Fix a bug with ListView with sticky headers + RefreshControl

Summary:The bug is caused by a weird race condition. What happens is that when calling `UIRefreshControl#endRefreshing` the `UIScrollView` delegate `scrollViewDidScroll` function is called synchronously and then `dockClosestSectionHeader` crashes because the sticky header indexes are updated but not the contentView children.

I fixed it by adding an updating property on `RCTRefreshControl` and setting it before calling `endRefreshing` so we can know not to call `dockClosestSectionHeader` at that moment.

Tested with both `RefreshControl` and `onRefreshStart` prop.

I reproduced the bug by replacing ListViewExample.js in UIExplorer with https://gist.github.com/janicduplessis/05fc58e852f3e80e51b9

Fixes react#5440

cc nicklockwood
Closes react#5445

Differential Revision: D2953984

Pulled By: nicklockwood

fb-gh-sync-id: c17a6a75ab31ef54d478706ed17a8115a11d734e
shipit-source-id: c17a6a75ab31ef54d478706ed17a8115a11d734e

Author: janicduplessis

React/Views/RCTRefreshControl.m

@@ -30,7 +30,7 @@ - (instancetype)init
 - (void)layoutSubviews
 {
   [super layoutSubviews];
-  
+
   // If the control is refreshing when mounted we need to call
   // beginRefreshing in layoutSubview or it doesn't work.
   if (_isInitialRender && _initialRefreshingState) {
@@ -46,9 +46,7 @@ - (void)beginRefreshing
   CGPoint offset = {scrollView.contentOffset.x, scrollView.contentOffset.y - self.frame.size.height};
   // Don't animate when the prop is set initialy.
   if (_isInitialRender) {
-    // Must use `[scrollView setContentOffset:offset animated:NO]` instead of just setting
-    // `scrollview.contentOffset` or it doesn't work, don't ask me why!
-    [scrollView setContentOffset:offset animated:NO];
+    scrollView.contentOffset = offset;
     [super beginRefreshing];
   } else {
     // `beginRefreshing` must be called after the animation is done. This is why it is impossible

React/Views/RCTScrollView.m

@@ -145,7 +145,7 @@ @interface RCTCustomScrollView : UIScrollView<UIGestureRecognizerDelegate>
 
 @property (nonatomic, copy) NSIndexSet *stickyHeaderIndices;
 @property (nonatomic, assign) BOOL centerContent;
-@property (nonatomic, strong) UIRefreshControl *refreshControl;
+@property (nonatomic, strong) RCTRefreshControl *refreshControl;
 
 @end
 
@@ -287,10 +287,12 @@ - (void)dockClosestSectionHeader
   __block UIView *nextHeader = nil;
   NSUInteger subviewCount = contentView.reactSubviews.count;
   [_stickyHeaderIndices enumerateIndexesWithOptions:0 usingBlock:
-   ^(NSUInteger idx, __unused BOOL *stop) {
+   ^(NSUInteger idx, BOOL *stop) {
 
+    // If the subviews are out of sync with the sticky header indices don't
+    // do anything.
     if (idx >= subviewCount) {
-      RCTLogError(@"Sticky header index %zd was outside the range {0, %zd}", idx, subviewCount);
+      *stop = YES;
       return;
     }
 
@@ -365,7 +367,7 @@ - (UIView *)hitTest:(CGPoint)point withEvent:(UIEvent *)event
   return hitView ?: [super hitTest:point withEvent:event];
 }
 
-- (void)setRefreshControl:(UIRefreshControl *)refreshControl
+- (void)setRefreshControl:(RCTRefreshControl *)refreshControl
 {
   if (_refreshControl) {
     [_refreshControl removeFromSuperview];
@@ -826,6 +828,17 @@ - (void)reactBridgeDidFinishTransaction
     _scrollView.contentSize = contentSize;
     _scrollView.contentOffset = newOffset;
   }
+
+  if (RCT_DEBUG) {
+    // Validate that sticky headers are not out of range.
+    NSUInteger subviewCount = _scrollView.contentView.reactSubviews.count;
+    NSUInteger lastIndex = _scrollView.stickyHeaderIndices.lastIndex;
+    if (lastIndex != NSNotFound && lastIndex >= subviewCount) {
+      RCTLogWarn(@"Sticky header index %zd was outside the range {0, %zd}",
+                 lastIndex, subviewCount);
+    }
+  }
+
   [_scrollView dockClosestSectionHeader];
 }
 
@@ -888,7 +901,7 @@ - (void)setOnRefreshStart:(RCTDirectEventBlock)onRefreshStart
   _onRefreshStart = [onRefreshStart copy];
 
   if (!_scrollView.refreshControl) {
-    UIRefreshControl *refreshControl = [[UIRefreshControl alloc] init];
+    RCTRefreshControl *refreshControl = [[RCTRefreshControl alloc] init];
     [refreshControl addTarget:self action:@selector(refreshControlValueChanged) forControlEvents:UIControlEventValueChanged];
     _scrollView.refreshControl = refreshControl;
   }