ScrollView: Always fire onScroll event for the resting scroll position

Summary:
When throttling scroll events with `scrollEventThrottle`, `onScroll`
is not guaranteed to be fired for the final scroll position
of the `ScrollView`. This can cause a component to render UI that
is consistent with the resting scroll position of the `ScrollView`.

This commit guarantees that an `onScroll` event will be fired for
the resting scroll position of the `ScrollView`.

**Test plan (required)**

Verified commit fixes a reduced repro. Also tested fix in a larger app.

Adam Comella
Microsoft Corp.
Closes #7366

Differential Revision: D3269303

Pulled By: javache

fb-gh-sync-id: f68ecb7e9c18d1ac255c6f872fb7eb4aadd07799
fbshipit-source-id: f68ecb7e9c18d1ac255c6f872fb7eb4aadd07799

React/Views/RCTScrollView.m

@@ -399,7 +399,7 @@ - (instancetype)initWithEventDispatcher:(RCTEventDispatcher *)eventDispatcher
     _lastClippedToRect = CGRectNull;
 
     _scrollEventThrottle = 0.0;
-    _lastScrollDispatchTime = CACurrentMediaTime();
+    _lastScrollDispatchTime = 0;
     _cachedChildFrames = [NSMutableArray new];
 
     [self addSubview:_scrollView];
@@ -544,6 +544,8 @@ - (void)scrollToOffset:(CGPoint)offset
 - (void)scrollToOffset:(CGPoint)offset animated:(BOOL)animated
 {
   if (!CGPointEqualToPoint(_scrollView.contentOffset, offset)) {
+    // Ensure at least one scroll event will fire
+    _allowNextScrollNoMatterWhat = YES;
     [_scrollView setContentOffset:offset animated:animated];
   }
 }
@@ -579,9 +581,7 @@ - (void)delegateMethod:(UIScrollView *)scrollView           \
   RCT_FORWARD_SCROLL_EVENT(delegateMethod:scrollView);      \
 }
 
-RCT_SCROLL_EVENT_HANDLER(scrollViewDidEndScrollingAnimation, onMomentumScrollEnd) //TODO: shouldn't this be onScrollAnimationEnd?
 RCT_SCROLL_EVENT_HANDLER(scrollViewWillBeginDecelerating, onMomentumScrollBegin)
-RCT_SCROLL_EVENT_HANDLER(scrollViewDidEndDecelerating, onMomentumScrollEnd)
 RCT_SCROLL_EVENT_HANDLER(scrollViewDidZoom, onScroll)
 
 - (void)scrollViewDidScroll:(UIScrollView *)scrollView
@@ -725,6 +725,28 @@ - (void)scrollViewDidEndZooming:(UIScrollView *)scrollView withView:(UIView *)vi
   RCT_FORWARD_SCROLL_EVENT(scrollViewDidEndZooming:scrollView withView:view atScale:scale);
 }
 
+- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
+{
+  // Fire a final scroll event
+  _allowNextScrollNoMatterWhat = YES;
+  [self scrollViewDidScroll:scrollView];
+
+  // Fire the end deceleration event
+  RCT_SEND_SCROLL_EVENT(onMomentumScrollEnd, nil);
+  RCT_FORWARD_SCROLL_EVENT(scrollViewDidEndDecelerating:scrollView);
+}
+
+- (void)scrollViewDidEndScrollingAnimation:(UIScrollView *)scrollView
+{
+  // Fire a final scroll event
+  _allowNextScrollNoMatterWhat = YES;
+  [self scrollViewDidScroll:scrollView];
+
+  // Fire the end deceleration event
+  RCT_SEND_SCROLL_EVENT(onMomentumScrollEnd, nil); //TODO: shouldn't this be onScrollAnimationEnd? 
+  RCT_FORWARD_SCROLL_EVENT(scrollViewDidEndScrollingAnimation:scrollView);
+}
+
 - (BOOL)scrollViewShouldScrollToTop:(UIScrollView *)scrollView
 {
   if ([_nativeScrollDelegate respondsToSelector:_cmd]) {