Add `onScrollToTop` to ScrollView for iOS (#21204)

Summary:
This PR exposes the `onScrollToTop` event on iOS using the same event-forwarding infrastructure as other ScrollView events. (As such, its `nativeEvent` object reflects the same fields as other ScrollView events.)

Motivation:
----------

If your app is only interested in knowing the position of a ScrollView after a scroll has completed, it can use `onScrollEndDrag` and `onMomentumScrollEnd` to inspect the `contentOffset` after a drag-initiated scroll has finished. (This is much less expensive than observing the `onScroll` event if you only want to know the end position.) However, neither of these `End` events fire if the ScrollView is scrolled to the top by tapping the status bar.

By exposing `onScrollToTop`, it is now possible for an app to cheaply know when such a scroll has completed.
Pull Request resolved: https://github.com/facebook/react-native/pull/21204

Differential Revision: D9943618

Pulled By: hramos

fbshipit-source-id: ac5ee42b7f12d94655ffda617f8f811138da7f6f

Libraries/Components/ScrollView/ScrollView.js

@@ -217,6 +217,11 @@ type IOSProps = $ReadOnly<{|
    * @platform ios
    */
   scrollsToTop?: ?boolean,
+  /**
+    * Fires when the scroll view scrolls to top after the status bar has been tapped
+    * @platform ios
+    */
+  onScrollToTop?: ?Function,
   /**
    * When true, shows a horizontal scroll indicator.
    * The default value is true.

React/Views/ScrollView/RCTScrollView.h

@@ -55,6 +55,7 @@
 // need to be coalesced before sending, for performance reasons.
 @property (nonatomic, copy) RCTDirectEventBlock onScrollBeginDrag;
 @property (nonatomic, copy) RCTDirectEventBlock onScroll;
+@property (nonatomic, copy) RCTDirectEventBlock onScrollToTop;
 @property (nonatomic, copy) RCTDirectEventBlock onScrollEndDrag;
 @property (nonatomic, copy) RCTDirectEventBlock onMomentumScrollBegin;
 @property (nonatomic, copy) RCTDirectEventBlock onMomentumScrollEnd;

React/Views/ScrollView/RCTScrollView.m

@@ -648,6 +648,7 @@ for (NSObject<UIScrollViewDelegate> *scrollViewListener in _scrollListeners) { \
 
 RCT_SCROLL_EVENT_HANDLER(scrollViewWillBeginDecelerating, onMomentumScrollBegin)
 RCT_SCROLL_EVENT_HANDLER(scrollViewDidZoom, onScroll)
+RCT_SCROLL_EVENT_HANDLER(scrollViewDidScrollToTop, onScrollToTop)
 
 - (void)addScrollListener:(NSObject<UIScrollViewDelegate> *)scrollListener
 {

React/Views/ScrollView/RCTScrollViewManager.m

@@ -88,6 +88,7 @@ RCT_EXPORT_VIEW_PROPERTY(snapToAlignment, NSString)
 RCT_REMAP_VIEW_PROPERTY(contentOffset, scrollView.contentOffset, CGPoint)
 RCT_EXPORT_VIEW_PROPERTY(onScrollBeginDrag, RCTDirectEventBlock)
 RCT_EXPORT_VIEW_PROPERTY(onScroll, RCTDirectEventBlock)
+RCT_EXPORT_VIEW_PROPERTY(onScrollToTop, RCTDirectEventBlock)
 RCT_EXPORT_VIEW_PROPERTY(onScrollEndDrag, RCTDirectEventBlock)
 RCT_EXPORT_VIEW_PROPERTY(onMomentumScrollBegin, RCTDirectEventBlock)
 RCT_EXPORT_VIEW_PROPERTY(onMomentumScrollEnd, RCTDirectEventBlock)