Fix for incorrect contentSize reported by RCTScrollView

ReactKit/Views/RCTScrollView.h

@@ -22,19 +22,25 @@
 - (instancetype)initWithEventDispatcher:(RCTEventDispatcher *)eventDispatcher NS_DESIGNATED_INITIALIZER;
 
 /**
- * If the `contentSize` is not provided, then the `contentSize` will
- * automatically be determined by the size of the `RKScrollView` subview.
- *
  * The `RCTScrollView` may have at most one single subview. This will ensure
  * that the scroll view's `contentSize` will be efficiently set to the size of
  * the single subview's frame. That frame size will be determined somewhat
  * efficiently since it will have already been computed by the off-main-thread
  * layout system.
  */
-@property (nonatomic, readonly) UIScrollView *scrollView;
 @property (nonatomic, readonly) UIView *contentView;
 
+/**
+ * If the `contentSize` is not specified (or is specified as {0, 0}, then the
+ * `contentSize` will automatically be determined by the size of the subview.
+ */
 @property (nonatomic, assign) CGSize contentSize;
+
+/**
+ * The underlying scrollView (TODO: can we remove this?)
+ */
+@property (nonatomic, readonly) UIScrollView *scrollView;
+
 @property (nonatomic, assign) UIEdgeInsets contentInset;
 @property (nonatomic, assign) BOOL automaticallyAdjustContentInsets;
 @property (nonatomic, assign) NSUInteger throttleScrollCallbackMS;

ReactKit/Views/RCTScrollView.m

@@ -253,7 +253,6 @@ CGFloat const ZINDEX_STICKY_HEADER = 50;
 @implementation RCTScrollView
 {
   RCTEventDispatcher *_eventDispatcher;
-  BOOL _contentSizeManuallySet;
   RCTCustomScrollView *_scrollView;
   UIView *_contentView;
   NSTimeInterval _lastScrollDispatchTime;
@@ -273,6 +272,7 @@ CGFloat const ZINDEX_STICKY_HEADER = 50;
     _scrollView.delaysContentTouches = NO;
     _automaticallyAdjustContentInsets = YES;
     _contentInset = UIEdgeInsetsZero;
+    _contentSize = CGSizeZero;
 
     _throttleScrollCallbackMS = 0;
     _lastScrollDispatchTime = CACurrentMediaTime();
@@ -319,16 +319,6 @@ CGFloat const ZINDEX_STICKY_HEADER = 50;
   _scrollView.stickyHeaderIndices = headerIndices;
 }
 
-/**
- * Once you set the `contentSize`, it's assumed to be managed by you forever
- * and we'll never automatically compute the size for you.
- */
-- (void)setContentSize:(CGSize)contentSize
-{
-  _contentSize = contentSize;
-  _contentSizeManuallySet = YES;
-}
-
 - (void)dealloc
 {
   _scrollView.delegate = nil;
@@ -556,29 +546,40 @@ RCT_SCROLL_EVENT_HANDLER(scrollViewDidZoom, RCTScrollEventTypeMove)
   return newOffset;
 }
 
-- (void)reactBridgeDidFinishTransaction
+/**
+ * Once you set the `contentSize`, to a nonzero value, it is assumed to be
+ * managed by you, and we'll never automatically compute the size for you,
+ * unless you manually reset it back to {0, 0}
+ */
+- (CGSize)contentSize
 {
-  if (_contentSizeManuallySet) {
-    _scrollView.contentSize = _contentSize;
+  if (!CGSizeEqualToSize(_contentSize, CGSizeZero)) {
+    return _contentSize;
   } else if (!_contentView) {
-    _scrollView.contentSize = CGSizeZero;
+    return CGSizeZero;
   } else {
     CGSize singleSubviewSize = _contentView.frame.size;
     CGPoint singleSubviewPosition = _contentView.frame.origin;
-    CGSize fittedSize = {
+    return (CGSize){
       singleSubviewSize.width + singleSubviewPosition.x,
       singleSubviewSize.height + singleSubviewPosition.y
     };
-    if (!CGSizeEqualToSize(_scrollView.contentSize, fittedSize)) {
-      // When contentSize is set manually, ScrollView internals will reset contentOffset to 0,0. Since
-      // we potentially set contentSize whenever anything in the ScrollView updates, we workaround this
-      // issue by manually adjusting contentOffset whenever this happens
-      CGPoint newOffset = [self calculateOffsetForContentSize:fittedSize];
-      _scrollView.contentSize = fittedSize;
+  }
+}
+
+- (void)reactBridgeDidFinishTransaction
+{
+  CGSize contentSize = self.contentSize;
+  if (!CGSizeEqualToSize(_scrollView.contentSize, contentSize)) {
+    // When contentSize is set manually, ScrollView internals will reset
+    // contentOffset to  {0, 0}. Since we potentially set contentSize whenever
+    // anything in the ScrollView updates, we workaround this issue by manually
+    // adjusting contentOffset whenever this happens
+    CGPoint newOffset = [self calculateOffsetForContentSize:contentSize];
+    _scrollView.contentSize = contentSize;
     _scrollView.contentOffset = newOffset;
   }
   [_scrollView dockClosestSectionHeader];
-  }
 }
 
 // Note: setting several properties of UIScrollView has the effect of