper {
       mScrollAnimationDuration = duration;
     }
   }
+
+  public static void addScrollListener(ScrollListener listener) {
+    sScrollListeners.add(listener);
+  }
+
+  public static void removeScrollListener(ScrollListener listener) {
+    sScrollListeners.remove(listener);
+  }
 }