Fixes to baseline stack alignment

1) Set the ascender/descender of an ASTextNode when the attributeString is set. Previously ascender/descender were only being computed in `setValuesFromLayoutable` and only when the attribute string was not nil. May make sense to remove the computation from `setValuesFromLayoutable` entirely.
2) Remove ability to allow different children of a stack spec to aling to different baselines. This wasn't working before and I'm not convinced it is possible to do properly/useful enough to invest the time.
3) Have all stack spec run `ASStackBaselinePositionedLayout::compute` to compute the stack's ascender and descender. Even if the stack isn't aligning its children to a baseline, the stack itself may be a child of another stack that IS aligning to a baseline.

AsyncDisplayKit/ASTextNode.mm

@@ -17,6 +17,7 @@
 #import <AsyncDisplayKit/ASTextNodeTextKitHelpers.h>
 #import <AsyncDisplayKit/ASDisplayNodeExtras.h>
 
+#import "ASInternalHelpers.h"
 #import "ASTextNodeRenderer.h"
 #import "ASTextNodeShadower.h"
 #import "ASEqualityHelpers.h"
@@ -344,6 +345,12 @@ static NSString *ASTextNodeTruncationTokenAttributeName = @"ASTextNodeTruncation
       self.isAccessibilityElement = YES;
     }
   });
+  
+  if (attributedString.length > 0) {
+    CGFloat screenScale = ASScreenScale();
+    self.ascender = round([[attributedString attribute:NSFontAttributeName atIndex:0 effectiveRange:NULL] ascender] * screenScale)/screenScale;
+    self.descender = round([[attributedString attribute:NSFontAttributeName atIndex:attributedString.length - 1 effectiveRange:NULL] descender] * screenScale)/screenScale;
+  }
 }
 
 #pragma mark - Text Layout

AsyncDisplayKit/Layout/ASStackLayoutDefines.h

@@ -45,9 +45,9 @@ typedef NS_ENUM(NSUInteger, ASStackLayoutAlignItems) {
   ASStackLayoutAlignItemsCenter,
   /** Expand children to fill cross axis */
   ASStackLayoutAlignItemsStretch,
-  /** Children align to their first baseline. Only available for horizontal stack nodes */
+  /** Children align to their first baseline. Only available for horizontal stack spec */
   ASStackLayoutAlignItemsBaselineFirst,
-  /** Children align to their last baseline. Only available for horizontal stack nodes */
+  /** Children align to their last baseline. Only available for horizontal stack spec */
   ASStackLayoutAlignItemsBaselineLast,
 };
 
@@ -66,8 +66,4 @@ typedef NS_ENUM(NSUInteger, ASStackLayoutAlignSelf) {
   ASStackLayoutAlignSelfCenter,
   /** Expand to fill cross axis */
   ASStackLayoutAlignSelfStretch,
-  /** Children align to their first baseline. Only available for horizontal stack nodes */
-  ASStackLayoutAlignSelfBaselineFirst,
-  /** Children align to their last baseline. Only available for horizontal stack nodes */
-  ASStackLayoutAlignSelfBaselineLast,
 };

AsyncDisplayKit/Layout/ASStackLayoutSpec.mm

@@ -101,7 +101,6 @@
   std::vector<id<ASLayoutable>> stackChildren = std::vector<id<ASLayoutable>>();
   for (id<ASLayoutable> child in self.children) {
     stackChildren.push_back(child);
-    needsBaselinePass |= child.alignSelf == ASStackLayoutAlignSelfBaselineFirst || child.alignSelf == ASStackLayoutAlignSelfBaselineLast;
   }
   
   const auto unpositionedLayout = ASStackUnpositionedLayout::compute(stackChildren, style, constrainedSize);
@@ -109,12 +108,21 @@
   
   CGSize finalSize = CGSizeZero;
   NSArray *sublayouts = nil;
-  if (needsBaselinePass) {
+  
+  // regardless of whether or not this stack aligns to baseline, we should let ASStackBaselinePositionedLayout::compute find the max ascender
+  // and min descender in case this spec is a child in another spec that wants to align to a baseline.
   const auto baselinePositionedLayout = ASStackBaselinePositionedLayout::compute(positionedLayout, style, constrainedSize);
+  if (self.direction == ASStackLayoutDirectionVertical) {
+    ASDN::MutexLocker l(_propertyLock);
+    self.ascender = [[self.children firstObject] ascender];
+    self.descender = [[self.children lastObject] descender];
+  } else {
     ASDN::MutexLocker l(_propertyLock);
     self.ascender = baselinePositionedLayout.ascender;
     self.descender = baselinePositionedLayout.descender;
+  }
   
+  if (needsBaselinePass) {
     finalSize = directionSize(style.direction, unpositionedLayout.stackDimensionSum, baselinePositionedLayout.crossSize);
     sublayouts = [NSArray arrayWithObjects:&baselinePositionedLayout.sublayouts[0] count:baselinePositionedLayout.sublayouts.size()];
   } else {

AsyncDisplayKit/Private/ASStackBaselinePositionedLayout.mm

@@ -93,7 +93,7 @@ ASStackBaselinePositionedLayout ASStackBaselinePositionedLayout::compute(const A
   const auto ascenderIt = std::max_element(positionedLayout.sublayouts.begin(), positionedLayout.sublayouts.end(), [&](const ASLayout *a, const ASLayout *b){
     return a.layoutableObject.ascender < b.layoutableObject.ascender;
   });
-  const CGFloat maxAscender = baselineIt == positionedLayout.sublayouts.end() ? 0 : (*ascenderIt).layoutableObject.ascender;
+  const CGFloat maxAscender = ascenderIt == positionedLayout.sublayouts.end() ? 0 : (*ascenderIt).layoutableObject.ascender;
   
   /*
     Step 3: Take each child and update its layout position based on the baseline offset.
@@ -103,9 +103,13 @@ ASStackBaselinePositionedLayout ASStackBaselinePositionedLayout::compute(const A
     spacing between the two nodes is from the baseline, not the bounding box.
    
   */
+  std::vector<ASLayout *> stackedChildren;
+  // Only change positions of layouts this stackSpec is aligning to a baseline. Otherwise we are only here to
+  // compute the min/max descender/ascender for this stack spec.
+  if (style.baselineRelativeArrangement || style.alignItems == ASStackLayoutAlignItemsBaselineFirst || style.alignItems == ASStackLayoutAlignItemsBaselineLast) {
     CGPoint p = CGPointZero;
     BOOL first = YES;
-  auto stackedChildren = AS::map(positionedLayout.sublayouts, [&](ASLayout *l) -> ASLayout *{
+    stackedChildren = AS::map(positionedLayout.sublayouts, [&](ASLayout *l) -> ASLayout *{
       __weak id<ASLayoutable> child = l.layoutableObject;
       p = p + directionPoint(style.direction, child.spacingBefore, 0);
       if (first) {
@@ -130,6 +134,9 @@ ASStackBaselinePositionedLayout ASStackBaselinePositionedLayout::compute(const A
       
       return l;
     });
+  } else {
+    stackedChildren = positionedLayout.sublayouts;
+  }
   
   /*
     Step 4: Since we have been mucking with positions, there is a chance that our cross size has changed. Imagine a node with a font size of 40

AsyncDisplayKit/Private/ASStackLayoutSpecUtilities.h

@@ -63,10 +63,6 @@ inline ASStackLayoutAlignItems alignment(ASStackLayoutAlignSelf childAlignment,
       return ASStackLayoutAlignItemsStart;
     case ASStackLayoutAlignSelfStretch:
       return ASStackLayoutAlignItemsStretch;
-    case ASStackLayoutAlignSelfBaselineFirst:
-      return ASStackLayoutAlignItemsBaselineFirst;
-    case ASStackLayoutAlignSelfBaselineLast:
-      return ASStackLayoutAlignItemsBaselineLast;
     case ASStackLayoutAlignSelfAuto:
     default:
       return stackAlignment;