Implement "space between" and "space around" options of stack layout justify content

AsyncDisplayKit/Layout/ASStackLayoutDefines.h

@@ -33,6 +33,21 @@ typedef NS_ENUM(NSUInteger, ASStackLayoutJustifyContent) {
    On underflow, children are right/bottom-aligned within this spec's bounds.
    */
   ASStackLayoutJustifyContentEnd,
+  /**
+   On overflow or if the stack has only 1 child, this value is identical to ASStackLayoutJustifyContentStart.
+   Otherwise, the starting edge of the first child is at the starting edge of the stack, 
+   the ending edge of the last child is at the ending edge of the stack, and the remaining children
+   are distributed so that the spacing between any two adjacent ones is the same.
+   If there is a remaining space after spacing division, it is combined with the last spacing (i.e the one between the last 2 children).
+   */
+  ASStackLayoutJustifyContentSpaceBetween,
+  /**
+   On overflow or if the stack has only 1 child, this value is identical to ASStackLayoutJustifyContentCenter.
+   Otherwise, children are distributed such that the spacing between any two adjacent ones is the same,
+   and the spacing between the first/last child and the stack edges is half the size of the spacing between children.
+   If there is a remaining space after spacing division, it is combined with the last spacing (i.e the one between the last child and the stack ending edge).
+   */
+  ASStackLayoutJustifyContentSpaceAround
 };
 
 /** Orientation of children along cross axis */

AsyncDisplayKit/Private/ASStackPositionedLayout.mm

@@ -15,6 +15,7 @@
 #import "ASStackLayoutSpecUtilities.h"
 #import "ASLayoutable.h"
 #import "ASLayoutOptions.h"
+#import "ASAssert.h"
 
 static CGFloat crossOffset(const ASStackLayoutSpecStyle &style,
                            const ASStackUnpositionedItem &l,
@@ -33,8 +34,20 @@ static CGFloat crossOffset(const ASStackLayoutSpecStyle &style,
   }
 }
 
+/**
+ * Positions children according to the stack style and positioning properties.
+ *
+ * @param style The layout style of the overall stack layout
+ * @param firstChildOffset Offset of the first child
+ * @param extraSpacing Spacing between children, in addition to spacing set to the stack's layout style
+ * @param lastChildOffset Offset of the last child
+ * @param unpositionedLayout Unpositioned children of the stack
+ * @param constrainedSize Constrained size of the stack
+ */
 static ASStackPositionedLayout stackedLayout(const ASStackLayoutSpecStyle &style,
-                                             const CGFloat offset,
+                                             const CGFloat firstChildOffset,
+                                             const CGFloat extraSpacing,
+                                             const CGFloat lastChildOffset,
                                              const ASStackUnpositionedLayout &unpositionedLayout,
                                              const ASSizeRange &constrainedSize)
 {
@@ -48,12 +61,16 @@ static ASStackPositionedLayout stackedLayout(const ASStackLayoutSpecStyle &style
   const auto maxCrossSize = crossDimension(style.direction, constrainedSize.max);
   const CGFloat crossSize = MIN(MAX(minCrossSize, largestChildCrossSize), maxCrossSize);
   
-  CGPoint p = directionPoint(style.direction, offset, 0);
+  CGPoint p = directionPoint(style.direction, firstChildOffset, 0);
   BOOL first = YES;
+  const auto lastChild = unpositionedLayout.items.back().child;
+  CGFloat offset = 0;
+  
   auto stackedChildren = AS::map(unpositionedLayout.items, [&](const ASStackUnpositionedItem &l) -> ASLayout *{
-    p = p + directionPoint(style.direction, l.child.spacingBefore, 0);
+    offset = (l.child == lastChild) ? lastChildOffset : 0;
+    p = p + directionPoint(style.direction, l.child.spacingBefore + offset, 0);
     if (!first) {
-      p = p + directionPoint(style.direction, style.spacing, 0);
+      p = p + directionPoint(style.direction, style.spacing + extraSpacing, 0);
     }
     first = NO;
     l.layout.position = p + directionPoint(style.direction, 0, crossOffset(style, l, crossSize));
@@ -64,16 +81,45 @@ static ASStackPositionedLayout stackedLayout(const ASStackLayoutSpecStyle &style
   return {stackedChildren, crossSize};
 }
 
+static ASStackPositionedLayout stackedLayout(const ASStackLayoutSpecStyle &style,
+                                             const CGFloat firstChildOffset,
+                                             const ASStackUnpositionedLayout &unpositionedLayout,
+                                             const ASSizeRange &constrainedSize)
+{
+  return stackedLayout(style, firstChildOffset, 0, 0, unpositionedLayout, constrainedSize);
+}
+
 ASStackPositionedLayout ASStackPositionedLayout::compute(const ASStackUnpositionedLayout &unpositionedLayout,
                                                          const ASStackLayoutSpecStyle &style,
                                                          const ASSizeRange &constrainedSize)
 {
-  switch (style.justifyContent) {
+  const auto numOfItems = unpositionedLayout.items.size();
+  ASDisplayNodeCAssertTrue(numOfItems > 0);
+  const CGFloat violation = unpositionedLayout.violation;
+  ASStackLayoutJustifyContent justifyContent = style.justifyContent;
+  
+  // Handle edge cases of "space between" and "space around"
+  if (justifyContent == ASStackLayoutJustifyContentSpaceBetween && (violation < 0 || numOfItems == 1)) {
+    justifyContent = ASStackLayoutJustifyContentStart;
+  } else if (justifyContent == ASStackLayoutJustifyContentSpaceAround && (violation < 0 || numOfItems == 1)) {
+    justifyContent = ASStackLayoutJustifyContentCenter;
+  }
+  
+  switch (justifyContent) {
     case ASStackLayoutJustifyContentStart:
       return stackedLayout(style, 0, unpositionedLayout, constrainedSize);
     case ASStackLayoutJustifyContentCenter:
-      return stackedLayout(style, floorf(unpositionedLayout.violation / 2), unpositionedLayout, constrainedSize);
+      return stackedLayout(style, floorf(violation / 2), unpositionedLayout, constrainedSize);
     case ASStackLayoutJustifyContentEnd:
-      return stackedLayout(style, unpositionedLayout.violation, unpositionedLayout, constrainedSize);
+      return stackedLayout(style, violation, unpositionedLayout, constrainedSize);
+    case ASStackLayoutJustifyContentSpaceBetween: {
+      const auto numOfSpacings = numOfItems - 1;
+      return stackedLayout(style, 0, floorf(violation / numOfSpacings), fmodf(violation, numOfSpacings), unpositionedLayout, constrainedSize);
+    }
+    case ASStackLayoutJustifyContentSpaceAround: {
+      // Spacing between items are twice the spacing on the edges
+      CGFloat spacingUnit = floorf(violation / (numOfItems * 2));
+      return stackedLayout(style, spacingUnit, spacingUnit * 2, 0, unpositionedLayout, constrainedSize);
+    }
   }
 }

AsyncDisplayKitTests/ASStackLayoutSpecSnapshotTests.mm

@@ -98,6 +98,8 @@ static NSArray *defaultSubnodesWithSameSize(CGSize subnodeSize, BOOL flex)
   [self testStackLayoutSpecWithJustify:ASStackLayoutJustifyContentCenter flex:NO sizeRange:kSize identifier:@"justifyCenter"];
   [self testStackLayoutSpecWithJustify:ASStackLayoutJustifyContentEnd flex:NO sizeRange:kSize identifier:@"justifyEnd"];
   [self testStackLayoutSpecWithJustify:ASStackLayoutJustifyContentStart flex:YES sizeRange:kSize identifier:@"flex"];
+  [self testStackLayoutSpecWithJustify:ASStackLayoutJustifyContentSpaceBetween flex:NO sizeRange:kSize identifier:@"justifySpaceBetween"];
+  [self testStackLayoutSpecWithJustify:ASStackLayoutJustifyContentSpaceAround flex:NO sizeRange:kSize identifier:@"justifySpaceAround"];
 }
 
 - (void)testOverflowBehaviors
@@ -108,6 +110,10 @@ static NSArray *defaultSubnodesWithSameSize(CGSize subnodeSize, BOOL flex)
   [self testStackLayoutSpecWithJustify:ASStackLayoutJustifyContentCenter flex:NO sizeRange:kSize identifier:@"justifyCenter"];
   [self testStackLayoutSpecWithJustify:ASStackLayoutJustifyContentEnd flex:NO sizeRange:kSize identifier:@"justifyEnd"];
   [self testStackLayoutSpecWithJustify:ASStackLayoutJustifyContentStart flex:YES sizeRange:kSize identifier:@"flex"];
+  // On overflow, "space between" is identical to "content start"
+  [self testStackLayoutSpecWithJustify:ASStackLayoutJustifyContentSpaceBetween flex:NO sizeRange:kSize identifier:@"justifyStart"];
+  // On overflow, "space around" is identical to "content center"
+  [self testStackLayoutSpecWithJustify:ASStackLayoutJustifyContentSpaceAround flex:NO sizeRange:kSize identifier:@"justifyCenter"];
 }
 
 - (void)testOverflowBehaviorsWhenAllFlexShrinkChildrenHaveBeenClampedToZeroButViolationStillExists
@@ -246,6 +252,50 @@ static NSArray *defaultSubnodesWithSameSize(CGSize subnodeSize, BOOL flex)
   [self testStackLayoutSpecWithStyle:style sizeRange:kVariableHeight subnodes:subnodes identifier:@"variableHeight"];
 }
 
+- (void)testJustifiedSpaceBetweenWithOneChild
+{
+  ASStackLayoutSpecStyle style = {
+    .direction = ASStackLayoutDirectionHorizontal,
+    .justifyContent = ASStackLayoutJustifyContentSpaceBetween
+  };
+
+  ASStaticSizeDisplayNode *child = ASDisplayNodeWithBackgroundColor([UIColor redColor]);
+  child.staticSize = {50, 50};
+  
+  // width 300px; height 0-INF
+  static ASSizeRange kVariableHeight = {{300, 0}, {300, INFINITY}};
+  [self testStackLayoutSpecWithStyle:style sizeRange:kVariableHeight subnodes:@[child] identifier:nil];
+}
+
+- (void)testJustifiedSpaceAroundWithOneChild
+{
+  ASStackLayoutSpecStyle style = {
+    .direction = ASStackLayoutDirectionHorizontal,
+    .justifyContent = ASStackLayoutJustifyContentSpaceAround
+  };
+  
+  ASStaticSizeDisplayNode *child = ASDisplayNodeWithBackgroundColor([UIColor redColor]);
+  child.staticSize = {50, 50};
+  
+  // width 300px; height 0-INF
+  static ASSizeRange kVariableHeight = {{300, 0}, {300, INFINITY}};
+  [self testStackLayoutSpecWithStyle:style sizeRange:kVariableHeight subnodes:@[child] identifier:nil];
+}
+
+- (void)testJustifiedSpaceBetweenWithRemainingSpace
+{
+  // width 301px; height 0-300px; 1px remaining
+  static ASSizeRange kSize = {{301, 0}, {301, 300}};
+  [self testStackLayoutSpecWithJustify:ASStackLayoutJustifyContentSpaceBetween flex:NO sizeRange:kSize identifier:nil];
+}
+
+- (void)testJustifiedSpaceAroundWithRemainingSpace
+{
+  // width 305px; height 0-300px; 5px remaining
+  static ASSizeRange kSize = {{305, 0}, {305, 300}};
+  [self testStackLayoutSpecWithJustify:ASStackLayoutJustifyContentSpaceAround flex:NO sizeRange:kSize identifier:nil];
+}
+
 - (void)testChildThatChangesCrossSizeWhenMainSizeIsFlexed
 {
   ASStackLayoutSpecStyle style = {.direction = ASStackLayoutDirectionHorizontal};