[ASLayoutSpec] Remove use of dictionary to hold children

AsyncDisplayKit/Layout/ASBackgroundLayoutSpec.mm

@@ -14,7 +14,8 @@
 #import "ASBaseDefines.h"
 #import "ASLayout.h"
 
-static NSString * const kBackgroundChildKey = @"kBackgroundChildKey";
+static NSUInteger const kForegroundChildIndex = 0;
+static NSUInteger const kBackgroundChildIndex = 1;
 
 @interface ASBackgroundLayoutSpec ()
 @end
@@ -28,7 +29,7 @@ - (instancetype)initWithChild:(id<ASLayoutable>)child background:(id<ASLayoutabl
   }
 
   ASDisplayNodeAssertNotNil(child, @"Child cannot be nil");
-  [self setChild:child];
+  [self setChild:child forIndex:kForegroundChildIndex];
   self.background = background;
   return self;
 }
@@ -63,12 +64,12 @@ - (ASLayout *)measureWithSizeRange:(ASSizeRange)constrainedSize
 
 - (void)setBackground:(id<ASLayoutable>)background
 {
-  [super setChild:background forIdentifier:kBackgroundChildKey];
+  [super setChild:background forIndex:kBackgroundChildIndex];
 }
 
 - (id<ASLayoutable>)background
 {
-  return [super childForIdentifier:kBackgroundChildKey];
+  return [super childForIndex:kBackgroundChildIndex];
 }
 
 @end

AsyncDisplayKit/Layout/ASLayoutSpec.h

@@ -40,10 +40,9 @@ NS_ASSUME_NONNULL_BEGIN
  * only require a single child.
  *
  * For layout specs that require a known number of children (ASBackgroundLayoutSpec, for example)
- * a subclass should use this method to set the "primary" child. It can then use setChild:forIdentifier:
- * to set any other required children. Ideally a subclass would hide this from the user, and use the
- * setChild:forIdentifier: internally. For example, ASBackgroundLayoutSpec exposes a backgroundChild
- * property that behind the scenes is calling setChild:forIdentifier:.
+ * a subclass should use this method to set the "primary" child. This is actually the same as calling 
+ * setChild:forIdentifier:0. All other children should be set by defining convenience methods
+ * that call setChild:forIdentifier behind the scenes.
  */
 - (void)setChild:(id<ASLayoutable>)child;
 
@@ -52,19 +51,19 @@ NS_ASSUME_NONNULL_BEGIN
  *
  * @param child A child to be added.
  *
- * @param identifier An identifier associated with the child.
+ * @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 should use the setChild method to set the "primary" child. It can then use this method
+ * 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:forIdentifier: internally. For example, ASBackgroundLayoutSpec exposes a backgroundChild
- * property that behind the scenes is calling setChild:forIdentifier:.
+ * setChild:forIndex: internally. For example, ASBackgroundLayoutSpec exposes a backgroundChild
+ * property that behind the scenes is calling setChild:forIndex:.
  */
-- (void)setChild:(id<ASLayoutable>)child forIdentifier:(NSString *)identifier;
+- (void)setChild:(id<ASLayoutable>)child forIndex:(NSUInteger)index;
 
 /**
  * Adds childen to this layout spec.
@@ -94,11 +93,11 @@ NS_ASSUME_NONNULL_BEGIN
 - (nullable id<ASLayoutable>)child;
 
 /**
- * Returns the child added to this layout spec using the given identifier.
+ * Returns the child added to this layout spec using the given index.
  *
- * @param identifier An identifier associated withe the child.
+ * @param index An identifier associated withe the child.
  */
-- (nullable id<ASLayoutable>)childForIdentifier:(NSString *)identifier;
+- (nullable id<ASLayoutable>)childForIndex:(NSUInteger)index;
 
 /**
  * Returns all children added to this layout spec.

AsyncDisplayKit/Layout/ASLayoutSpec.mm

@@ -20,14 +20,15 @@
 #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() {
   ASEnvironmentState _environmentState;
   ASDN::RecursiveMutex _propertyLock;
-
-  NSArray *_children;
-  NSMutableDictionary *_childrenWithIdentifier;
+  ASChildMap _children;
 }
 @end
 
@@ -45,7 +46,6 @@ - (instancetype)init
   }
   _isMutable = YES;
   _environmentState = ASEnvironmentStateMakeDefault();
-  _children = [NSArray array];
   return self;
 }
 
@@ -102,14 +102,6 @@ - (ASLayout *)measureWithSizeRange:(ASSizeRange)constrainedSize
   return child;
 }
 
-- (NSMutableDictionary *)childrenWithIdentifier
-{
-  if (!_childrenWithIdentifier) {
-    _childrenWithIdentifier = [NSMutableDictionary dictionary];
-  }
-  return _childrenWithIdentifier;
-}
-
 - (void)setParent:(id<ASLayoutable>)parent
 {
   // FIXME: Locking should be evaluated here.  _parent is not widely used yet, though.
@@ -126,34 +118,23 @@ - (void)setChild:(id<ASLayoutable>)child
   if (child) {
     id<ASLayoutable> finalLayoutable = [self layoutableToAddFromLayoutable:child];
     if (finalLayoutable) {
-      _children = @[finalLayoutable];
+      _children[0] = finalLayoutable;
       [self propagateUpLayoutable:finalLayoutable];
     }
   } else {
-    // remove the only child
-    _children = [NSArray array];
+    _children.erase(0);
   }
 }
 
-- (void)setChild:(id<ASLayoutable>)child forIdentifier:(NSString *)identifier
+- (void)setChild:(id<ASLayoutable>)child forIndex:(NSUInteger)index
 {
   ASDisplayNodeAssert(self.isMutable, @"Cannot set properties when layout spec is not mutable");
   if (child) {
     id<ASLayoutable> finalLayoutable = [self layoutableToAddFromLayoutable:child];
-    self.childrenWithIdentifier[identifier] = finalLayoutable;
-    if (finalLayoutable) {
-      _children = [_children arrayByAddingObject:finalLayoutable];
-    }
+    _children[index] = finalLayoutable;
   } else {
-    id<ASLayoutable> oldChild = self.childrenWithIdentifier[identifier];
-    if (oldChild) {
-      self.childrenWithIdentifier[identifier] = nil;
-      NSMutableArray *mutableChildren = [_children mutableCopy];
-      [mutableChildren removeObject:oldChild];
-      _children = [mutableChildren copy];
-    }
+    _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
   // [self propagateUpLayoutable:finalLayoutable];
@@ -163,32 +144,33 @@ - (void)setChildren:(NSArray<id<ASLayoutable>> *)children
 {
   ASDisplayNodeAssert(self.isMutable, @"Cannot set properties when layout spec is not mutable");
 
-  std::vector<id<ASLayoutable>> finalChildren;
-  for (id<ASLayoutable> child in children) {
-    finalChildren.push_back([self layoutableToAddFromLayoutable:child]);
-  }
-
-  _children = nil;
-  if (finalChildren.size() > 0) {
-    _children = [NSArray arrayWithObjects:&finalChildren[0] count:finalChildren.size()];
-  } else {
-    _children = [NSArray array];
-  }
+  _children.clear();
+  [children enumerateObjectsUsingBlock:^(id<ASLayoutable>  _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) {
+    _children[idx] = obj;
+  }];
 }
 
-- (id<ASLayoutable>)childForIdentifier:(NSString *)identifier
+- (id<ASLayoutable>)childForIndex:(NSUInteger)index
 {
-  return self.childrenWithIdentifier[identifier];
+  if (index < _children.size()) {
+    return _children[index];
+  }
+  return nil;
 }
 
 - (id<ASLayoutable>)child
 {
-  return [_children firstObject];
+  return _children[0];
 }
 
 - (NSArray *)children
 {
-  return _children;
+  std::vector<ASLayout *> children;
+  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 - ASEnvironment

AsyncDisplayKit/Layout/ASOverlayLayoutSpec.mm

@@ -14,7 +14,8 @@
 #import "ASBaseDefines.h"
 #import "ASLayout.h"
 
-static NSString * const kOverlayChildKey = @"kOverlayChildKey";
+static NSUInteger const kUnderlayChildIndex = 0;
+static NSUInteger const kOverlayChildIndex = 1;
 
 @implementation ASOverlayLayoutSpec
 
@@ -25,7 +26,7 @@ - (instancetype)initWithChild:(id<ASLayoutable>)child overlay:(id<ASLayoutable>)
   }
   ASDisplayNodeAssertNotNil(child, @"Child that will be overlayed on shouldn't be nil");
   self.overlay = overlay;
-  [self setChild:child];
+  [self setChild:child forIndex:kUnderlayChildIndex];
   return self;
 }
 
@@ -36,12 +37,12 @@ + (instancetype)overlayLayoutSpecWithChild:(id<ASLayoutable>)child overlay:(id<A
 
 - (void)setOverlay:(id<ASLayoutable>)overlay
 {
-  [super setChild:overlay forIdentifier:kOverlayChildKey];
+  [super setChild:overlay forIndex:kOverlayChildIndex];
 }
 
 - (id<ASLayoutable>)overlay
 {
-  return [super childForIdentifier:kOverlayChildKey];
+  return [super childForIndex:kOverlayChildIndex];
 }
 
 /**