[Layout] Add RTL support to LayoutSpecs

Source/ASDisplayNode+LayoutSpec.mm

@@ -107,6 +107,27 @@ - (ASLayout *)calculateLayoutLayoutSpec:(ASSizeRange)constrainedSize
   }
   layout = [layout filteredNodeLayoutTree];
 
+  // Flip layout if layout should be rendered right-to-left
+  BOOL shouldRenderRTLLayout = [UIView userInterfaceLayoutDirectionForSemanticContentAttribute:_semanticContentAttribute] == UIUserInterfaceLayoutDirectionRightToLeft;
+  if (shouldRenderRTLLayout) {
+      for (ASLayout *sublayout in layout.sublayouts) {
+          switch (_semanticContentAttribute) {
+              case UISemanticContentAttributeUnspecified:
+              case UISemanticContentAttributeForceRightToLeft: {
+                  // Flip
+                CGPoint flippedPosition = CGPointMake(layout.size.width - CGRectGetWidth(sublayout.frame) - sublayout.position.x, sublayout.position.y);
+                sublayout.position = flippedPosition;
+              }
+              case UISemanticContentAttributePlayback:
+              case UISemanticContentAttributeForceLeftToRight:
+              case UISemanticContentAttributeSpatial:
+                  // Don't flip
+                  break;
+          }
+      }
+  }
+
+
   return layout;
 }
 

Source/ASDisplayNode.mm

@@ -1625,6 +1625,21 @@ - (void)updateCornerRoundingWithType:(ASCornerRoundingType)newRoundingType
   });
 }
 
+- (void)updateSemanticContentAttributeWithAttribute:(UISemanticContentAttribute)attribute
+{
+  __instanceLock__.lock();
+  UISemanticContentAttribute oldAttribute = _semanticContentAttribute;
+  _semanticContentAttribute = attribute;
+  __instanceLock__.unlock();
+
+  ASPerformBlockOnMainThread(^{
+    // If the value has changed we should attempt to relayout.
+    if (attribute != oldAttribute) {
+      [self setNeedsLayout];
+    }
+  });
+}
+
 - (void)recursivelySetDisplaySuspended:(BOOL)flag
 {
   _recursivelySetDisplaySuspended(self, nil, flag);

Source/Private/ASBatchFetching.h

@@ -67,6 +67,7 @@ ASDK_EXTERN BOOL ASDisplayShouldFetchBatchForContext(ASBatchContext *context,
                                                 CGPoint targetOffset,
                                                 CGFloat leadingScreens,
                                                 BOOL visible,
+                                                BOOL shouldRenderRTLLayout,
                                                 CGPoint velocity,
                                                 _Nullable id<ASBatchFetchingDelegate> delegate);
 

Source/Private/ASBatchFetching.mm

@@ -29,7 +29,8 @@ BOOL ASDisplayShouldFetchBatchForScrollView(UIScrollView<ASBatchFetchingScrollVi
   CGFloat leadingScreens = scrollView.leadingScreensForBatching;
   id<ASBatchFetchingDelegate> delegate = scrollView.batchFetchingDelegate;
   BOOL visible = (scrollView.window != nil);
-  return ASDisplayShouldFetchBatchForContext(context, scrollDirection, scrollableDirections, bounds, contentSize, contentOffset, leadingScreens, visible, velocity, delegate);
+  BOOL shouldRenderRTLLayout = [UIView userInterfaceLayoutDirectionForSemanticContentAttribute:scrollView.semanticContentAttribute] == UIUserInterfaceLayoutDirectionRightToLeft;
+  return ASDisplayShouldFetchBatchForContext(context, scrollDirection, scrollableDirections, bounds, contentSize, contentOffset, leadingScreens, visible, shouldRenderRTLLayout, velocity, delegate);
 }
 
 BOOL ASDisplayShouldFetchBatchForContext(ASBatchContext *context,
@@ -40,6 +41,7 @@ BOOL ASDisplayShouldFetchBatchForContext(ASBatchContext *context,
                                          CGPoint targetOffset,
                                          CGFloat leadingScreens,
                                          BOOL visible,
+                                         BOOL shouldRenderRTLLayout,
                                          CGPoint velocity,
                                          id<ASBatchFetchingDelegate> delegate)
 {
@@ -79,13 +81,18 @@ BOOL ASDisplayShouldFetchBatchForContext(ASBatchContext *context,
   }
 
   // If they are scrolling toward the head of content, don't batch fetch.
-  BOOL isScrollingTowardHead = (ASScrollDirectionContainsUp(scrollDirection) || ASScrollDirectionContainsLeft(scrollDirection));
+  BOOL isScrollingTowardHead = (ASScrollDirectionContainsUp(scrollDirection) || (shouldRenderRTLLayout ? ASScrollDirectionContainsRight(scrollDirection) : ASScrollDirectionContainsLeft(scrollDirection)));
   if (isScrollingTowardHead) {
     return NO;
   }
 
   CGFloat triggerDistance = viewLength * leadingScreens;
   CGFloat remainingDistance = contentLength - viewLength - offset;
+  if (shouldRenderRTLLayout && ASScrollDirectionContainsHorizontalDirection(scrollableDirections)) {
+      remainingDistance = offset;
+  } else {
+      remainingDistance = contentLength - viewLength - offset;
+  }
   BOOL result = remainingDistance <= triggerDistance;
 
   if (delegate != nil && velocityLength > 0.0) {

Source/Private/ASDisplayNode+UIViewBridge.mm

@@ -944,14 +944,15 @@ - (void)setEdgeAntialiasingMask:(CAEdgeAntialiasingMask)edgeAntialiasingMask
 
 - (UISemanticContentAttribute)semanticContentAttribute
 {
-  _bridge_prologue_read;
-  return _getFromViewOnly(semanticContentAttribute);
+  AS::MutexLocker l(__instanceLock__);
+  return _semanticContentAttribute;
 }
 
 - (void)setSemanticContentAttribute:(UISemanticContentAttribute)semanticContentAttribute
 {
-  _bridge_prologue_write;
+  AS::MutexLocker l(__instanceLock__);
   _setToViewOnly(semanticContentAttribute, semanticContentAttribute);
+  _semanticContentAttribute = semanticContentAttribute;
 #if YOGA
   [self semanticContentAttributeDidChange:semanticContentAttribute];
 #endif

Source/Private/ASDisplayNodeInternal.h

@@ -258,7 +258,8 @@ static constexpr CACornerMask kASCACornerAllCorners =
   // These properties are used on iOS 10 and lower, where safe area is not supported by UIKit.
   UIEdgeInsets _fallbackSafeAreaInsets;
 
-
+  // Right-to-Left layout support
+  UISemanticContentAttribute _semanticContentAttribute;
 
 #pragma mark - ASDisplayNode (Debugging)
   ASLayout *_unflattenedLayout;
@@ -332,6 +333,9 @@ static constexpr CACornerMask kASCACornerAllCorners =
                         cornerRadius:(CGFloat)newCornerRadius
                        maskedCorners:(CACornerMask)newMaskedCorners;
 
+/// Update the Semantic Content Attribute. Trigger layout if this value has changed.
+- (void)updateSemanticContentAttributeWithAttribute:(UISemanticContentAttribute)attribute;
+
 /// Alternative initialiser for backing with a custom view class.  Supports asynchronous display with _ASDisplayView subclasses.
 - (instancetype)initWithViewClass:(Class)viewClass;