Added method to ASLayoutable to allow a layoutable to override how it will be added to the layoutSpec.

Also moved ASStaticLayoutSpec to act more like the other layouts.

AsyncDisplayKit.xcodeproj/project.pbxproj

@@ -233,6 +233,8 @@
 		9C3061091B857EC400D0530B /* ASBaselineLayoutSpec.mm in Sources */ = {isa = PBXBuildFile; fileRef = 9C3061051B857EC400D0530B /* ASBaselineLayoutSpec.mm */; };
 		9C49C36F1B853957000B0DD5 /* ASStackLayoutable.h in Headers */ = {isa = PBXBuildFile; fileRef = 9C49C36E1B853957000B0DD5 /* ASStackLayoutable.h */; settings = {ATTRIBUTES = (Public, ); }; };
 		9C49C3701B853961000B0DD5 /* ASStackLayoutable.h in Headers */ = {isa = PBXBuildFile; fileRef = 9C49C36E1B853957000B0DD5 /* ASStackLayoutable.h */; settings = {ATTRIBUTES = (Public, ); }; };
+		9C6BB3B21B8CC9C200F13F52 /* ASStaticLayoutable.h in Headers */ = {isa = PBXBuildFile; fileRef = 9C6BB3B01B8CC9C200F13F52 /* ASStaticLayoutable.h */; settings = {ATTRIBUTES = (Public, ); }; };
+		9C6BB3B31B8CC9C200F13F52 /* ASStaticLayoutable.h in Headers */ = {isa = PBXBuildFile; fileRef = 9C6BB3B01B8CC9C200F13F52 /* ASStaticLayoutable.h */; settings = {ATTRIBUTES = (Public, ); }; };
 		9F06E5CD1B4CAF4200F015D8 /* ASCollectionViewTests.m in Sources */ = {isa = PBXBuildFile; fileRef = 9F06E5CC1B4CAF4200F015D8 /* ASCollectionViewTests.m */; };
 		AC21EC101B3D0BF600C8B19A /* ASStackLayoutDefines.h in Headers */ = {isa = PBXBuildFile; fileRef = AC21EC0F1B3D0BF600C8B19A /* ASStackLayoutDefines.h */; settings = {ATTRIBUTES = (Public, ); }; };
 		AC3C4A511A1139C100143C57 /* ASCollectionView.h in Headers */ = {isa = PBXBuildFile; fileRef = AC3C4A4F1A1139C100143C57 /* ASCollectionView.h */; settings = {ATTRIBUTES = (Public, ); }; };
@@ -580,6 +582,7 @@
 		9C3061041B857EC400D0530B /* ASBaselineLayoutSpec.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; name = ASBaselineLayoutSpec.h; path = AsyncDisplayKit/Layout/ASBaselineLayoutSpec.h; sourceTree = "<group>"; };
 		9C3061051B857EC400D0530B /* ASBaselineLayoutSpec.mm */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.objcpp; name = ASBaselineLayoutSpec.mm; path = AsyncDisplayKit/Layout/ASBaselineLayoutSpec.mm; sourceTree = "<group>"; };
 		9C49C36E1B853957000B0DD5 /* ASStackLayoutable.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; name = ASStackLayoutable.h; path = AsyncDisplayKit/Layout/ASStackLayoutable.h; sourceTree = "<group>"; };
+		9C6BB3B01B8CC9C200F13F52 /* ASStaticLayoutable.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; name = ASStaticLayoutable.h; path = AsyncDisplayKit/Layout/ASStaticLayoutable.h; sourceTree = "<group>"; };
 		9F06E5CC1B4CAF4200F015D8 /* ASCollectionViewTests.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = ASCollectionViewTests.m; sourceTree = "<group>"; };
 		AC21EC0F1B3D0BF600C8B19A /* ASStackLayoutDefines.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; name = ASStackLayoutDefines.h; path = AsyncDisplayKit/Layout/ASStackLayoutDefines.h; sourceTree = "<group>"; };
 		AC3C4A4F1A1139C100143C57 /* ASCollectionView.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = ASCollectionView.h; sourceTree = "<group>"; };
@@ -993,6 +996,7 @@
 				AC21EC0F1B3D0BF600C8B19A /* ASStackLayoutDefines.h */,
 				ACF6ED161B17843500DA7C62 /* ASStackLayoutSpec.h */,
 				ACF6ED171B17843500DA7C62 /* ASStackLayoutSpec.mm */,
+				9C6BB3B01B8CC9C200F13F52 /* ASStaticLayoutable.h */,
 				ACF6ED181B17843500DA7C62 /* ASStaticLayoutSpec.h */,
 				ACF6ED191B17843500DA7C62 /* ASStaticLayoutSpec.mm */,
 				9C3061041B857EC400D0530B /* ASBaselineLayoutSpec.h */,
@@ -1046,6 +1050,7 @@
 				ACF6ED201B17843500DA7C62 /* ASDimension.h in Headers */,
 				ACF6ED2B1B17843500DA7C62 /* ASOverlayLayoutSpec.h in Headers */,
 				ACF6ED1C1B17843500DA7C62 /* ASCenterLayoutSpec.h in Headers */,
+				9C6BB3B21B8CC9C200F13F52 /* ASStaticLayoutable.h in Headers */,
 				ACF6ED2A1B17843500DA7C62 /* ASLayoutable.h in Headers */,
 				ACF6ED311B17843500DA7C62 /* ASStaticLayoutSpec.h in Headers */,
 				ACF6ED241B17843500DA7C62 /* ASLayout.h in Headers */,
@@ -1161,6 +1166,7 @@
 			isa = PBXHeadersBuildPhase;
 			buildActionMask = 2147483647;
 			files = (
+				9C6BB3B31B8CC9C200F13F52 /* ASStaticLayoutable.h in Headers */,
 				B35062321B010EFD0018CF92 /* ASTextNodeShadower.h in Headers */,
 				34EFC7651B701CCC00AD841F /* ASRelativeSize.h in Headers */,
 				B35062431B010EFD0018CF92 /* UIView+ASConvenience.h in Headers */,

AsyncDisplayKit/ASDisplayNode.mm

@@ -677,6 +677,11 @@ static inline BOOL _ASDisplayNodeIsAncestorOfDisplayNode(ASDisplayNode *possible
   return NO;
 }
 
+- (id<ASLayoutable>)finalLayoutable
+{
+  return self;
+}
+
 /**
  * NOTE: It is an error to try to convert between nodes which do not share a common ancestor. This behavior is
  * disallowed in UIKit documentation and the behavior is left undefined. The output does not have a rigorously defined

AsyncDisplayKit/Layout/ASBackgroundLayoutSpec.mm

@@ -14,11 +14,9 @@
 #import "ASBaseDefines.h"
 #import "ASLayout.h"
 
+static NSString * const kBackgroundChildKey = @"kBackgroundChildKey";
+
 @interface ASBackgroundLayoutSpec ()
-{
-  id<ASLayoutable> _child;
-  id<ASLayoutable> _background;
-}
 @end
 
 @implementation ASBackgroundLayoutSpec
@@ -30,12 +28,11 @@
   }
   
   ASDisplayNodeAssertNotNil(child, @"Child cannot be nil");
-  _child = child;
-  _background = background;
+  [self setChild:child];
+  self.background = background;
   return self;
 }
 
-
 + (instancetype)backgroundLayoutSpecWithChild:(id<ASLayoutable>)child background:(id<ASLayoutable>)background;
 {
   return [[self alloc] initWithChild:child background:background];
@@ -46,12 +43,12 @@
  */
 - (ASLayout *)measureWithSizeRange:(ASSizeRange)constrainedSize
 {
-  ASLayout *contentsLayout = [_child measureWithSizeRange:constrainedSize];
+  ASLayout *contentsLayout = [[self child] measureWithSizeRange:constrainedSize];
 
   NSMutableArray *sublayouts = [NSMutableArray arrayWithCapacity:2];
-  if (_background) {
+  if (self.background) {
     // Size background to exactly the same size.
-    ASLayout *backgroundLayout = [_background measureWithSizeRange:{contentsLayout.size, contentsLayout.size}];
+    ASLayout *backgroundLayout = [self.background measureWithSizeRange:{contentsLayout.size, contentsLayout.size}];
     backgroundLayout.position = CGPointZero;
     [sublayouts addObject:backgroundLayout];
   }
@@ -63,14 +60,12 @@
 
 - (void)setBackground:(id<ASLayoutable>)background
 {
-  ASDisplayNodeAssert(self.isMutable, @"Cannot set properties when layout spec is not mutable");
-  _background = background;
+  [super setChild:background forIdentifier:kBackgroundChildKey];
 }
 
-- (void)setChild:(id<ASLayoutable>)child
+- (id<ASLayoutable>)background
 {
-  ASDisplayNodeAssert(self.isMutable, @"Cannot set properties when layout spec is not mutable");
-  _child = child;
+  return [super childForIdentifier:kBackgroundChildKey];
 }
 
 @end

AsyncDisplayKit/Layout/ASBaselineLayoutSpec.h

@@ -42,9 +42,6 @@ typedef NS_ENUM(NSUInteger, ASBaselineLayoutBaselineAlignment) {
 /** The type of baseline alignment */
 @property (nonatomic, assign) ASBaselineLayoutBaselineAlignment baselineAlignment;
 
-- (void)addChild:(id<ASBaselineLayoutable>)child;
-- (void)addChildren:(NSArray *)children;
-
 /**
  @param direction The direction of the stack view (horizontal or vertical)
  @param spacing The spacing between the children

AsyncDisplayKit/Layout/ASBaselineLayoutSpec.mm

@@ -27,7 +27,6 @@
 
 @implementation ASBaselineLayoutSpec
 {
-  std::vector<id<ASStackLayoutable>> _children;
   ASDN::RecursiveMutex _propertyLock;
 }
 
@@ -47,10 +46,7 @@
   _justifyContent = justifyContent;
   _baselineAlignment = baselineAlignment;
   
-  _children = std::vector<id<ASStackLayoutable>>();
-  for (id<ASStackLayoutable> child in children) {
-    _children.push_back(child);
-  }
+  [self setChildren:children];
   return self;
 }
 
@@ -65,7 +61,12 @@
   ASStackLayoutSpecStyle stackStyle = {.direction = _direction, .spacing = _spacing, .justifyContent = _justifyContent, .alignItems = _alignItems};
   ASBaselineLayoutSpecStyle style = { .baselineAlignment = _baselineAlignment, .stackLayoutStyle = stackStyle };
   
-  const auto unpositionedLayout = ASStackUnpositionedLayout::compute(_children, stackStyle, constrainedSize);
+  std::vector<id<ASStackLayoutable>> stackChildren = std::vector<id<ASStackLayoutable>>();
+  for (id<ASStackLayoutable> child in self.children) {
+    stackChildren.push_back(child);
+  }
+  
+  const auto unpositionedLayout = ASStackUnpositionedLayout::compute(stackChildren, stackStyle, constrainedSize);
   const auto positionedLayout = ASStackPositionedLayout::compute(unpositionedLayout, stackStyle, constrainedSize);
   const auto baselinePositionedLayout = ASBaselinePositionedLayout::compute(positionedLayout, style, constrainedSize);
   
@@ -82,16 +83,19 @@
                                    sublayouts:sublayouts];
 }
 
-- (void)addChild:(id<ASBaselineLayoutable>)child
+- (void)setChildren:(NSArray *)children
 {
-  _children.push_back(child);
+  [super setChildren:children];
+#if DEBUG
+  for (id<ASBaselineLayoutable> child in children) {
+    NSAssert(([child finalLayoutable] == child && [child conformsToProtocol:@protocol(ASBaselineLayoutable)]) || ([child finalLayoutable] != child && [[child finalLayoutable] conformsToProtocol:@protocol(ASBaselineLayoutable)]), @"child must conform to ASBaselineLayoutable");
+  }
+#endif
 }
 
-- (void)addChildren:(NSArray *)children
+- (void)setChild:(id<ASLayoutable>)child forIdentifier:(NSString *)identifier
 {
-  for (id<ASBaselineLayoutable> child in children) {
-    [self addChild:child];
-  }
+  ASDisplayNodeAssert(NO, @"ASBaselineLayoutSpec only supports setChildren");
 }
 
 @end

AsyncDisplayKit/Layout/ASCenterLayoutSpec.mm

@@ -17,7 +17,6 @@
 {
   ASCenterLayoutSpecCenteringOptions _centeringOptions;
   ASCenterLayoutSpecSizingOptions _sizingOptions;
-  id<ASLayoutable> _child;
 }
 
 - (instancetype)initWithCenteringOptions:(ASCenterLayoutSpecCenteringOptions)centeringOptions
@@ -30,7 +29,7 @@
   ASDisplayNodeAssertNotNil(child, @"Child cannot be nil");
   _centeringOptions = centeringOptions;
   _sizingOptions = sizingOptions;
-  _child = child;
+  [self setChild:child];
   return self;
 }
 
@@ -41,12 +40,6 @@
   return [[self alloc] initWithCenteringOptions:centeringOptions sizingOptions:sizingOptions child:child];
 }
 
-- (void)setChild:(id<ASLayoutable>)child
-{
-  ASDisplayNodeAssert(self.isMutable, @"Cannot set properties when layout spec is not mutable");
-  _child = child;
-}
-
 - (void)setCenteringOptions:(ASCenterLayoutSpecCenteringOptions)centeringOptions
 {
   ASDisplayNodeAssert(self.isMutable, @"Cannot set properties when layout spec is not mutable");
@@ -71,7 +64,7 @@
     (_centeringOptions & ASCenterLayoutSpecCenteringX) != 0 ? 0 : constrainedSize.min.width,
     (_centeringOptions & ASCenterLayoutSpecCenteringY) != 0 ? 0 : constrainedSize.min.height,
   };
-  ASLayout *sublayout = [_child measureWithSizeRange:ASSizeRangeMake(minChildSize, constrainedSize.max)];
+  ASLayout *sublayout = [self.child measureWithSizeRange:ASSizeRangeMake(minChildSize, constrainedSize.max)];
 
   // If we have an undetermined height or width, use the child size to define the layout
   // size

AsyncDisplayKit/Layout/ASInsetLayoutSpec.mm

@@ -19,7 +19,6 @@
 @interface ASInsetLayoutSpec ()
 {
   UIEdgeInsets _insets;
-  id<ASLayoutable> _child;
 }
 @end
 
@@ -50,7 +49,7 @@ static CGFloat centerInset(CGFloat outer, CGFloat inner)
   }
   ASDisplayNodeAssertNotNil(child, @"Child cannot be nil");
   _insets = insets;
-  _child = child;
+  [self setChild:child];
   return self;
 }
 
@@ -59,12 +58,6 @@ static CGFloat centerInset(CGFloat outer, CGFloat inner)
   return [[self alloc] initWithInsets:insets child:child];
 }
 
-- (void)setChild:(id<ASLayoutable>)child
-{
-  ASDisplayNodeAssert(self.isMutable, @"Cannot set properties when layout spec is not mutable");
-  _child = child;
-}
-
 - (void)setInsets:(UIEdgeInsets)insets
 {
   ASDisplayNodeAssert(self.isMutable, @"Cannot set properties when layout spec is not mutable");
@@ -95,7 +88,7 @@ static CGFloat centerInset(CGFloat outer, CGFloat inner)
       MAX(0, constrainedSize.max.height - insetsY),
     }
   };
-  ASLayout *sublayout = [_child measureWithSizeRange:insetConstrainedSize];
+  ASLayout *sublayout = [self.child measureWithSizeRange:insetConstrainedSize];
 
   const CGSize computedSize = ASSizeRangeClamp(constrainedSize, {
     finite(sublayout.size.width + _insets.left + _insets.right, constrainedSize.max.width),

AsyncDisplayKit/Layout/ASLayoutSpec.h

@@ -22,4 +22,13 @@
 
 - (instancetype)init;
 
+- (void)setChild:(id<ASLayoutable>)child;
+- (id<ASLayoutable>)child;
+
+- (void)setChild:(id<ASLayoutable>)child forIdentifier:(NSString *)identifier;
+- (id<ASLayoutable>)childForIdentifier:(NSString *)identifier;
+
+- (void)setChildren:(NSArray *)children;
+- (NSArray *)children;
+
 @end

AsyncDisplayKit/Layout/ASLayoutSpec.mm

@@ -16,6 +16,13 @@
 #import "ASInternalHelpers.h"
 #import "ASLayout.h"
 
+static NSString * const kDefaultChildKey = @"kDefaultChildKey";
+static NSString * const kDefaultChildrenKey = @"kDefaultChildrenKey";
+
+@interface ASLayoutSpec()
+@property (nonatomic, strong) NSMutableDictionary *layoutChildren;
+@end
+
 @implementation ASLayoutSpec
 
 @synthesize spacingBefore = _spacingBefore;
@@ -24,12 +31,14 @@
 @synthesize flexShrink = _flexShrink;
 @synthesize flexBasis = _flexBasis;
 @synthesize alignSelf = _alignSelf;
+@synthesize layoutChildren = _layoutChildren;
 
 - (instancetype)init
 {
   if (!(self = [super init])) {
     return nil;
   }
+  _layoutChildren = [NSMutableDictionary dictionary];
   _flexBasis = ASRelativeDimensionUnconstrained;
   _isMutable = YES;
   return self;
@@ -42,4 +51,45 @@
   return [ASLayout layoutWithLayoutableObject:self size:constrainedSize.min];
 }
 
+- (id<ASLayoutable>)finalLayoutable
+{
+  return self;
+}
+
+- (void)setChild:(id<ASLayoutable>)child;
+{
+  [self setChild:child forIdentifier:kDefaultChildKey];
+}
+
+- (id<ASLayoutable>)child
+{
+  return self.layoutChildren[kDefaultChildKey];
+}
+
+- (void)setChild:(id<ASLayoutable>)child forIdentifier:(NSString *)identifier
+{
+  ASDisplayNodeAssert(self.isMutable, @"Cannot set properties when layout spec is not mutable");
+  self.layoutChildren[identifier] = [child finalLayoutable];
+}
+
+- (id<ASLayoutable>)childForIdentifier:(NSString *)identifier
+{
+  return self.layoutChildren[identifier];
+}
+
+- (void)setChildren:(NSArray *)children
+{
+  ASDisplayNodeAssert(self.isMutable, @"Cannot set properties when layout spec is not mutable");
+  NSMutableArray *finalChildren = [NSMutableArray arrayWithCapacity:children.count];
+  for (id<ASLayoutable> child in children) {
+    [finalChildren addObject:[child finalLayoutable]];
+  }
+  self.layoutChildren[kDefaultChildrenKey] = [NSArray arrayWithArray:finalChildren];
+}
+
+- (NSArray *)children
+{
+  return self.layoutChildren[kDefaultChildrenKey];
+}
+
 @end

AsyncDisplayKit/Layout/ASLayoutable.h

@@ -12,6 +12,7 @@
 #import <AsyncDisplayKit/ASStackLayoutDefines.h>
 
 @class ASLayout;
+@class ASLayoutSpec;
 
 /** 
  * The ASLayoutable protocol declares a method for measuring the layout of an object. A class must implement the method
@@ -29,4 +30,16 @@
  */
 - (ASLayout *)measureWithSizeRange:(ASSizeRange)constrainedSize;
 
+/**
+ @abstract Give this object a last chance to add itself to a container ASLayoutable (most likely an ASLayoutSpec) before
+ being added to a ASLayoutSpec.
+ 
+ For example, consider a node whose superclass is laid out via calculateLayoutThatFits:. The subclass cannot implement
+ layoutSpecThatFits: since its ASLayout is already being created by calculateLayoutThatFits:. By implementing this method
+ a subclass can wrap itself in an ASLayoutSpec right before it is added to a layout spec.
+ 
+ It is rare that a class will need to implement this method.
+ */
+- (id<ASLayoutable>)finalLayoutable;
+
 @end

AsyncDisplayKit/Layout/ASOverlayLayoutSpec.mm

@@ -14,11 +14,9 @@
 #import "ASBaseDefines.h"
 #import "ASLayout.h"
 
+static NSString * const kOverlayChildKey = @"kOverlayChildKey";
+
 @implementation ASOverlayLayoutSpec
-{
-  id<ASLayoutable> _overlay;
-  id<ASLayoutable> _child;
-}
 
 - (instancetype)initWithChild:(id<ASLayoutable>)child overlay:(id<ASLayoutable>)overlay
 {
@@ -26,8 +24,8 @@
     return nil;
   }
   ASDisplayNodeAssertNotNil(child, @"Child that will be overlayed on shouldn't be nil");
-  _overlay = overlay;
-  _child = child;
+  self.overlay = overlay;
+  [self setChild:child];
   return self;
 }
 
@@ -36,16 +34,14 @@
   return [[self alloc] initWithChild:child overlay:overlay];
 }
 
-- (void)setChild:(id<ASLayoutable>)child
-{
-  ASDisplayNodeAssert(self.isMutable, @"Cannot set properties when layout spec is not mutable");
-  _child = child;
-}
-
 - (void)setOverlay:(id<ASLayoutable>)overlay
 {
-  ASDisplayNodeAssert(self.isMutable, @"Cannot set properties when layout spec is not mutable");
-  _overlay = overlay;
+  [super setChild:overlay forIdentifier:kOverlayChildKey];
+}
+
+- (id<ASLayoutable>)overlay
+{
+  return [super childForIdentifier:kOverlayChildKey];
 }
 
 /**
@@ -56,8 +52,8 @@
   ASLayout *contentsLayout = [_child measureWithSizeRange:constrainedSize];
   contentsLayout.position = CGPointZero;
   NSMutableArray *sublayouts = [NSMutableArray arrayWithObject:contentsLayout];
-  if (_overlay) {
-    ASLayout *overlayLayout = [_overlay measureWithSizeRange:{contentsLayout.size, contentsLayout.size}];
+  if (self.overlay) {
+    ASLayout *overlayLayout = [self.overlay measureWithSizeRange:{contentsLayout.size, contentsLayout.size}];
     overlayLayout.position = CGPointZero;
     [sublayouts addObject:overlayLayout];
   }

AsyncDisplayKit/Layout/ASRatioLayoutSpec.mm

@@ -22,7 +22,6 @@
 @implementation ASRatioLayoutSpec
 {
   CGFloat _ratio;
-  id<ASLayoutable> _child;
 }
 
 + (instancetype)ratioLayoutSpecWithRatio:(CGFloat)ratio child:(id<ASLayoutable>)child
@@ -38,16 +37,10 @@
   ASDisplayNodeAssertNotNil(child, @"Child cannot be nil");
   ASDisplayNodeAssert(ratio > 0, @"Ratio should be strictly positive, but received %f", ratio);
   _ratio = ratio;
-  _child = child;
+  [self setChild:child];
   return self;
 }
 
-- (void)setChild:(id<ASLayoutable>)child
-{
-  ASDisplayNodeAssert(self.isMutable, @"Cannot set properties when layout spec is not mutable");
-  _child = child;
-}
-
 - (void)setRatio:(CGFloat)ratio
 {
   ASDisplayNodeAssert(self.isMutable, @"Cannot set properties when layout spec is not mutable");
@@ -77,7 +70,7 @@
 
   // If there is no max size in *either* dimension, we can't apply the ratio, so just pass our size range through.
   const ASSizeRange childRange = (bestSize == sizeOptions.end()) ? constrainedSize : ASSizeRangeMake(*bestSize, *bestSize);
-  ASLayout *sublayout = [_child measureWithSizeRange:childRange];
+  ASLayout *sublayout = [self.child measureWithSizeRange:childRange];
   sublayout.position = CGPointZero;
   return [ASLayout layoutWithLayoutableObject:self size:sublayout.size sublayouts:@[sublayout]];
 }

AsyncDisplayKit/Layout/ASStackLayoutSpec.h

@@ -55,7 +55,4 @@
  */
 + (instancetype)stackLayoutSpecWithDirection:(ASStackLayoutDirection)direction spacing:(CGFloat)spacing justifyContent:(ASStackLayoutJustifyContent)justifyContent alignItems:(ASStackLayoutAlignItems)alignItems children:(NSArray *)children;
 
-- (void)addChild:(id<ASStackLayoutable>)child;
-- (void)addChildren:(NSArray *)children;
-
 @end

AsyncDisplayKit/Layout/ASStackLayoutSpec.mm

@@ -23,9 +23,6 @@
 #import "ASThread.h"
 
 @implementation ASStackLayoutSpec
-{
-  std::vector<id<ASStackLayoutable>> _children;
-}
 
 - (instancetype)init
 {
@@ -47,26 +44,10 @@
   _spacing = spacing;
   _justifyContent = justifyContent;
   
-  _children = std::vector<id<ASStackLayoutable>>();
-  for (id<ASStackLayoutable> child in children) {
-    _children.push_back(child);
-  }
+  [self setChildren:children];
   return self;
 }
 
-- (void)addChild:(id<ASStackLayoutable>)child
-{
-  ASDisplayNodeAssert(self.isMutable, @"Cannot set properties when layout spec is not mutable");
-  _children.push_back(child);
-}
-
-- (void)addChildren:(NSArray *)children
-{
-  for (id<ASStackLayoutable> child in children) {
-    [self addChild:child];
-  }
-}
-
 - (void)setDirection:(ASStackLayoutDirection)direction
 {
   ASDisplayNodeAssert(self.isMutable, @"Cannot set properties when layout spec is not mutable");
@@ -91,10 +72,32 @@
   _spacing = spacing;
 }
 
+- (void)setChildren:(NSArray *)children
+{
+  [super setChildren:children];
+  
+#if DEBUG
+  for (id<ASStackLayoutable> child in children) {
+    ASDisplayNodeAssert(([child finalLayoutable] == child && [child conformsToProtocol:@protocol(ASStackLayoutable)]) || ([child finalLayoutable] != child && [[child finalLayoutable] conformsToProtocol:@protocol(ASStackLayoutable)]), @"child must conform to ASBaselineLayoutable");
+  }
+#endif
+}
+
+- (void)setChild:(id<ASLayoutable>)child forIdentifier:(NSString *)identifier
+{
+  ASDisplayNodeAssert(NO, @"ASStackLayoutSpec only supports setChildren");
+}
+
 - (ASLayout *)measureWithSizeRange:(ASSizeRange)constrainedSize
 {
   ASStackLayoutSpecStyle style = {.direction = _direction, .spacing = _spacing, .justifyContent = _justifyContent, .alignItems = _alignItems};
-  const auto unpositionedLayout = ASStackUnpositionedLayout::compute(_children, style, constrainedSize);
+  std::vector<id<ASStackLayoutable>> stackChildren = std::vector<id<ASStackLayoutable>>();
+  for (id<ASStackLayoutable> child in self.children) {
+    NSAssert([child conformsToProtocol:@protocol(ASStackLayoutable)], @"Child must implement ASStackLayoutable");
+    stackChildren.push_back(child);
+  }
+  
+  const auto unpositionedLayout = ASStackUnpositionedLayout::compute(stackChildren, style, constrainedSize);
   const auto positionedLayout = ASStackPositionedLayout::compute(unpositionedLayout, style, constrainedSize);
   const CGSize finalSize = directionSize(style.direction, unpositionedLayout.stackDimensionSum, positionedLayout.crossSize);
   NSArray *sublayouts = [NSArray arrayWithObjects:&positionedLayout.sublayouts[0] count:positionedLayout.sublayouts.size()];

AsyncDisplayKit/Layout/ASStaticLayoutSpec.h

@@ -11,43 +11,6 @@
 #import <AsyncDisplayKit/ASLayoutSpec.h>
 #import <AsyncDisplayKit/ASRelativeSize.h>
 
-/** 
- * An ASStaticLayoutSpecChild object wraps an ASLayoutable object and provides position and size information,
- * to be used as a child of an ASStaticLayoutSpec. 
- */
-@interface ASStaticLayoutSpecChild : NSObject
-
-@property (nonatomic, readonly) CGPoint position;
-@property (nonatomic, readonly) id<ASLayoutable> layoutableObject;
-
-/**
- If specified, the child's size is restricted according to this size. Percentages are resolved relative to the static layout spec.
- */
-@property (nonatomic, readonly) ASRelativeSizeRange size;
-
-/**
- * Initializer.
- *
- * @param position The position of this child within its parent spec.
- *
- * @param layoutableObject The backing ASLayoutable object of this child.
- *
- * @param size The size range that this child's size is trstricted according to.
- */
-+ (instancetype)staticLayoutChildWithPosition:(CGPoint)position layoutableObject:(id<ASLayoutable>)layoutableObject size:(ASRelativeSizeRange)size;
-
-/**
- * Convenience initializer with default size is Unconstrained in both dimensions, which sets the child's min size to zero
- * and max size to the maximum available space it can consume without overflowing the spec's bounds.
- *
- * @param position The position of this child within its parent spec.
- *
- * @param layoutableObject The backing ASLayoutable object of this child.
- */
-+ (instancetype)staticLayoutChildWithPosition:(CGPoint)position layoutableObject:(id<ASLayoutable>)layoutableObject;
-
-@end
-
 /**
  * A layout spec that positions children at fixed positions.
  * 
@@ -56,10 +19,8 @@
 @interface ASStaticLayoutSpec : ASLayoutSpec
 
 /**
- @param children Children to be positioned at fixed positions, each is of type ASStaticLayoutSpecChild.
+ @param children Children to be positioned at fixed positions, each conforms to ASStaticLayoutable
  */
 + (instancetype)staticLayoutSpecWithChildren:(NSArray *)children;
 
-- (void)addChild:(ASStaticLayoutSpecChild *)child;
-
 @end

AsyncDisplayKit/Layout/ASStaticLayoutSpec.mm

@@ -13,31 +13,9 @@
 #import "ASLayoutSpecUtilities.h"
 #import "ASInternalHelpers.h"
 #import "ASLayout.h"
-
-@implementation ASStaticLayoutSpecChild
-
-+ (instancetype)staticLayoutChildWithPosition:(CGPoint)position layoutableObject:(id<ASLayoutable>)layoutableObject size:(ASRelativeSizeRange)size;
-{
-  ASStaticLayoutSpecChild *c = [[super alloc] init];
-  if (c) {
-    c->_position = position;
-    c->_layoutableObject = layoutableObject;
-    c->_size = size;
-  }
-  return c;
-}
-
-+ (instancetype)staticLayoutChildWithPosition:(CGPoint)position layoutableObject:(id<ASLayoutable>)layoutableObject
-{
-  return [self staticLayoutChildWithPosition:position layoutableObject:layoutableObject size:ASRelativeSizeRangeUnconstrained];
-}
-
-@end
+#import "ASStaticLayoutable.h"
 
 @implementation ASStaticLayoutSpec
-{
-  NSArray *_children;
-}
 
 + (instancetype)staticLayoutSpecWithChildren:(NSArray *)children
 {
@@ -54,14 +32,19 @@
   if (!(self = [super init])) {
     return nil;
   }
-  _children = children;
+  self.children = children;
   return self;
 }
 
-- (void)addChild:(ASStaticLayoutSpecChild *)child
+- (void)setChildren:(NSArray *)children
 {
-  ASDisplayNodeAssert(self.isMutable, @"Cannot set properties when layout spec is not mutable");
-  _children = [_children arrayByAddingObject:child];
+  [super setChildren:children];
+  
+#if DEBUG
+  for (id<ASStaticLayoutable> child in children) {
+    ASDisplayNodeAssert(([child finalLayoutable] == child && [child conformsToProtocol:@protocol(ASStaticLayoutable)]) || ([child finalLayoutable] != child && [[child finalLayoutable] conformsToProtocol:@protocol(ASStaticLayoutable)]), @"child must conform to ASStaticLayoutable");
+  }
+#endif
 }
 
 - (ASLayout *)measureWithSizeRange:(ASSizeRange)constrainedSize
@@ -71,16 +54,16 @@
     constrainedSize.max.height
   };
 
-  NSMutableArray *sublayouts = [NSMutableArray arrayWithCapacity:_children.count];
-  for (ASStaticLayoutSpecChild *child in _children) {
+  NSMutableArray *sublayouts = [NSMutableArray arrayWithCapacity:self.children.count];
+  for (id<ASStaticLayoutable> child in self.children) {
     CGSize autoMaxSize = {
       constrainedSize.max.width - child.position.x,
       constrainedSize.max.height - child.position.y
     };
-    ASSizeRange childConstraint = ASRelativeSizeRangeEqualToRelativeSizeRange(ASRelativeSizeRangeUnconstrained, child.size)
+    ASSizeRange childConstraint = ASRelativeSizeRangeEqualToRelativeSizeRange(ASRelativeSizeRangeUnconstrained, child.sizeRange)
       ? ASSizeRangeMake({0, 0}, autoMaxSize)
-      : ASRelativeSizeRangeResolve(child.size, size);
-    ASLayout *sublayout = [child.layoutableObject measureWithSizeRange:childConstraint];
+      : ASRelativeSizeRangeResolve(child.sizeRange, size);
+    ASLayout *sublayout = [child measureWithSizeRange:childConstraint];
     sublayout.position = child.position;
     [sublayouts addObject:sublayout];
   }

AsyncDisplayKit/Layout/ASStaticLayoutable.h

@@ -0,0 +1,24 @@
+/*
+ *  Copyright (c) 2014-present, Facebook, Inc.
+ *  All rights reserved.
+ *
+ *  This source code is licensed under the BSD-style license found in the
+ *  LICENSE file in the root directory of this source tree. An additional grant
+ *  of patent rights can be found in the PATENTS file in the same directory.
+ *
+ */
+
+#import <AsyncDisplayKit/ASLayoutable.h>
+#import <AsyncDisplayKit/ASRelativeSize.h>
+
+@protocol ASStaticLayoutable<ASLayoutable>
+
+/**
+ If specified, the child's size is restricted according to this size. Percentages are resolved relative to the static layout spec.
+ */
+@property (nonatomic, assign) ASRelativeSizeRange sizeRange;
+
+/** The position of this object within its parent spec. */
+@property (nonatomic, assign) CGPoint position;
+
+@end