fix: incorrect ScrollView offset on update (#30647)

Summary:
This pull request is to fix #30258.

After an investigation, I found out the scroll offset of the list seems to be calculated incorrectly due to a workaround in the source code.

Instead of fixing the `calculateOffsetForContentSize`, I chose to remove it, the reason why I do so is because this workaround is for fixing the offset when `contentSize` is set manually, but according to the source code, there is no interface for a react-native user to set the `contentSize` of ScrollView, it is just set to `GCSizeZero` and will never be changed ([ref](https://github.com/facebook/react-native/pull/30647/files#diff-cf6f991f585ebf4cfdd555fe474e1f9ce40c2e4f823fc3f42b549414639c8c30L304)).

Also I changed the function name from `updateContentOffsetIfNeeded` to  `updateContentSizeIfNeeded` according what the function is actually doing.

## Changelog

<!-- Help reviewers and the release process by writing your own changelog entry. For an example, see:
https://github.com/facebook/react-native/wiki/Changelog
-->

[iOS] [Fixed] - Incorrect ScrollView offset on update

Pull Request resolved: #30647

Test Plan:
1. Create a fresh new project named `testapp` using npx create-react-native-app
2. Apply example code to `testapp` from https://snack.expo.io/lokomass/flatlist
3. Run `testapp` on iOS emulator and reproduce the bug
4. Make changes to files in `testapp/node_modules/react-native/`
5. Rebuild `testapp` and run on iOS emulator again, the bug is no more exist
6. Apply changes from step 4 to react-native, make a pull request.

#### Screenshots

Before: The scroll offset is incorrect after children of FlatList has changed

https://user-images.githubusercontent.com/48589760/103155130-b54a0580-47d7-11eb-97af-bdfd3e728714.mov

After: No more incorrect scroll offset if children of FlatList has changed

https://user-images.githubusercontent.com/48589760/103155091-6ef4a680-47d7-11eb-89fa-6f708bfef1c9.mov

Reviewed By: sammy-SC

Differential Revision: D25732958

Pulled By: shergin

fbshipit-source-id: dac6eff15ac3bbfec502452ac14b3d49fee76c29

React/Views/ScrollView/RCTScrollContentView.m

@@ -26,7 +26,7 @@ - (void)reactSetFrame:(CGRect)frame
 
   RCTAssert([scrollView isKindOfClass:[RCTScrollView class]], @"Unexpected view hierarchy of RCTScrollView component.");
 
-  [scrollView updateContentOffsetIfNeeded];
+  [scrollView updateContentSizeIfNeeded];
 }
 
 @end

React/Views/ScrollView/RCTScrollView.h

@@ -28,12 +28,6 @@
  */
 @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?)
  */
@@ -68,7 +62,7 @@
 
 @interface RCTScrollView (Internal)
 
-- (void)updateContentOffsetIfNeeded;
+- (void)updateContentSizeIfNeeded;
 
 @end
 

React/Views/ScrollView/RCTScrollView.m

@@ -301,7 +301,6 @@ - (instancetype)initWithEventDispatcher:(id<RCTEventDispatcherProtocol>)eventDis
 
     _automaticallyAdjustContentInsets = YES;
     _contentInset = UIEdgeInsetsZero;
-    _contentSize = CGSizeZero;
     _lastClippedToRect = CGRectNull;
 
     _scrollEventThrottle = 0.0;
@@ -381,7 +380,7 @@ - (void)didUpdateReactSubviews
 - (void)didSetProps:(NSArray<NSString *> *)changedProps
 {
   if ([changedProps containsObject:@"contentSize"]) {
-    [self updateContentOffsetIfNeeded];
+    [self updateContentSizeIfNeeded];
   }
 }
 
@@ -799,8 +798,6 @@ - (UIView *)viewForZoomingInScrollView:(__unused UIScrollView *)scrollView
   return _contentView;
 }
 
-#pragma mark - Setters
-
 - (CGSize)_calculateViewportSize
 {
   CGSize viewportSize = self.bounds.size;
@@ -813,71 +810,16 @@ - (CGSize)_calculateViewportSize
   return viewportSize;
 }
 
-- (CGPoint)calculateOffsetForContentSize:(CGSize)newContentSize
-{
-  CGPoint oldOffset = _scrollView.contentOffset;
-  CGPoint newOffset = oldOffset;
-
-  CGSize oldContentSize = _scrollView.contentSize;
-  CGSize viewportSize = [self _calculateViewportSize];
-
-  BOOL fitsinViewportY = oldContentSize.height <= viewportSize.height && newContentSize.height <= viewportSize.height;
-  if (newContentSize.height < oldContentSize.height && !fitsinViewportY) {
-    CGFloat offsetHeight = oldOffset.y + viewportSize.height;
-    if (oldOffset.y < 0) {
-      // overscrolled on top, leave offset alone
-    } else if (offsetHeight > oldContentSize.height) {
-      // overscrolled on the bottom, preserve overscroll amount
-      newOffset.y = MAX(0, oldOffset.y - (oldContentSize.height - newContentSize.height));
-    } else if (offsetHeight > newContentSize.height) {
-      // offset falls outside of bounds, scroll back to end of list
-      newOffset.y = MAX(0, newContentSize.height - viewportSize.height);
-    }
-  }
-
-  BOOL fitsinViewportX = oldContentSize.width <= viewportSize.width && newContentSize.width <= viewportSize.width;
-  if (newContentSize.width < oldContentSize.width && !fitsinViewportX) {
-    CGFloat offsetHeight = oldOffset.x + viewportSize.width;
-    if (oldOffset.x < 0) {
-      // overscrolled at the beginning, leave offset alone
-    } else if (offsetHeight > oldContentSize.width && newContentSize.width > viewportSize.width) {
-      // overscrolled at the end, preserve overscroll amount as much as possible
-      newOffset.x = MAX(0, oldOffset.x - (oldContentSize.width - newContentSize.width));
-    } else if (offsetHeight > newContentSize.width) {
-      // offset falls outside of bounds, scroll back to end
-      newOffset.x = MAX(0, newContentSize.width - viewportSize.width);
-    }
-  }
-
-  // all other cases, offset doesn't change
-  return newOffset;
-}
-
-/**
- * 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 (!CGSizeEqualToSize(_contentSize, CGSizeZero)) {
-    return _contentSize;
-  }
-
   return _contentView.frame.size;
 }
 
-- (void)updateContentOffsetIfNeeded
+- (void)updateContentSizeIfNeeded
 {
   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;
   }
 }