[Layout] Move [ASLayoutSpec children] from std::map to NSMutableArray (#2253)

* Initial commit to move [ASLayoutSpec children] from std::map to NSMutableArray

* Add NSFastEnumeration to ASLayoutable

* ASNullLayoutSpec is a Singleton now

* Move ASLayoutSpecPrivate in Private folder

* Move to NSArrayPointer and remove ASNullLayoutSpec

* Revert "Move to NSArrayPointer and remove ASNullLayoutSpec"

This reverts commit 9ab9cf7024b1f6e1984d84fe58af2b84e84cdf94.

* Move to childAtIndex: and setChild:atIndex:

AsyncDisplayKit.xcodeproj/project.pbxproj

@@ -206,6 +206,9 @@
 		69708BA61D76386D005C3CF9 /* ASEqualityHashHelpers.h in Headers */ = {isa = PBXBuildFile; fileRef = 69708BA41D76386D005C3CF9 /* ASEqualityHashHelpers.h */; };
 		69708BA71D76386D005C3CF9 /* ASEqualityHashHelpers.mm in Sources */ = {isa = PBXBuildFile; fileRef = 69708BA51D76386D005C3CF9 /* ASEqualityHashHelpers.mm */; };
 		69708BA81D76386D005C3CF9 /* ASEqualityHashHelpers.mm in Sources */ = {isa = PBXBuildFile; fileRef = 69708BA51D76386D005C3CF9 /* ASEqualityHashHelpers.mm */; };
+		6977965F1D8AC8D3007E93D7 /* ASLayoutSpec+Subclasses.h in Headers */ = {isa = PBXBuildFile; fileRef = 6977965D1D8AC8D3007E93D7 /* ASLayoutSpec+Subclasses.h */; };
+		697796601D8AC8D3007E93D7 /* ASLayoutSpec+Subclasses.mm in Sources */ = {isa = PBXBuildFile; fileRef = 6977965E1D8AC8D3007E93D7 /* ASLayoutSpec+Subclasses.mm */; };
+		697796611D8AC8D3007E93D7 /* ASLayoutSpec+Subclasses.mm in Sources */ = {isa = PBXBuildFile; fileRef = 6977965E1D8AC8D3007E93D7 /* ASLayoutSpec+Subclasses.mm */; };
 		697B315A1CFE4B410049936F /* ASEditableTextNodeTests.m in Sources */ = {isa = PBXBuildFile; fileRef = 697B31591CFE4B410049936F /* ASEditableTextNodeTests.m */; };
 		697C0DE41CF38F28001DE0D4 /* ASLayoutValidation.h in Headers */ = {isa = PBXBuildFile; fileRef = 697C0DE11CF38F28001DE0D4 /* ASLayoutValidation.h */; };
 		697C0DE51CF38F28001DE0D4 /* ASLayoutValidation.mm in Sources */ = {isa = PBXBuildFile; fileRef = 697C0DE21CF38F28001DE0D4 /* ASLayoutValidation.mm */; };
@@ -220,6 +223,7 @@
 		69E1006E1CA89CB600D88C1B /* ASEnvironmentInternal.h in Headers */ = {isa = PBXBuildFile; fileRef = 69E100691CA89CB600D88C1B /* ASEnvironmentInternal.h */; };
 		69E1006F1CA89CB600D88C1B /* ASEnvironmentInternal.mm in Sources */ = {isa = PBXBuildFile; fileRef = 69E1006A1CA89CB600D88C1B /* ASEnvironmentInternal.mm */; };
 		69E100701CA89CB600D88C1B /* ASEnvironmentInternal.mm in Sources */ = {isa = PBXBuildFile; fileRef = 69E1006A1CA89CB600D88C1B /* ASEnvironmentInternal.mm */; };
+		69EEA0A11D9AB43900B46420 /* ASLayoutSpecPrivate.h in Headers */ = {isa = PBXBuildFile; fileRef = 69EEA0A01D9AB43900B46420 /* ASLayoutSpecPrivate.h */; };
 		69F10C871C84C35D0026140C /* ASRangeControllerUpdateRangeProtocol+Beta.h in Headers */ = {isa = PBXBuildFile; fileRef = 69F10C851C84C35D0026140C /* ASRangeControllerUpdateRangeProtocol+Beta.h */; settings = {ATTRIBUTES = (Public, ); }; };
 		69FEE53D1D95A9AF0086F066 /* ASLayoutableStyleTests.m in Sources */ = {isa = PBXBuildFile; fileRef = 69FEE53C1D95A9AF0086F066 /* ASLayoutableStyleTests.m */; };
 		7630FFA81C9E267E007A7C0E /* ASVideoNode.h in Headers */ = {isa = PBXBuildFile; fileRef = AEEC47DF1C20C2DD00EC1693 /* ASVideoNode.h */; settings = {ATTRIBUTES = (Public, ); }; };
@@ -985,6 +989,8 @@
 		696FCB301D6E46050093471E /* ASBackgroundLayoutSpecSnapshotTests.mm */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.objcpp; path = ASBackgroundLayoutSpecSnapshotTests.mm; sourceTree = "<group>"; };
 		69708BA41D76386D005C3CF9 /* ASEqualityHashHelpers.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; name = ASEqualityHashHelpers.h; path = TextKit/ASEqualityHashHelpers.h; sourceTree = "<group>"; };
 		69708BA51D76386D005C3CF9 /* ASEqualityHashHelpers.mm */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.objcpp; name = ASEqualityHashHelpers.mm; path = TextKit/ASEqualityHashHelpers.mm; sourceTree = "<group>"; };
+		6977965D1D8AC8D3007E93D7 /* ASLayoutSpec+Subclasses.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; name = "ASLayoutSpec+Subclasses.h"; path = "AsyncDisplayKit/Layout/ASLayoutSpec+Subclasses.h"; sourceTree = "<group>"; };
+		6977965E1D8AC8D3007E93D7 /* ASLayoutSpec+Subclasses.mm */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.objcpp; name = "ASLayoutSpec+Subclasses.mm"; path = "AsyncDisplayKit/Layout/ASLayoutSpec+Subclasses.mm"; sourceTree = "<group>"; };
 		697B31591CFE4B410049936F /* ASEditableTextNodeTests.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = ASEditableTextNodeTests.m; sourceTree = "<group>"; };
 		697C0DE11CF38F28001DE0D4 /* ASLayoutValidation.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; name = ASLayoutValidation.h; path = AsyncDisplayKit/Layout/ASLayoutValidation.h; sourceTree = "<group>"; };
 		697C0DE21CF38F28001DE0D4 /* ASLayoutValidation.mm */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.objcpp; name = ASLayoutValidation.mm; path = AsyncDisplayKit/Layout/ASLayoutValidation.mm; sourceTree = "<group>"; };
@@ -996,6 +1002,7 @@
 		69CB62AA1CB8165900024920 /* _ASDisplayViewAccessiblity.mm */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.objcpp; path = _ASDisplayViewAccessiblity.mm; sourceTree = "<group>"; };
 		69E100691CA89CB600D88C1B /* ASEnvironmentInternal.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = ASEnvironmentInternal.h; sourceTree = "<group>"; };
 		69E1006A1CA89CB600D88C1B /* ASEnvironmentInternal.mm */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.objcpp; path = ASEnvironmentInternal.mm; sourceTree = "<group>"; };
+		69EEA0A01D9AB43900B46420 /* ASLayoutSpecPrivate.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = ASLayoutSpecPrivate.h; sourceTree = "<group>"; };
 		69F10C851C84C35D0026140C /* ASRangeControllerUpdateRangeProtocol+Beta.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = "ASRangeControllerUpdateRangeProtocol+Beta.h"; sourceTree = "<group>"; };
 		69FEE53C1D95A9AF0086F066 /* ASLayoutableStyleTests.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = ASLayoutableStyleTests.m; sourceTree = "<group>"; };
 		6BDC61F51978FEA400E50D21 /* AsyncDisplayKit.h */ = {isa = PBXFileReference; explicitFileType = sourcecode.c.h; path = AsyncDisplayKit.h; sourceTree = "<group>"; };
@@ -1527,9 +1534,6 @@
 		058D0A01195D050800B7D73C /* Private */ = {
 			isa = PBXGroup;
 			children = (
-				CC446A2D1D80AAE00071FD03 /* ASObjectDescriptionHelpers.h */,
-				CC446A2E1D80AAE00071FD03 /* ASObjectDescriptionHelpers.m */,
-				CC54A81B1D70077A00296A24 /* ASDispatch.h */,
 				058D0A02195D050800B7D73C /* _AS-objc-internal.h */,
 				058D0A03195D050800B7D73C /* _ASCoreAnimationExtras.h */,
 				058D0A04195D050800B7D73C /* _ASCoreAnimationExtras.mm */,
@@ -1548,6 +1552,7 @@
 				8B0768B21CE752EC002E1453 /* ASDefaultPlaybackButton.m */,
 				AEB7B0181C5962EA00662EF4 /* ASDefaultPlayButton.h */,
 				AEB7B0191C5962EA00662EF4 /* ASDefaultPlayButton.m */,
+				CC54A81B1D70077A00296A24 /* ASDispatch.h */,
 				058D0A08195D050800B7D73C /* ASDisplayNode+AsyncDisplay.mm */,
 				058D0A09195D050800B7D73C /* ASDisplayNode+DebugTiming.h */,
 				058D0A0A195D050800B7D73C /* ASDisplayNode+DebugTiming.mm */,
@@ -1563,11 +1568,14 @@
 				058D0A0E195D050800B7D73C /* ASImageNode+CGExtras.m */,
 				ACF6ED431B17847A00DA7C62 /* ASInternalHelpers.h */,
 				ACF6ED441B17847A00DA7C62 /* ASInternalHelpers.m */,
+				69EEA0A01D9AB43900B46420 /* ASLayoutSpecPrivate.h */,
 				ACF6ED451B17847A00DA7C62 /* ASLayoutSpecUtilities.h */,
 				E52405B41C8FEF16004DC8E7 /* ASLayoutTransition.h */,
 				E52405B21C8FEF03004DC8E7 /* ASLayoutTransition.mm */,
 				0442850B1BAA64EC00D16268 /* ASMultidimensionalArrayUtils.h */,
 				0442850C1BAA64EC00D16268 /* ASMultidimensionalArrayUtils.mm */,
+				CC446A2D1D80AAE00071FD03 /* ASObjectDescriptionHelpers.h */,
+				CC446A2E1D80AAE00071FD03 /* ASObjectDescriptionHelpers.m */,
 				CC3B20811C3F76D600798563 /* ASPendingStateController.h */,
 				CC3B20821C3F76D600798563 /* ASPendingStateController.mm */,
 				058D0A10195D050800B7D73C /* ASSentinel.h */,
@@ -1677,12 +1685,14 @@
 				ACF6ED0A1B17843500DA7C62 /* ASInsetLayoutSpec.mm */,
 				ACF6ED0B1B17843500DA7C62 /* ASLayout.h */,
 				ACF6ED0C1B17843500DA7C62 /* ASLayout.mm */,
+				9CDC18CB1B910E12004965E2 /* ASLayoutablePrivate.h */,
 				ACF6ED111B17843500DA7C62 /* ASLayoutable.h */,
 				E55D86311CA8A14000A0C26F /* ASLayoutable.mm */,
 				698C8B601CAB49FC0052DC3F /* ASLayoutableExtensibility.h */,
-				9CDC18CB1B910E12004965E2 /* ASLayoutablePrivate.h */,
 				ACF6ED0D1B17843500DA7C62 /* ASLayoutSpec.h */,
 				ACF6ED0E1B17843500DA7C62 /* ASLayoutSpec.mm */,
+				6977965D1D8AC8D3007E93D7 /* ASLayoutSpec+Subclasses.h */,
+				6977965E1D8AC8D3007E93D7 /* ASLayoutSpec+Subclasses.mm */,
 				697C0DE11CF38F28001DE0D4 /* ASLayoutValidation.h */,
 				697C0DE21CF38F28001DE0D4 /* ASLayoutValidation.mm */,
 				ACF6ED121B17843500DA7C62 /* ASOverlayLayoutSpec.h */,
@@ -1775,6 +1785,7 @@
 				254C6B7C1BF94DF4003EC431 /* ASTextKitRenderer+TextChecking.h in Headers */,
 				34EFC7611B701C9C00AD841F /* ASBackgroundLayoutSpec.h in Headers */,
 				68AF37DB1CBEF4D80077BF76 /* ASImageNode+AnimatedImagePrivate.h in Headers */,
+				69EEA0A11D9AB43900B46420 /* ASLayoutSpecPrivate.h in Headers */,
 				B35062591B010F070018CF92 /* ASBaseDefines.h in Headers */,
 				B35062131B010EFD0018CF92 /* ASBasicImageDownloader.h in Headers */,
 				B35062461B010EFD0018CF92 /* ASBasicImageDownloaderInternal.h in Headers */,
@@ -1819,6 +1830,7 @@
 				AC7A2C181BDE11DF0093FE1A /* ASTableViewInternal.h in Headers */,
 				B35062531B010EFD0018CF92 /* ASImageNode+CGExtras.h in Headers */,
 				254C6B7F1BF94DF4003EC431 /* ASTextKitTruncating.h in Headers */,
+				6977965F1D8AC8D3007E93D7 /* ASLayoutSpec+Subclasses.h in Headers */,
 				7AB338671C55B3460055FDE8 /* ASRelativeLayoutSpec.h in Headers */,
 				B35062021B010EFD0018CF92 /* ASImageNode.h in Headers */,
 				B350621F1B010EFD0018CF92 /* ASImageProtocols.h in Headers */,
@@ -2227,6 +2239,7 @@
 				205F0E221B376416007741D0 /* CGRect+ASConvenience.m in Sources */,
 				257754B21BEE44CD00737CA5 /* ASTextKitShadower.mm in Sources */,
 				9CFFC6BE1CCAC52B006A6476 /* ASEnvironment.mm in Sources */,
+				697796601D8AC8D3007E93D7 /* ASLayoutSpec+Subclasses.mm in Sources */,
 				058D0A21195D050800B7D73C /* NSMutableAttributedString+TextKitAdditions.m in Sources */,
 				205F0E101B371875007741D0 /* UICollectionViewLayout+ASConvenience.m in Sources */,
 				CC7FD9DF1BB5E962005CCB2B /* ASPhotosFrameworkImageRequest.m in Sources */,
@@ -2411,6 +2424,7 @@
 				254C6B871BF94F8A003EC431 /* ASTextKitEntityAttribute.m in Sources */,
 				34566CB31BC1213700715E6B /* ASPhotosFrameworkImageRequest.m in Sources */,
 				254C6B831BF94F8A003EC431 /* ASTextKitCoreTextAdditions.m in Sources */,
+				697796611D8AC8D3007E93D7 /* ASLayoutSpec+Subclasses.mm in Sources */,
 				B350623B1B010EFD0018CF92 /* NSMutableAttributedString+TextKitAdditions.m in Sources */,
 				044284FD1BAA365100D16268 /* UICollectionViewLayout+ASConvenience.m in Sources */,
 				254C6B8A1BF94F8A003EC431 /* ASTextKitRenderer+TextChecking.mm in Sources */,

AsyncDisplayKit/ASDisplayNode.mm

@@ -3364,6 +3364,13 @@ static const char *ASDisplayNodeDrawingPriorityKey = "ASDrawingPriority";
   }
 }
 
+#pragma mark - NSFastEnumeration
+
+- (NSUInteger)countByEnumeratingWithState:(NSFastEnumerationState *)state objects:(id __unsafe_unretained _Nullable [_Nonnull])buffer count:(NSUInteger)len
+{
+  return [self.subnodes countByEnumeratingWithState:state objects:buffer count:len];
+}
+
 #pragma mark - ASEnvironment
 
 - (ASEnvironmentState)environmentState

AsyncDisplayKit/Layout/ASBackgroundLayoutSpec.h

@@ -17,11 +17,16 @@ NS_ASSUME_NONNULL_BEGIN
  */
 @interface ASBackgroundLayoutSpec : ASLayoutSpec
 
+/**
+ * Background layoutable for this layout spec
+ */
 @property (nullable, nonatomic, strong) id<ASLayoutable> background;
 
 /**
- @param child A child that is laid out to determine the size of this spec.
+ * Creates and returns an ASBackgroundLayoutSpec object
- @param background A layoutable object that is laid out behind the child. If this is nil, the background is omitted.
+ *
+ * @param child A child that is laid out to determine the size of this spec.
+ * @param background A layoutable object that is laid out behind the child. If this is nil, the background is omitted.
  */
 + (instancetype)backgroundLayoutSpecWithChild:(id<ASLayoutable>)child background:(nullable id<ASLayoutable>)background;
 

AsyncDisplayKit/Layout/ASBackgroundLayoutSpec.mm

@@ -9,6 +9,7 @@
 //
 
 #import "ASBackgroundLayoutSpec.h"
+#import "ASLayoutSpec+Subclasses.h"
 
 #import "ASAssert.h"
 #import "ASLayout.h"
@@ -16,11 +17,17 @@
 static NSUInteger const kForegroundChildIndex = 0;
 static NSUInteger const kBackgroundChildIndex = 1;
 
-@interface ASBackgroundLayoutSpec ()
-@end
-
 @implementation ASBackgroundLayoutSpec
 
+#pragma mark - Class
+
++ (instancetype)backgroundLayoutSpecWithChild:(id<ASLayoutable>)child background:(id<ASLayoutable>)background;
+{
+  return [[self alloc] initWithChild:child background:background];
+}
+
+#pragma mark - Lifecycle
+
 - (instancetype)initWithChild:(id<ASLayoutable>)child background:(id<ASLayoutable>)background
 {
   if (!(self = [super init])) {
@@ -28,15 +35,12 @@ static NSUInteger const kBackgroundChildIndex = 1;
   }
   
   ASDisplayNodeAssertNotNil(child, @"Child cannot be nil");
-  [self setChild:child forIndex:kForegroundChildIndex];
+  [self setChild:child atIndex:kForegroundChildIndex];
   self.background = background;
   return self;
 }
 
-+ (instancetype)backgroundLayoutSpecWithChild:(id<ASLayoutable>)child background:(id<ASLayoutable>)background;
+#pragma mark - ASLayoutSpec
-{
-  return [[self alloc] initWithChild:child background:background];
-}
 
 /**
  * First layout the contents, then fit the background image.
@@ -45,7 +49,7 @@ static NSUInteger const kBackgroundChildIndex = 1;
                      restrictedToSize:(ASLayoutableSize)size
                  relativeToParentSize:(CGSize)parentSize
 {
-  ASLayout *contentsLayout = [self.child layoutThatFits:constrainedSize parentSize:parentSize];
+  ASLayout *contentsLayout = [[super childAtIndex:kForegroundChildIndex] layoutThatFits:constrainedSize parentSize:parentSize];
 
   NSMutableArray *sublayouts = [NSMutableArray arrayWithCapacity:2];
   if (self.background) {
@@ -61,14 +65,16 @@ static NSUInteger const kBackgroundChildIndex = 1;
   return [ASLayout layoutWithLayoutable:self size:contentsLayout.size sublayouts:sublayouts];
 }
 
+#pragma mark - Background
+
 - (void)setBackground:(id<ASLayoutable>)background
 {
-  [super setChild:background forIndex:kBackgroundChildIndex];
+  [super setChild:background atIndex:kBackgroundChildIndex];
 }
 
 - (id<ASLayoutable>)background
 {
-  return [super childForIndex:kBackgroundChildIndex];
+  return [super childAtIndex:kBackgroundChildIndex];
 }
 
 @end

AsyncDisplayKit/Layout/ASLayoutSpec+Subclasses.h

@@ -0,0 +1,64 @@
+//
+//  ASLayoutSpec+Subclasses.h
+//  AsyncDisplayKit
+//
+//  Created by Michael Schneider on 9/15/16.
+//  Copyright © 2016 Facebook. All rights reserved.
+//
+
+#import <AsyncDisplayKit/AsyncDisplayKit.h>
+
+NS_ASSUME_NONNULL_BEGIN
+
+@interface ASLayoutSpec (Subclassing)
+
+/**
+ * Helper method for finalLayoutable support
+ *
+ * @warning If you are getting recursion crashes here after implementing finalLayoutable, make sure
+ * that you are setting isFinalLayoutable flag to YES. This must be one BEFORE adding a child
+ * to the new ASLayoutable.
+ *
+ * For example:
+ * - (id<ASLayoutable>)finalLayoutable
+ * {
+ *   ASInsetLayoutSpec *insetSpec = [[ASInsetLayoutSpec alloc] init];
+ *   insetSpec.insets = UIEdgeInsetsMake(10,10,10,10);
+ *   insetSpec.isFinalLayoutable = YES;
+ *   [insetSpec setChild:self];
+ *   return insetSpec;
+ * }
+ *
+ * @see finalLayoutable
+ */
+- (id<ASLayoutable>)layoutableToAddFromLayoutable:(id<ASLayoutable>)child;
+
+/**
+ * Adds a child with the given identifier to this layout spec.
+ *
+ * @param child A child to be added.
+ *
+ * @param index An index associated with the child.
+ *
+ * @discussion Every ASLayoutSpec must act on at least one child. The ASLayoutSpec base class takes the
+ * responsibility of holding on to the spec children. Some layout specs, like ASInsetLayoutSpec,
+ * only require a single child.
+ *
+ * For layout specs that require a known number of children (ASBackgroundLayoutSpec, for example)
+ * a subclass can use the setChild method to set the "primary" child. It should then use this method
+ * to set any other required children. Ideally a subclass would hide this from the user, and use the
+ * setChild:forIndex: internally. For example, ASBackgroundLayoutSpec exposes a backgroundChild
+ * property that behind the scenes is calling setChild:forIndex:.
+ */
+- (void)setChild:(id<ASLayoutable>)child atIndex:(NSUInteger)index;
+
+/**
+ * Returns the child added to this layout spec using the given index.
+ *
+ * @param index An identifier associated with the the child.
+ */
+- (nullable id<ASLayoutable>)childAtIndex:(NSUInteger)index;
+
+@end
+
+NS_ASSUME_NONNULL_END

AsyncDisplayKit/Layout/ASLayoutSpec+Subclasses.mm

@@ -0,0 +1,110 @@
+//
+//  ASLayoutSpec+Subclasses.m
+//  AsyncDisplayKit
+//
+//  Created by Michael Schneider on 9/15/16.
+//  Copyright © 2016 Facebook. All rights reserved.
+//
+
+#import "ASLayoutSpec+Subclasses.h"
+#import "ASLayoutSpec.h"
+#import "ASLayoutSpecPrivate.h"
+
+#pragma mark - ASNullLayoutSpec
+
+@interface ASNullLayoutSpec : ASLayoutSpec
+- (instancetype)init __unavailable;
++ (ASNullLayoutSpec *)null;
+@end
+
+@implementation ASNullLayoutSpec : ASLayoutSpec
+
++ (ASNullLayoutSpec *)null
+{
+  static ASNullLayoutSpec *sharedNullLayoutSpec = nil;
+  static dispatch_once_t onceToken;
+  dispatch_once(&onceToken, ^{
+    sharedNullLayoutSpec = [[self alloc] init];
+  });
+  return sharedNullLayoutSpec;
+}
+
+- (BOOL)isMutable
+{
+  return NO;
+}
+
+- (ASLayout *)calculateLayoutThatFits:(ASSizeRange)constrainedSize
+{
+  return [ASLayout layoutWithLayoutable:self size:CGSizeZero];
+}
+
+@end
+
+
+#pragma mark - ASLayoutSpec (Subclassing)
+
+@implementation ASLayoutSpec (Subclassing)
+
+#pragma mark - Final layoutable
+
+- (id<ASLayoutable>)layoutableToAddFromLayoutable:(id<ASLayoutable>)child
+{
+  if (self.isFinalLayoutable == NO) {
+    id<ASLayoutable> finalLayoutable = [child finalLayoutable];
+    if (finalLayoutable != child) {
+      if (ASEnvironmentStatePropagationEnabled()) {
+        ASEnvironmentStatePropagateUp(finalLayoutable, child.environmentState.layoutOptionsState);
+      } else {
+        // If state propagation is not enabled the layout options state needs to be copied manually
+        ASEnvironmentState finalLayoutableEnvironmentState = finalLayoutable.environmentState;
+        finalLayoutableEnvironmentState.layoutOptionsState = child.environmentState.layoutOptionsState;
+        finalLayoutable.environmentState = finalLayoutableEnvironmentState;
+      }
+      return finalLayoutable;
+    }
+  }
+  return child;
+}
+
+#pragma mark - Child with index
+
+- (void)setChild:(id<ASLayoutable>)child atIndex:(NSUInteger)index
+{
+  ASDisplayNodeAssert(self.isMutable, @"Cannot set properties when layout spec is not mutable");
+  
+  id<ASLayoutable> layoutable = child ? [self layoutableToAddFromLayoutable:child] : [ASNullLayoutSpec null];
+  
+  if (child) {
+    if (_childrenArray.count < index) {
+      // Fill up the array with null objects until the index
+      NSInteger i = _childrenArray.count;
+      while (i < index) {
+        _childrenArray[i] = [ASNullLayoutSpec null];
+        i++;
+      }
+    }
+  }
+  
+  // Replace object at the given index with the layoutable
+  _childrenArray[index] = layoutable;
+  
+  // TODO: Should we propagate up the layoutable at it could happen that multiple children will propagated up their
+  //       layout options and one child will overwrite values from another child
+  // [self propagateUpLayoutable:finalLayoutable];
+}
+
+- (id<ASLayoutable>)childAtIndex:(NSUInteger)index
+{
+  id<ASLayoutable> layoutable = nil;
+  if (index < _childrenArray.count) {
+    layoutable = _childrenArray[index];
+  }
+  
+  // Null layoutable should not be accessed
+  ASDisplayNodeAssert(layoutable != [ASNullLayoutSpec null], @"Access child at index without set a child at that index");
+
+  return layoutable;
+}
+
+@end

AsyncDisplayKit/Layout/ASLayoutSpec.h

@@ -25,8 +25,6 @@ NS_ASSUME_NONNULL_BEGIN
  */
 @property (nonatomic, assign) BOOL isMutable;
 
-- (instancetype)init;
-
 /**
  * Parent of the layout spec
  */
@@ -49,25 +47,6 @@ NS_ASSUME_NONNULL_BEGIN
  */
 @property (nullable, strong, nonatomic) id<ASLayoutable> child;
 
-/**
- * Adds a child with the given identifier to this layout spec.
- *
- * @param child A child to be added.
- *
- * @param index An index associated with the child.
- *
- * @discussion Every ASLayoutSpec must act on at least one child. The ASLayoutSpec base class takes the
- * responsibility of holding on to the spec children. Some layout specs, like ASInsetLayoutSpec,
- * only require a single child.
- *
- * For layout specs that require a known number of children (ASBackgroundLayoutSpec, for example)
- * a subclass can use the setChild method to set the "primary" child. It should then use this method
- * to set any other required children. Ideally a subclass would hide this from the user, and use the
- * setChild:forIndex: internally. For example, ASBackgroundLayoutSpec exposes a backgroundChild
- * property that behind the scenes is calling setChild:forIndex:.
- */
-- (void)setChild:(id<ASLayoutable>)child forIndex:(NSUInteger)index;
-
 /**
  * Adds childen to this layout spec.
  *
@@ -81,13 +60,6 @@ NS_ASSUME_NONNULL_BEGIN
  */
 @property (nullable, strong, nonatomic) NSArray<id<ASLayoutable>> *children;
 
-/**
- * Returns the child added to this layout spec using the given index.
- *
- * @param index An identifier associated withe the child.
- */
-- (nullable id<ASLayoutable>)childForIndex:(NSUInteger)index;
-
 @end
 
 /**
@@ -122,4 +94,3 @@ NS_ASSUME_NONNULL_BEGIN
 @end
 
 NS_ASSUME_NONNULL_END
-

AsyncDisplayKit/Layout/ASLayoutSpec.mm

@@ -9,28 +9,8 @@
 //
 
 #import "ASLayoutSpec.h"
-
+#import "ASLayoutSpecPrivate.h"
-#import "ASAssert.h"
+#import "ASLayoutSpec+Subclasses.h"
-#import "ASInternalHelpers.h"
-#import "ASEnvironmentInternal.h"
-
-#import "ASLayout.h"
-#import "ASThread.h"
-#import "ASTraitCollection.h"
-
-#import <objc/runtime.h>
-#import <map>
-#import <vector>
-
-typedef std::map<unsigned long, id<ASLayoutable>, std::less<unsigned long>> ASChildMap;
-
-@interface ASLayoutSpec() {
-  ASDN::RecursiveMutex __instanceLock__;
-  ASChildMap _children;
-  ASEnvironmentState _environmentState;
-  ASLayoutableStyle *_style;
-}
-@end
 
 @implementation ASLayoutSpec
 
@@ -60,6 +40,7 @@ typedef std::map<unsigned long, id<ASLayoutable>, std::less<unsigned long>> ASCh
   _isMutable = YES;
   _environmentState = ASEnvironmentStateMakeDefault();
   _style = [[ASLayoutableStyle alloc] init];
+  _childrenArray = [[NSMutableArray alloc] init];
   
   return self;
 }
@@ -74,6 +55,13 @@ typedef std::map<unsigned long, id<ASLayoutable>, std::less<unsigned long>> ASCh
   return YES;
 }
 
+#pragma mark - Final Layoutable
+
+- (id<ASLayoutable>)finalLayoutable
+{
+  return self;
+}
+
 #pragma mark - Style
 
 - (ASLayoutableStyle *)style
@@ -113,44 +101,8 @@ typedef std::map<unsigned long, id<ASLayoutable>, std::less<unsigned long>> ASCh
   return [ASLayout layoutWithLayoutable:self size:constrainedSize.min];
 }
 
-- (id<ASLayoutable>)finalLayoutable
-{
-  return self;
-}
 
-- (id<ASLayoutable>)layoutableToAddFromLayoutable:(id<ASLayoutable>)child
+#pragma mark - Parent
-{
-  if (self.isFinalLayoutable == NO) {
-    
-    // If you are getting recursion crashes here after implementing finalLayoutable, make sure
-    // that you are setting isFinalLayoutable flag to YES. This must be one BEFORE adding a child
-    // to the new ASLayoutable.
-    //
-    // For example:
-    //- (id<ASLayoutable>)finalLayoutable
-    //{
-    //  ASInsetLayoutSpec *insetSpec = [[ASInsetLayoutSpec alloc] init];
-    //  insetSpec.insets = UIEdgeInsetsMake(10,10,10,10);
-    //  insetSpec.isFinalLayoutable = YES;
-    //  [insetSpec setChild:self];
-    //  return insetSpec;
-    //}
-
-    id<ASLayoutable> finalLayoutable = [child finalLayoutable];
-    if (finalLayoutable != child) {
-      if (ASEnvironmentStatePropagationEnabled()) {
-        ASEnvironmentStatePropagateUp(finalLayoutable, child.environmentState.layoutOptionsState);
-      } else {
-        // If state propagation is not enabled the layout options state needs to be copied manually
-        ASEnvironmentState finalLayoutableEnvironmentState = finalLayoutable.environmentState;
-        finalLayoutableEnvironmentState.layoutOptionsState = child.environmentState.layoutOptionsState;
-        finalLayoutable.environmentState = finalLayoutableEnvironmentState;
-      }
-      return finalLayoutable;
-    }
-  }
-  return child;
-}
 
 - (void)setParent:(id<ASLayoutable>)parent
 {
@@ -162,67 +114,63 @@ typedef std::map<unsigned long, id<ASLayoutable>, std::less<unsigned long>> ASCh
   }
 }
 
+
+#pragma mark - Child
+
 - (void)setChild:(id<ASLayoutable>)child
 {
-  ASDisplayNodeAssert(self.isMutable, @"Cannot set properties when layout spec is not mutable");
+  ASDisplayNodeAssert(self.isMutable, @"Cannot set properties when layout spec is not mutable");;
+  ASDisplayNodeAssert(_childrenArray.count < 2, @"This layout spec does not support more than one child. Use the setChildren: or the setChild:AtIndex: API");
+  
   if (child) {
     id<ASLayoutable> finalLayoutable = [self layoutableToAddFromLayoutable:child];
     if (finalLayoutable) {
-      _children[0] = finalLayoutable;
+      _childrenArray[0] = finalLayoutable;
       [self propagateUpLayoutable:finalLayoutable];
     }
   } else {
-    _children.erase(0);
+    if (_childrenArray.count) {
+      [_childrenArray removeObjectAtIndex:0];
+    }
   }
 }
 
-- (void)setChild:(id<ASLayoutable>)child forIndex:(NSUInteger)index
+- (id<ASLayoutable>)child
 {
-  ASDisplayNodeAssert(self.isMutable, @"Cannot set properties when layout spec is not mutable");
+  ASDisplayNodeAssert(_childrenArray.count < 2, @"This layout spec does not support more than one child. Use the setChildren: or the setChild:AtIndex: API");
-  if (child) {
+  
-    id<ASLayoutable> finalLayoutable = [self layoutableToAddFromLayoutable:child];
+  if (_childrenArray.count) {
-    _children[index] = finalLayoutable;
+    return _childrenArray[0];
-  } else {
-    _children.erase(index);
   }
-  // TODO: Should we propagate up the layoutable at it could happen that multiple children will propagated up their
+  
-  //       layout options and one child will overwrite values from another child
+  return nil;
-  // [self propagateUpLayoutable:finalLayoutable];
 }
 
+#pragma mark - Children
+
 - (void)setChildren:(NSArray<id<ASLayoutable>> *)children
 {
   ASDisplayNodeAssert(self.isMutable, @"Cannot set properties when layout spec is not mutable");
   
-  _children.clear();
+  [_childrenArray removeAllObjects];
+  
   NSUInteger i = 0;
   for (id<ASLayoutable> child in children) {
-    _children[i] = [self layoutableToAddFromLayoutable:child];
+    _childrenArray[i] = [self layoutableToAddFromLayoutable:child];
     i += 1;
   }
 }
 
-- (id<ASLayoutable>)childForIndex:(NSUInteger)index
-{
-  if (index < _children.size()) {
-    return _children[index];
-  }
-  return nil;
-}
-
-- (id<ASLayoutable>)child
-{
-  return _children[0];
-}
-
 - (NSArray *)children
 {
-  std::vector<ASLayout *> children;
+  return [_childrenArray copy];
-  for (ASChildMap::iterator it = _children.begin(); it != _children.end(); ++it ) {
-    children.push_back(it->second);
 }
 
-  return [NSArray arrayWithObjects:&children[0] count:children.size()];
+#pragma mark - NSFastEnumeration
+
+- (NSUInteger)countByEnumeratingWithState:(NSFastEnumerationState *)state objects:(id __unsafe_unretained _Nullable [_Nonnull])buffer count:(NSUInteger)len
+{
+  return [_childrenArray countByEnumeratingWithState:state objects:buffer count:len];
 }
 
 #pragma mark - ASEnvironment
@@ -279,7 +227,6 @@ ASEnvironmentLayoutExtensibilityForwarding
 
 @end
 
-
 #pragma mark - ASWrapperLayoutSpec
 
 @implementation ASWrapperLayoutSpec

AsyncDisplayKit/Layout/ASLayoutable.h

@@ -50,7 +50,7 @@ NS_ASSUME_NONNULL_BEGIN
  * access to the options via convenience properties. If you are creating custom layout spec, then you can
  * extend the backing layout options class to accommodate any new layout options.
  */
-@protocol ASLayoutable <ASEnvironment, ASLayoutablePrivate, ASLayoutableExtensibility>
+@protocol ASLayoutable <ASEnvironment, ASLayoutablePrivate, ASLayoutableExtensibility, NSFastEnumeration>
 
 #pragma mark - Getter
 

AsyncDisplayKit/Layout/ASOverlayLayoutSpec.h

@@ -17,8 +17,17 @@ NS_ASSUME_NONNULL_BEGIN
  */
 @interface ASOverlayLayoutSpec : ASLayoutSpec
 
+/**
+ * Overlay layoutable of this layout spec
+ */
 @property (nullable, nonatomic, strong) id<ASLayoutable> overlay;
 
+/**
+ * Creates and returns an ASOverlayLayoutSpec object with a given child and an layoutable that act as overlay.
+ *
+ * @param child A child that is laid out to determine the size of this spec.
+ * @param overlay A layoutable object that is laid out over the child. If this is nil, the overlay is omitted.
+ */
 + (instancetype)overlayLayoutSpecWithChild:(id<ASLayoutable>)child overlay:(nullable id<ASLayoutable>)overlay;
 
 @end

AsyncDisplayKit/Layout/ASOverlayLayoutSpec.mm

@@ -9,6 +9,7 @@
 //
 
 #import "ASOverlayLayoutSpec.h"
+#import "ASLayoutSpec+Subclasses.h"
 
 #import "ASAssert.h"
 #import "ASLayout.h"
@@ -18,32 +19,40 @@ static NSUInteger const kOverlayChildIndex = 1;
 
 @implementation ASOverlayLayoutSpec
 
-- (instancetype)initWithChild:(id<ASLayoutable>)child overlay:(id<ASLayoutable>)overlay
+#pragma mark - Class
-{
-  if (!(self = [super init])) {
-    return nil;
-  }
-  ASDisplayNodeAssertNotNil(child, @"Child that will be overlayed on shouldn't be nil");
-  self.overlay = overlay;
-  [self setChild:child forIndex:kUnderlayChildIndex];
-  return self;
-}
 
 + (instancetype)overlayLayoutSpecWithChild:(id<ASLayoutable>)child overlay:(id<ASLayoutable>)overlay
 {
   return [[self alloc] initWithChild:child overlay:overlay];
 }
 
+#pragma mark - Lifecycle
+
+- (instancetype)initWithChild:(id<ASLayoutable>)child overlay:(id<ASLayoutable>)overlay
+{
+  if (!(self = [super init])) {
+    return nil;
+  }
+  ASDisplayNodeAssertNotNil(child, @"Child that will be overlayed on shouldn't be nil");
+  [self setChild:child atIndex:kUnderlayChildIndex];
+  self.overlay = overlay;
+  return self;
+}
+
+#pragma mark - Setter / Getter
+
 - (void)setOverlay:(id<ASLayoutable>)overlay
 {
-  [super setChild:overlay forIndex:kOverlayChildIndex];
+  [super setChild:overlay atIndex:kOverlayChildIndex];
 }
 
 - (id<ASLayoutable>)overlay
 {
-  return [super childForIndex:kOverlayChildIndex];
+  return [super childAtIndex:kOverlayChildIndex];
 }
 
+#pragma mark - ASLayoutSpec
+
 /**
  First layout the contents, then fit the overlay on top of it.
  */
@@ -51,7 +60,7 @@ static NSUInteger const kOverlayChildIndex = 1;
                      restrictedToSize:(ASLayoutableSize)size
                  relativeToParentSize:(CGSize)parentSize
 {
-  ASLayout *contentsLayout = [self.child layoutThatFits:constrainedSize parentSize:parentSize];
+  ASLayout *contentsLayout = [[super childAtIndex:kUnderlayChildIndex] layoutThatFits:constrainedSize parentSize:parentSize];
   contentsLayout.position = CGPointZero;
   NSMutableArray *sublayouts = [NSMutableArray arrayWithObject:contentsLayout];
   if (self.overlay) {

AsyncDisplayKit/Private/ASLayoutSpecPrivate.h

@@ -0,0 +1,25 @@
+//
+//  ASLayoutSpecPrivate.h
+//  AsyncDisplayKit
+//
+//  Created by Michael Schneider on 9/15/16.
+//
+//  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 "ASInternalHelpers.h"
+#import "ASEnvironmentInternal.h"
+#import "ASThread.h"
+
+@interface ASLayoutSpec() {
+  ASDN::RecursiveMutex __instanceLock__;
+  ASEnvironmentState _environmentState;
+  ASLayoutableStyle *_style;
+  NSMutableArray *_childrenArray;
+}
+@end
+

AsyncDisplayKitTests/ASDisplayNodeTests.m

@@ -2070,4 +2070,24 @@ static bool stringContainsPointer(NSString *description, id p) {
 #pragma clang diagnostic pop
 }
 
+- (void)testSubnodesFastEnumeration
+{
+  DeclareNodeNamed(parentNode);
+  DeclareNodeNamed(a);
+  DeclareNodeNamed(b);
+  DeclareNodeNamed(c);
+  DeclareViewNamed(d);
+
+  NSArray *subnodes = @[a, b, c, d];  
+  for (ASDisplayNode *node in subnodes) {
+    [parentNode addSubnode:node];
+  }
+  
+  NSInteger i = 0;
+  for (ASDisplayNode *subnode in parentNode.subnodes) {
+    XCTAssertEqualObjects(subnode, subnodes[i]);
+    i++;
+  }
+}
+
 @end