[RTL/Batching] Make ASDisplayShouldFetchBatchForScrollView aware of f…

…lipped CV layouts (#1985)

* [RTL/Batching] Make ASDisplayShouldFetchBatchForScrollView aware of flipped CV layouts

UICollectionViewLayout has a property called `flipsHorizontallyInOppositeLayoutDirection`. If this is set to `YES` then a RTL collectionView’s contentOffset behaves like it does in LTR. In other words, the first item is at contentOffset 0. In this case, the existing logic for `ASDisplayShouldFetchBatchForScrollView` works in RTL.

If you don’t override `flipsHorizontallyInOppositeLayoutDirection` to be `YES`, then it means that in RTL languages the first item in your collectionView will actually be at x offset `collectionView.contentSize.width - collectionView.frame.size.width`. As you scroll to the right, the content offset will decrease until you reach the end of the data at a content offset of 0,0. In this case, `ASDisplayShouldFetchBatchForScrollView` needs to know that you are in RTL and the layout is not flipped. It can then use the contentOffset as the `remainingDistance` to determine when to fetch.

* fix indentation

* assert that we are on main when accessing CV layout

Source/ASCollectionView.mm

@@ -1857,7 +1857,10 @@ - (void)_checkForBatchFetching
 
 - (void)_beginBatchFetchingIfNeededWithContentOffset:(CGPoint)contentOffset velocity:(CGPoint)velocity
 {
-  if (ASDisplayShouldFetchBatchForScrollView(self, self.scrollDirection, self.scrollableDirections, contentOffset, velocity)) {
+  // Since we are accessing self.collectionViewLayout, we should make sure we are on main
+  ASDisplayNodeAssertMainThread();
+
+  if (ASDisplayShouldFetchBatchForScrollView(self, self.scrollDirection, self.scrollableDirections, contentOffset, velocity, self.collectionViewLayout.flipsHorizontallyInOppositeLayoutDirection)) {
     [self _beginBatchFetching];
   }
 }

Source/ASTableView.mm

@@ -1486,7 +1486,7 @@ - (void)_checkForBatchFetching
 
 - (void)_beginBatchFetchingIfNeededWithContentOffset:(CGPoint)contentOffset velocity:(CGPoint)velocity
 {
-  if (ASDisplayShouldFetchBatchForScrollView(self, self.scrollDirection, ASScrollDirectionVerticalDirections, contentOffset, velocity)) {
+  if (ASDisplayShouldFetchBatchForScrollView(self, self.scrollDirection, ASScrollDirectionVerticalDirections, contentOffset, velocity, NO)) {
     [self _beginBatchFetching];
   }
 }

Source/Private/ASBatchFetching.h

@@ -34,13 +34,16 @@ NS_ASSUME_NONNULL_BEGIN
  @param scrollableDirections The possible scrolling directions of the scroll view.
  @param contentOffset The offset that the scrollview will scroll to.
  @param velocity The velocity of the scroll view (in points) at the moment the touch was released.
+ @param flipsHorizontallyInOppositeLayoutDirection Whether or not this scroll view flips its layout automatically in RTL.
+         See flipsHorizontallyInOppositeLayoutDirection in UICollectionViewLayout
  @return Whether or not the current state should proceed with batch fetching.
  */
 ASDK_EXTERN BOOL ASDisplayShouldFetchBatchForScrollView(UIScrollView<ASBatchFetchingScrollView> *scrollView,
                                             ASScrollDirection scrollDirection,
                                             ASScrollDirection scrollableDirections,
                                             CGPoint contentOffset,
-                                            CGPoint velocity);
+                                            CGPoint velocity,
+                                            BOOL flipsHorizontallyInOppositeLayoutDirection);
 
 
 /**
@@ -55,6 +58,8 @@ ASDK_EXTERN BOOL ASDisplayShouldFetchBatchForScrollView(UIScrollView<ASBatchFetc
  @param visible Whether the view is visible or not.
  @param velocity The velocity of the scroll view (in points) at the moment the touch was released.
  @param delegate The delegate to be consulted if needed.
+ @param flipsHorizontallyInOppositeLayoutDirection Whether or not this scroll view flips its layout automatically in RTL.
+         See flipsHorizontallyInOppositeLayoutDirection in UICollectionViewLayout
  @return Whether or not the current state should proceed with batch fetching.
  @discussion This method is broken into a category for unit testing purposes and should be used with the ASTableView and
  * ASCollectionView batch fetching API.
@@ -69,6 +74,7 @@ ASDK_EXTERN BOOL ASDisplayShouldFetchBatchForContext(ASBatchContext *context,
                                                 BOOL visible,
                                                 BOOL shouldRenderRTLLayout,
                                                 CGPoint velocity,
+                                                BOOL flipsHorizontallyInOppositeLayoutDirection,
                                                 _Nullable id<ASBatchFetchingDelegate> delegate);
 
 NS_ASSUME_NONNULL_END

Source/Private/ASBatchFetching.mm

@@ -15,7 +15,8 @@ BOOL ASDisplayShouldFetchBatchForScrollView(UIScrollView<ASBatchFetchingScrollVi
                                             ASScrollDirection scrollDirection,
                                             ASScrollDirection scrollableDirections,
                                             CGPoint contentOffset,
-                                            CGPoint velocity)
+                                            CGPoint velocity,
+                                            BOOL flipsHorizontallyInOppositeLayoutDirection)
 {
   // Don't fetch if the scroll view does not allow
   if (![scrollView canBatchFetch]) {
@@ -30,7 +31,7 @@ BOOL ASDisplayShouldFetchBatchForScrollView(UIScrollView<ASBatchFetchingScrollVi
   id<ASBatchFetchingDelegate> delegate = scrollView.batchFetchingDelegate;
   BOOL visible = (scrollView.window != nil);
   BOOL shouldRenderRTLLayout = [UIView userInterfaceLayoutDirectionForSemanticContentAttribute:scrollView.semanticContentAttribute] == UIUserInterfaceLayoutDirectionRightToLeft;
-  return ASDisplayShouldFetchBatchForContext(context, scrollDirection, scrollableDirections, bounds, contentSize, contentOffset, leadingScreens, visible, shouldRenderRTLLayout, velocity, delegate);
+  return ASDisplayShouldFetchBatchForContext(context, scrollDirection, scrollableDirections, bounds, contentSize, contentOffset, leadingScreens, visible, shouldRenderRTLLayout, velocity, flipsHorizontallyInOppositeLayoutDirection, delegate);
 }
 
 BOOL ASDisplayShouldFetchBatchForContext(ASBatchContext *context,
@@ -43,6 +44,7 @@ BOOL ASDisplayShouldFetchBatchForContext(ASBatchContext *context,
                                          BOOL visible,
                                          BOOL shouldRenderRTLLayout,
                                          CGPoint velocity,
+                                         BOOL flipsHorizontallyInOppositeLayoutDirection,
                                          id<ASBatchFetchingDelegate> delegate)
 {
   // Do not allow fetching if a batch is already in-flight and hasn't been completed or cancelled
@@ -87,11 +89,11 @@ BOOL ASDisplayShouldFetchBatchForContext(ASBatchContext *context,
   }
 
   CGFloat triggerDistance = viewLength * leadingScreens;
-  CGFloat remainingDistance = contentLength - viewLength - offset;
-  if (shouldRenderRTLLayout && ASScrollDirectionContainsHorizontalDirection(scrollableDirections)) {
-      remainingDistance = offset;
+  CGFloat remainingDistance = 0;
+  if (!flipsHorizontallyInOppositeLayoutDirection && shouldRenderRTLLayout && ASScrollDirectionContainsHorizontalDirection(scrollableDirections)) {
+    remainingDistance = offset;
   } else {
-      remainingDistance = contentLength - viewLength - offset;
+    remainingDistance = contentLength - viewLength - offset;
   }
   BOOL result = remainingDistance <= triggerDistance;