Improvements in ASCollectionGalleryLayoutDelegate (#496)

* Improvements in ASCollectionGalleryLayoutDelegate
- It now can handle section inset, as well as interitem and line spacings
- Other small changes

* Fix build failure and update file licenses

* Update CHANGELOG

* Minor change

* Another assertion on scrollable directions of gallery layout delegate

AsyncDisplayKit.xcodeproj/project.pbxproj

@@ -429,9 +429,11 @@
 		E54E81FC1EB357BD00FFE8E1 /* ASPageTable.h in Headers */ = {isa = PBXBuildFile; fileRef = E54E81FA1EB357BD00FFE8E1 /* ASPageTable.h */; };
 		E54E81FD1EB357BD00FFE8E1 /* ASPageTable.m in Sources */ = {isa = PBXBuildFile; fileRef = E54E81FB1EB357BD00FFE8E1 /* ASPageTable.m */; };
 		E55D86331CA8A14000A0C26F /* ASLayoutElement.mm in Sources */ = {isa = PBXBuildFile; fileRef = E55D86311CA8A14000A0C26F /* ASLayoutElement.mm */; };
+		E5667E8C1F33871300FA6FC0 /* _ASCollectionGalleryLayoutInfo.h in Headers */ = {isa = PBXBuildFile; fileRef = E5667E8B1F33871300FA6FC0 /* _ASCollectionGalleryLayoutInfo.h */; settings = {ATTRIBUTES = (Private, ); }; };
+		E5667E8E1F33872700FA6FC0 /* _ASCollectionGalleryLayoutInfo.m in Sources */ = {isa = PBXBuildFile; fileRef = E5667E8D1F33872700FA6FC0 /* _ASCollectionGalleryLayoutInfo.m */; };
 		E5711A2C1C840C81009619D4 /* ASCollectionElement.h in Headers */ = {isa = PBXBuildFile; fileRef = E5711A2A1C840C81009619D4 /* ASCollectionElement.h */; settings = {ATTRIBUTES = (Private, ); }; };
 		E5711A301C840C96009619D4 /* ASCollectionElement.mm in Sources */ = {isa = PBXBuildFile; fileRef = E5711A2D1C840C96009619D4 /* ASCollectionElement.mm */; };
-		E5775AFC1F13CE9F00CAC9BC /* _ASCollectionGalleryLayoutItem.h in Headers */ = {isa = PBXBuildFile; fileRef = E5775AFB1F13CE9F00CAC9BC /* _ASCollectionGalleryLayoutItem.h */; };
+		E5775AFC1F13CE9F00CAC9BC /* _ASCollectionGalleryLayoutItem.h in Headers */ = {isa = PBXBuildFile; fileRef = E5775AFB1F13CE9F00CAC9BC /* _ASCollectionGalleryLayoutItem.h */; settings = {ATTRIBUTES = (Private, ); }; };
 		E5775AFE1F13CF7400CAC9BC /* _ASCollectionGalleryLayoutItem.mm in Sources */ = {isa = PBXBuildFile; fileRef = E5775AFD1F13CF7400CAC9BC /* _ASCollectionGalleryLayoutItem.mm */; };
 		E5775B001F13D25400CAC9BC /* ASCollectionLayoutState+Private.h in Headers */ = {isa = PBXBuildFile; fileRef = E5775AFF1F13D25400CAC9BC /* ASCollectionLayoutState+Private.h */; settings = {ATTRIBUTES = (Private, ); }; };
 		E5775B021F16759300CAC9BC /* ASCollectionLayoutCache.h in Headers */ = {isa = PBXBuildFile; fileRef = E5775B011F16759300CAC9BC /* ASCollectionLayoutCache.h */; settings = {ATTRIBUTES = (Private, ); }; };
@@ -458,7 +460,7 @@
 		E5E281741E71C833006B67C2 /* ASCollectionLayoutState.h in Headers */ = {isa = PBXBuildFile; fileRef = E5E281731E71C833006B67C2 /* ASCollectionLayoutState.h */; settings = {ATTRIBUTES = (Public, ); }; };
 		E5E281761E71C845006B67C2 /* ASCollectionLayoutState.mm in Sources */ = {isa = PBXBuildFile; fileRef = E5E281751E71C845006B67C2 /* ASCollectionLayoutState.mm */; };
 		E5E2D72E1EA780C4005C24C6 /* ASCollectionGalleryLayoutDelegate.h in Headers */ = {isa = PBXBuildFile; fileRef = E5E2D72D1EA780C4005C24C6 /* ASCollectionGalleryLayoutDelegate.h */; settings = {ATTRIBUTES = (Public, ); }; };
-		E5E2D7301EA780DF005C24C6 /* ASCollectionGalleryLayoutDelegate.m in Sources */ = {isa = PBXBuildFile; fileRef = E5E2D72F1EA780DF005C24C6 /* ASCollectionGalleryLayoutDelegate.m */; };
+		E5E2D7301EA780DF005C24C6 /* ASCollectionGalleryLayoutDelegate.mm in Sources */ = {isa = PBXBuildFile; fileRef = E5E2D72F1EA780DF005C24C6 /* ASCollectionGalleryLayoutDelegate.mm */; };
 		F711994E1D20C21100568860 /* ASDisplayNodeExtrasTests.m in Sources */ = {isa = PBXBuildFile; fileRef = F711994D1D20C21100568860 /* ASDisplayNodeExtrasTests.m */; };
 /* End PBXBuildFile section */
 
@@ -917,6 +919,8 @@
 		E54E81FA1EB357BD00FFE8E1 /* ASPageTable.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = ASPageTable.h; sourceTree = "<group>"; };
 		E54E81FB1EB357BD00FFE8E1 /* ASPageTable.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = ASPageTable.m; sourceTree = "<group>"; };
 		E55D86311CA8A14000A0C26F /* ASLayoutElement.mm */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.objcpp; path = ASLayoutElement.mm; sourceTree = "<group>"; };
+		E5667E8B1F33871300FA6FC0 /* _ASCollectionGalleryLayoutInfo.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = _ASCollectionGalleryLayoutInfo.h; sourceTree = "<group>"; };
+		E5667E8D1F33872700FA6FC0 /* _ASCollectionGalleryLayoutInfo.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = _ASCollectionGalleryLayoutInfo.m; sourceTree = "<group>"; };
 		E5711A2A1C840C81009619D4 /* ASCollectionElement.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = ASCollectionElement.h; sourceTree = "<group>"; };
 		E5711A2D1C840C96009619D4 /* ASCollectionElement.mm */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.objcpp; path = ASCollectionElement.mm; sourceTree = "<group>"; };
 		E5775AFB1F13CE9F00CAC9BC /* _ASCollectionGalleryLayoutItem.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = _ASCollectionGalleryLayoutItem.h; sourceTree = "<group>"; };
@@ -946,7 +950,7 @@
 		E5E281731E71C833006B67C2 /* ASCollectionLayoutState.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = ASCollectionLayoutState.h; sourceTree = "<group>"; };
 		E5E281751E71C845006B67C2 /* ASCollectionLayoutState.mm */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.objcpp; path = ASCollectionLayoutState.mm; sourceTree = "<group>"; };
 		E5E2D72D1EA780C4005C24C6 /* ASCollectionGalleryLayoutDelegate.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = ASCollectionGalleryLayoutDelegate.h; sourceTree = "<group>"; };
-		E5E2D72F1EA780DF005C24C6 /* ASCollectionGalleryLayoutDelegate.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; lineEnding = 0; path = ASCollectionGalleryLayoutDelegate.m; sourceTree = "<group>"; xcLanguageSpecificationIdentifier = xcode.lang.objcpp; };
+		E5E2D72F1EA780DF005C24C6 /* ASCollectionGalleryLayoutDelegate.mm */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.objcpp; lineEnding = 0; path = ASCollectionGalleryLayoutDelegate.mm; sourceTree = "<group>"; xcLanguageSpecificationIdentifier = xcode.lang.objcpp; };
 		EFA731F0396842FF8AB635EE /* libPods-AsyncDisplayKitTests.a */ = {isa = PBXFileReference; explicitFileType = archive.ar; includeInIndex = 0; path = "libPods-AsyncDisplayKitTests.a"; sourceTree = BUILT_PRODUCTS_DIR; };
 		F711994D1D20C21100568860 /* ASDisplayNodeExtrasTests.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = ASDisplayNodeExtrasTests.m; sourceTree = "<group>"; };
 		FB07EABBCF28656C6297BC2D /* Pods-AsyncDisplayKitTests.debug.xcconfig */ = {isa = PBXFileReference; includeInIndex = 1; lastKnownFileType = text.xcconfig; name = "Pods-AsyncDisplayKitTests.debug.xcconfig"; path = "Pods/Target Support Files/Pods-AsyncDisplayKitTests/Pods-AsyncDisplayKitTests.debug.xcconfig"; sourceTree = "<group>"; };
@@ -1670,6 +1674,8 @@
 				E5855DEE1EBB4D83003639AE /* ASCollectionLayoutDefines.h */,
 				E5855DED1EBB4D83003639AE /* ASCollectionLayoutDefines.m */,
 				E5775AFF1F13D25400CAC9BC /* ASCollectionLayoutState+Private.h */,
+				E5667E8B1F33871300FA6FC0 /* _ASCollectionGalleryLayoutInfo.h */,
+				E5667E8D1F33872700FA6FC0 /* _ASCollectionGalleryLayoutInfo.m */,
 				E5775AFB1F13CE9F00CAC9BC /* _ASCollectionGalleryLayoutItem.h */,
 				E5775AFD1F13CF7400CAC9BC /* _ASCollectionGalleryLayoutItem.mm */,
 			);
@@ -1687,7 +1693,7 @@
 				E58E9E3D1E941D74004CFC59 /* ASCollectionFlowLayoutDelegate.h */,
 				E58E9E3E1E941D74004CFC59 /* ASCollectionFlowLayoutDelegate.m */,
 				E5E2D72D1EA780C4005C24C6 /* ASCollectionGalleryLayoutDelegate.h */,
-				E5E2D72F1EA780DF005C24C6 /* ASCollectionGalleryLayoutDelegate.m */,
+				E5E2D72F1EA780DF005C24C6 /* ASCollectionGalleryLayoutDelegate.mm */,
 				E54E81FA1EB357BD00FFE8E1 /* ASPageTable.h */,
 				E54E81FB1EB357BD00FFE8E1 /* ASPageTable.m */,
 			);
@@ -1824,6 +1830,8 @@
 				CC87BB951DA8193C0090E380 /* ASCellNode+Internal.h in Headers */,
 				E5775B021F16759300CAC9BC /* ASCollectionLayoutCache.h in Headers */,
 				E5775B001F13D25400CAC9BC /* ASCollectionLayoutState+Private.h in Headers */,
+				E5667E8C1F33871300FA6FC0 /* _ASCollectionGalleryLayoutInfo.h in Headers */,
+				E5775AFC1F13CE9F00CAC9BC /* _ASCollectionGalleryLayoutItem.h in Headers */,
 				E5855DF01EBB4D83003639AE /* ASCollectionLayoutDefines.h in Headers */,
 				E5B5B9D11E9BAD9800A6B726 /* ASCollectionLayoutContext+Private.h in Headers */,
 				9C8898BD1C738BB800D6B02E /* ASTextKitFontSizeAdjuster.h in Headers */,
@@ -1896,7 +1904,6 @@
 				254C6B751BF94DF4003EC431 /* ASTextKitComponents.h in Headers */,
 				B35062081B010EFD0018CF92 /* ASScrollNode.h in Headers */,
 				CCA282CC1E9EB73E0037E8B7 /* ASTipNode.h in Headers */,
-				E5775AFC1F13CE9F00CAC9BC /* _ASCollectionGalleryLayoutItem.h in Headers */,
 				25E327571C16819500A2170C /* ASPagerNode.h in Headers */,
 				CCCCCCDB1EC3EF060087FE10 /* ASTextLine.h in Headers */,
 				9C70F20E1CDBE9E5007D6C76 /* NSArray+Diffing.h in Headers */,
@@ -2251,6 +2258,7 @@
 				CCCCCCD61EC3EF060087FE10 /* ASTextDebugOption.m in Sources */,
 				34EFC75C1B701BD200AD841F /* ASDimension.mm in Sources */,
 				B350624E1B010EFD0018CF92 /* ASDisplayNode+AsyncDisplay.mm in Sources */,
+				E5667E8E1F33872700FA6FC0 /* _ASCollectionGalleryLayoutInfo.m in Sources */,
 				25E327591C16819500A2170C /* ASPagerNode.m in Sources */,
 				636EA1A41C7FF4EC00EE152F /* NSArray+Diffing.m in Sources */,
 				B35062501B010EFD0018CF92 /* ASDisplayNode+DebugTiming.mm in Sources */,
@@ -2286,7 +2294,7 @@
 				CCCCCCE01EC3EF060087FE10 /* ASTextRunDelegate.m in Sources */,
 				CCCCCCDA1EC3EF060087FE10 /* ASTextLayout.m in Sources */,
 				254C6B841BF94F8A003EC431 /* ASTextNodeWordKerner.m in Sources */,
-				E5E2D7301EA780DF005C24C6 /* ASCollectionGalleryLayoutDelegate.m in Sources */,
+				E5E2D7301EA780DF005C24C6 /* ASCollectionGalleryLayoutDelegate.mm in Sources */,
 				34EFC76B1B701CEB00AD841F /* ASLayoutSpec.mm in Sources */,
 				CC3B20861C3F76D600798563 /* ASPendingStateController.mm in Sources */,
 				254C6B8C1BF94F8A003EC431 /* ASTextKitTailTruncater.mm in Sources */,

CHANGELOG.md

@@ -7,7 +7,7 @@
 - [ASCollectionView] Add delegate bridging and index space translation for missing UICollectionViewLayout properties. [Scott Goodson](https://github.com/appleguy)
 - [ASTextNode2] Add initial implementation for link handling. [Scott Goodson](https://github.com/appleguy) [#396](https://github.com/TextureGroup/Texture/pull/396)
 - [ASTextNode2] Provide compile flag to globally enable new implementation of ASTextNode: ASTEXTNODE_EXPERIMENT_GLOBAL_ENABLE. [Scott Goodson](https://github.com/appleguy) [#396](https://github.com/TextureGroup/Texture/pull/410)
-- Add ASCollectionGalleryLayoutDelegate - an async collection layout that makes same-size collections (e.g photo galleries, pagers, etc) fast and lightweight! [Huy Nguyen](https://github.com/nguyenhuy/) [#76](https://github.com/TextureGroup/Texture/pull/76) [#451](https://github.com/TextureGroup/Texture/pull/451)
+- Add ASCollectionGalleryLayoutDelegate - an async collection layout that makes same-size collections (e.g photo galleries, pagers, etc) fast and lightweight! [Huy Nguyen](https://github.com/nguyenhuy/) [#76](https://github.com/TextureGroup/Texture/pull/76) [#451](https://github.com/TextureGroup/Texture/pull/451) [#496](https://github.com/TextureGroup/Texture/pull/496)
 - Fix an issue that causes infinite layout loop in ASDisplayNode after [#428](https://github.com/TextureGroup/Texture/pull/428) [Huy Nguyen](https://github.com/nguyenhuy) [#455](https://github.com/TextureGroup/Texture/pull/455) 
 - Fix an issue in layout transition that causes it to unexpectedly use the old layout [Huy Nguyen](https://github.com/nguyenhuy) [#464](https://github.com/TextureGroup/Texture/pull/464)
 - Add -[ASDisplayNode detailedLayoutDescription] property to aid debugging. [Adlai Holler](https://github.com/Adlai-Holler) [#476](https://github.com/TextureGroup/Texture/pull/476)

Source/ASPagerNode.m

@@ -29,7 +29,7 @@
 #import <AsyncDisplayKit/ASCollectionView+Undeprecated.h>
 #import <AsyncDisplayKit/UIResponder+AsyncDisplayKit.h>
 
-@interface ASPagerNode () <ASCollectionDataSource, ASCollectionDelegate, ASCollectionDelegateFlowLayout, ASDelegateProxyInterceptor, ASCollectionGalleryLayoutSizeProviding>
+@interface ASPagerNode () <ASCollectionDataSource, ASCollectionDelegate, ASCollectionDelegateFlowLayout, ASDelegateProxyInterceptor, ASCollectionGalleryLayoutPropertiesProviding>
 {
   __weak id <ASPagerDataSource> _pagerDataSource;
   ASPagerNodeProxy *_proxyDataSource;
@@ -75,7 +75,7 @@
   ASCollectionGalleryLayoutDelegate *layoutDelegate = [[ASCollectionGalleryLayoutDelegate alloc] initWithScrollableDirections:ASScrollDirectionHorizontalDirections];
   self = [super initWithLayoutDelegate:layoutDelegate layoutFacilitator:nil];
   if (self) {
-    layoutDelegate.sizeProvider = self;
+    layoutDelegate.propertiesProvider = self;
   }
   return self;
 }
@@ -137,7 +137,7 @@
   return indexPath.row;
 }
 
-#pragma mark - ASCollectionGalleryLayoutSizeProviding
+#pragma mark - ASCollectionGalleryLayoutPropertiesProviding
 
 - (CGSize)sizeForElements:(ASElementMap *)elements
 {

Source/Details/ASCollectionFlowLayoutDelegate.m

@@ -76,8 +76,9 @@
   ASSizeRange sizeRange = ASSizeRangeForCollectionLayoutThatFitsViewportSize(context.viewportSize, context.scrollableDirections);
   ASLayout *layout = [stackSpec layoutThatFits:sizeRange];
 
-  return [[ASCollectionLayoutState alloc] initWithContext:context layout:layout getElementBlock:^ASCollectionElement * _Nonnull(ASLayout * _Nonnull sublayout) {
+  return [[ASCollectionLayoutState alloc] initWithContext:context layout:layout getElementBlock:^ASCollectionElement * _Nullable(ASLayout * _Nonnull sublayout) {
-    return ((ASCellNode *)sublayout.layoutElement).collectionElement;
+    ASCellNode *node = ASDynamicCast(sublayout.layoutElement, ASCellNode);
+    return node ? node.collectionElement : nil;
   }];
 }
 

Source/Details/ASCollectionGalleryLayoutDelegate.h

@@ -17,7 +17,7 @@
 
 NS_ASSUME_NONNULL_BEGIN
 
-@protocol ASCollectionGalleryLayoutSizeProviding <NSObject>
+@protocol ASCollectionGalleryLayoutPropertiesProviding <NSObject>
 
 /**
  * Returns the fixed size of each and every element.
@@ -30,6 +30,51 @@ NS_ASSUME_NONNULL_BEGIN
  */
 - (CGSize)sizeForElements:(ASElementMap *)elements;
 
+@optional
+
+/**
+ * Returns the minumum spacing to use between lines of items.
+ *
+ * @discussion This method will only be called on main thread.
+ *
+ * @discussion For a vertically scrolling layout, this value represents the minimum spacing between rows.
+ * For a horizontally scrolling one, it represents the minimum spacing between columns.
+ * It is not applied between the first line and the header, or between the last line and the footer.
+ * This is the same behavior as UICollectionViewFlowLayout's minimumLineSpacing.
+ *
+ * @param elements All elements in the layout.
+ *
+ * @return The interitem spacing
+ */
+- (CGFloat)minimumLineSpacingForElements:(ASElementMap *)elements;
+
+/**
+ * Returns the minumum spacing to use between items in the same row or column, depending on the scroll directions.
+ *
+ * @discussion This method will only be called on main thread.
+ *
+ * @discussion For a vertically scrolling layout, this value represents the minimum spacing between items in the same row. 
+ * For a horizontally scrolling one, it represents the minimum spacing between items in the same column.
+ * It is considered while fitting items into lines, but the actual final spacing between some items might be larger.
+ * This is the same behavior as UICollectionViewFlowLayout's minimumInteritemSpacing.
+ *
+ * @param elements All elements in the layout.
+ *
+ * @return The interitem spacing
+ */
+- (CGFloat)minimumInteritemSpacingForElements:(ASElementMap *)elements;
+
+/**
+ * Returns the margins of each section.
+ *
+ * @discussion This method will only be called on main thread.
+ *
+ * @param elements All elements in the layout.
+ *
+ * @return The margins used to layout content in a section
+ */
+- (UIEdgeInsets)sectionInsetForElements:(ASElementMap *)elements;
+
 @end
 
 /**
@@ -40,8 +85,13 @@ NS_ASSUME_NONNULL_BEGIN
 AS_SUBCLASSING_RESTRICTED
 @interface ASCollectionGalleryLayoutDelegate : NSObject <ASCollectionLayoutDelegate>
 
-@property (nonatomic, weak) id<ASCollectionGalleryLayoutSizeProviding> sizeProvider;
+@property (nonatomic, weak) id<ASCollectionGalleryLayoutPropertiesProviding> propertiesProvider;
 
+/**
+ * Designated initializer.
+ *
+ * @param scrollableDirections The scrollable directions of this layout. Must be either vertical or horizontal directions.
+ */
 - (instancetype)initWithScrollableDirections:(ASScrollDirection)scrollableDirections NS_DESIGNATED_INITIALIZER;
 
 - (instancetype)init __unavailable;

Source/Details/ASCollectionGalleryLayoutDelegate.m

@@ -1,93 +0,0 @@
-//
-//  ASCollectionGalleryLayoutDelegate.m
-//  Texture
-//
-//  Copyright (c) 2017-present, Pinterest, Inc.  All rights reserved.
-//  Licensed under the Apache License, Version 2.0 (the "License");
-//  you may not use this file except in compliance with the License.
-//  You may obtain a copy of the License at
-//
-//      http://www.apache.org/licenses/LICENSE-2.0
-//
-
-#import <AsyncDisplayKit/ASCollectionGalleryLayoutDelegate.h>
-
-#import <AsyncDisplayKit/_ASCollectionGalleryLayoutItem.h>
-#import <AsyncDisplayKit/ASAssert.h>
-#import <AsyncDisplayKit/ASCellNode.h>
-#import <AsyncDisplayKit/ASCollectionElement.h>
-#import <AsyncDisplayKit/ASCollectionLayoutContext.h>
-#import <AsyncDisplayKit/ASCollectionLayoutDefines.h>
-#import <AsyncDisplayKit/ASCollectionLayoutState.h>
-#import <AsyncDisplayKit/ASElementMap.h>
-#import <AsyncDisplayKit/ASLayout.h>
-#import <AsyncDisplayKit/ASLayoutRangeType.h>
-#import <AsyncDisplayKit/ASStackLayoutSpec.h>
-
-#pragma mark - ASCollectionGalleryLayoutDelegate
-
-@implementation ASCollectionGalleryLayoutDelegate {
-  ASScrollDirection _scrollableDirections;
-  CGSize _itemSize;
-}
-
-- (instancetype)initWithScrollableDirections:(ASScrollDirection)scrollableDirections
-{
-  self = [super init];
-  if (self) {
-    _scrollableDirections = scrollableDirections;
-  }
-  return self;
-}
-
-- (ASScrollDirection)scrollableDirections
-{
-  ASDisplayNodeAssertMainThread();
-  return _scrollableDirections;
-}
-
-- (id)additionalInfoForLayoutWithElements:(ASElementMap *)elements
-{
-  ASDisplayNodeAssertMainThread();
-  if (_sizeProvider == nil) {
-    return nil;
-  }
-
-  return [NSValue valueWithCGSize:[_sizeProvider sizeForElements:elements]];
-}
-
-+ (ASCollectionLayoutState *)calculateLayoutWithContext:(ASCollectionLayoutContext *)context
-{
-  ASElementMap *elements = context.elements;
-  CGSize pageSize = context.viewportSize;
-  ASScrollDirection scrollableDirections = context.scrollableDirections;
-
-  CGSize itemSize = context.additionalInfo ? ((NSValue *)context.additionalInfo).CGSizeValue : CGSizeZero;
-  if (CGSizeEqualToSize(CGSizeZero, itemSize)) {
-    return [[ASCollectionLayoutState alloc] initWithContext:context];
-  }
-
-  NSMutableArray<_ASGalleryLayoutItem *> *children = ASArrayByFlatMapping(elements.itemElements,
-                                                                          ASCollectionElement *element,
-                                                                          [[_ASGalleryLayoutItem alloc] initWithItemSize:itemSize collectionElement:element]);
-  if (children.count == 0) {
-    return [[ASCollectionLayoutState alloc] initWithContext:context];
-  }
-  
-  // Use a stack spec to calculate layout content size and frames of all elements without actually measuring each element
-  ASStackLayoutSpec *stackSpec = [ASStackLayoutSpec stackLayoutSpecWithDirection:ASStackLayoutDirectionHorizontal
-                                                                         spacing:0
-                                                                  justifyContent:ASStackLayoutJustifyContentStart
-                                                                      alignItems:ASStackLayoutAlignItemsStart
-                                                                        flexWrap:ASStackLayoutFlexWrapWrap
-                                                                    alignContent:ASStackLayoutAlignContentStart
-                                                                        children:children];
-  stackSpec.concurrent = YES;
-  ASLayout *layout = [stackSpec layoutThatFits:ASSizeRangeForCollectionLayoutThatFitsViewportSize(pageSize, scrollableDirections)];
-  
-  return [[ASCollectionLayoutState alloc] initWithContext:context layout:layout getElementBlock:^ASCollectionElement *(ASLayout *sublayout) {
-    return ((_ASGalleryLayoutItem *)sublayout.layoutElement).collectionElement;
-  }];
-}
-
-@end

Source/Details/ASCollectionGalleryLayoutDelegate.mm

@@ -0,0 +1,140 @@
+//
+//  ASCollectionGalleryLayoutDelegate.mm
+//  Texture
+//
+//  Copyright (c) 2017-present, Pinterest, Inc.  All rights reserved.
+//  Licensed under the Apache License, Version 2.0 (the "License");
+//  you may not use this file except in compliance with the License.
+//  You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+
+#import <AsyncDisplayKit/ASCollectionGalleryLayoutDelegate.h>
+
+#import <AsyncDisplayKit/_ASCollectionGalleryLayoutInfo.h>
+#import <AsyncDisplayKit/_ASCollectionGalleryLayoutItem.h>
+#import <AsyncDisplayKit/ASAssert.h>
+#import <AsyncDisplayKit/ASCellNode.h>
+#import <AsyncDisplayKit/ASCollectionElement.h>
+#import <AsyncDisplayKit/ASCollectionLayoutContext.h>
+#import <AsyncDisplayKit/ASCollectionLayoutDefines.h>
+#import <AsyncDisplayKit/ASCollectionLayoutState.h>
+#import <AsyncDisplayKit/ASElementMap.h>
+#import <AsyncDisplayKit/ASLayout.h>
+#import <AsyncDisplayKit/ASLayoutRangeType.h>
+#import <AsyncDisplayKit/ASInsetLayoutSpec.h>
+#import <AsyncDisplayKit/ASStackLayoutSpec.h>
+
+#pragma mark - ASCollectionGalleryLayoutDelegate
+
+@implementation ASCollectionGalleryLayoutDelegate {
+  ASScrollDirection _scrollableDirections;
+
+  struct {
+    unsigned int minimumLineSpacingForElements:1;
+    unsigned int minimumInteritemSpacingForElements:1;
+    unsigned int sectionInsetForElements:1;
+  } _propertiesProviderFlags;
+}
+
+- (instancetype)initWithScrollableDirections:(ASScrollDirection)scrollableDirections
+{
+  self = [super init];
+  if (self) {
+    // Scrollable directions must be either vertical or horizontal, but not both
+    ASDisplayNodeAssertTrue(ASScrollDirectionContainsVerticalDirection(scrollableDirections)
+                            || ASScrollDirectionContainsHorizontalDirection(scrollableDirections));
+    ASDisplayNodeAssertFalse(ASScrollDirectionContainsVerticalDirection(scrollableDirections)
+                             && ASScrollDirectionContainsHorizontalDirection(scrollableDirections));
+    _scrollableDirections = scrollableDirections;
+  }
+  return self;
+}
+
+- (ASScrollDirection)scrollableDirections
+{
+  ASDisplayNodeAssertMainThread();
+  return _scrollableDirections;
+}
+
+- (void)setPropertiesProvider:(id<ASCollectionGalleryLayoutPropertiesProviding>)propertiesProvider
+{
+  ASDisplayNodeAssertMainThread();
+  if (propertiesProvider == nil) {
+    _propertiesProvider = nil;
+    _propertiesProviderFlags = {};
+  } else {
+    _propertiesProvider = propertiesProvider;
+    _propertiesProviderFlags.minimumLineSpacingForElements = [_propertiesProvider respondsToSelector:@selector(minimumLineSpacingForElements:)];
+    _propertiesProviderFlags.minimumInteritemSpacingForElements = [_propertiesProvider respondsToSelector:@selector(minimumInteritemSpacingForElements:)];
+    _propertiesProviderFlags.sectionInsetForElements = [_propertiesProvider respondsToSelector:@selector(sectionInsetForElements:)];
+  }
+}
+
+- (id)additionalInfoForLayoutWithElements:(ASElementMap *)elements
+{
+  ASDisplayNodeAssertMainThread();
+  id<ASCollectionGalleryLayoutPropertiesProviding> propertiesProvider = _propertiesProvider;
+  if (propertiesProvider == nil) {
+    return nil;
+  }
+
+  CGSize itemSize = [propertiesProvider sizeForElements:elements];
+  UIEdgeInsets sectionInset = _propertiesProviderFlags.sectionInsetForElements ? [propertiesProvider sectionInsetForElements:elements] : UIEdgeInsetsZero;
+  CGFloat lineSpacing = _propertiesProviderFlags.minimumLineSpacingForElements ? [propertiesProvider minimumLineSpacingForElements:elements] : 0.0;
+  CGFloat interitemSpacing = _propertiesProviderFlags.minimumInteritemSpacingForElements ? [propertiesProvider minimumInteritemSpacingForElements:elements] : 0.0;
+  return [[_ASCollectionGalleryLayoutInfo alloc] initWithItemSize:itemSize
+                                               minimumLineSpacing:lineSpacing
+                                          minimumInteritemSpacing:interitemSpacing
+                                                     sectionInset:sectionInset];
+}
+
++ (ASCollectionLayoutState *)calculateLayoutWithContext:(ASCollectionLayoutContext *)context
+{
+  ASElementMap *elements = context.elements;
+  CGSize pageSize = context.viewportSize;
+  ASScrollDirection scrollableDirections = context.scrollableDirections;
+
+  _ASCollectionGalleryLayoutInfo *info = ASDynamicCast(context.additionalInfo, _ASCollectionGalleryLayoutInfo);
+  CGSize itemSize = info.itemSize;
+  if (info == nil || CGSizeEqualToSize(CGSizeZero, itemSize)) {
+    return [[ASCollectionLayoutState alloc] initWithContext:context];
+  }
+
+  NSMutableArray<_ASGalleryLayoutItem *> *children = ASArrayByFlatMapping(elements.itemElements,
+                                                                          ASCollectionElement *element,
+                                                                          [[_ASGalleryLayoutItem alloc] initWithItemSize:itemSize collectionElement:element]);
+  if (children.count == 0) {
+    return [[ASCollectionLayoutState alloc] initWithContext:context];
+  }
+
+  // Use a stack spec to calculate layout content size and frames of all elements without actually measuring each element
+  ASStackLayoutDirection stackDirection = ASScrollDirectionContainsVerticalDirection(scrollableDirections)
+                                              ? ASStackLayoutDirectionHorizontal
+                                              : ASStackLayoutDirectionVertical;
+  ASStackLayoutSpec *stackSpec = [ASStackLayoutSpec stackLayoutSpecWithDirection:stackDirection
+                                                                         spacing:info.minimumInteritemSpacing
+                                                                  justifyContent:ASStackLayoutJustifyContentStart
+                                                                      alignItems:ASStackLayoutAlignItemsStart
+                                                                        flexWrap:ASStackLayoutFlexWrapWrap
+                                                                    alignContent:ASStackLayoutAlignContentStart
+                                                                     lineSpacing:info.minimumLineSpacing
+                                                                        children:children];
+  stackSpec.concurrent = YES;
+
+  ASLayoutSpec *finalSpec = stackSpec;
+  UIEdgeInsets sectionInset = info.sectionInset;
+  if (UIEdgeInsetsEqualToEdgeInsets(sectionInset, UIEdgeInsetsZero) == NO) {
+    finalSpec = [ASInsetLayoutSpec insetLayoutSpecWithInsets:sectionInset child:stackSpec];
+  }
+
+  ASLayout *layout = [finalSpec layoutThatFits:ASSizeRangeForCollectionLayoutThatFitsViewportSize(pageSize, scrollableDirections)];
+
+  return [[ASCollectionLayoutState alloc] initWithContext:context layout:layout getElementBlock:^ASCollectionElement * _Nullable(ASLayout * _Nonnull sublayout) {
+    _ASGalleryLayoutItem *item = ASDynamicCast(sublayout.layoutElement, _ASGalleryLayoutItem);
+    return item ? item.collectionElement : nil;
+  }];
+}
+
+@end

Source/Details/ASCollectionLayoutState.h

@@ -23,6 +23,8 @@
 
 NS_ASSUME_NONNULL_BEGIN
 
+typedef ASCollectionElement * _Nullable (^ASCollectionLayoutStateGetElementBlock)(ASLayout *);
+
 @interface NSMapTable (ASCollectionLayoutConvenience)
 
 + (NSMapTable<ASCollectionElement *, UICollectionViewLayoutAttributes *> *)elementToLayoutAttributesTable;
@@ -70,11 +72,11 @@ AS_SUBCLASSING_RESTRICTED
  *
  * @param layout The layout describes size and position of all elements.
  *
- * @param getElementBlock A block that can retrieve the collection element from a direct sublayout of the root layout.
+ * @param getElementBlock A block that can retrieve the collection element from a sublayout of the root layout.
  */
 - (instancetype)initWithContext:(ASCollectionLayoutContext *)context
                          layout:(ASLayout *)layout
-                getElementBlock:(ASCollectionElement *(^)(ASLayout *))getElementBlock;
+                getElementBlock:(ASCollectionLayoutStateGetElementBlock)getElementBlock;
 
 /**
  * Returns all layout attributes present in this object.

Source/Details/ASCollectionLayoutState.mm

@@ -20,9 +20,12 @@
 #import <AsyncDisplayKit/ASDisplayNode+Subclasses.h>
 #import <AsyncDisplayKit/ASElementMap.h>
 #import <AsyncDisplayKit/ASLayout.h>
+#import <AsyncDisplayKit/ASLayoutSpecUtilities.h>
 #import <AsyncDisplayKit/ASPageTable.h>
 #import <AsyncDisplayKit/ASThread.h>
 
+#import <queue>
+
 @implementation NSMapTable (ASCollectionLayoutConvenience)
 
 + (NSMapTable<ASCollectionElement *, UICollectionViewLayoutAttributes *> *)elementToLayoutAttributesTable
@@ -48,30 +51,49 @@ elementToLayoutAttributesTable:[NSMapTable elementToLayoutAttributesTable]];
 
 - (instancetype)initWithContext:(ASCollectionLayoutContext *)context
                          layout:(ASLayout *)layout
-                getElementBlock:(ASCollectionElement *(^)(ASLayout *))getElementBlock
+                getElementBlock:(ASCollectionLayoutStateGetElementBlock)getElementBlock
 {
   ASElementMap *elements = context.elements;
   NSMapTable *table = [NSMapTable elementToLayoutAttributesTable];
 
-  for (ASLayout *sublayout in layout.sublayouts) {
+  // Traverse the given layout tree in breadth first fashion. Generate layout attributes for all included elements along the way. 
-    ASCollectionElement *element = getElementBlock(sublayout);
+  struct Context {
-    if (element == nil) {
+    ASLayout *layout;
-      ASDisplayNodeFailAssert(@"Element not found!");
+    CGPoint absolutePosition;
-      continue;
+  };
+
+  std::queue<Context> queue;
+  queue.push({layout, CGPointZero});
+
+  while (!queue.empty()) {
+    Context context = queue.front();
+    queue.pop();
+
+    ASLayout *layout = context.layout;
+    const CGPoint absolutePosition = context.absolutePosition;
+
+    ASCollectionElement *element = getElementBlock(layout);
+    if (element != nil) {
+      NSIndexPath *indexPath = [elements indexPathForElement:element];
+      NSString *supplementaryElementKind = element.supplementaryElementKind;
+
+      UICollectionViewLayoutAttributes *attrs;
+      if (supplementaryElementKind == nil) {
+        attrs = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:indexPath];
+      } else {
+        attrs = [UICollectionViewLayoutAttributes layoutAttributesForSupplementaryViewOfKind:supplementaryElementKind withIndexPath:indexPath];
+      }
+
+      CGRect frame = layout.frame;
+      frame.origin = absolutePosition;
+      attrs.frame = frame;
+      [table setObject:attrs forKey:element];
     }
 
-    NSIndexPath *indexPath = [elements indexPathForElement:element];
+    // Add all sublayouts to process in next step
-    NSString *supplementaryElementKind = element.supplementaryElementKind;
+    for (ASLayout *sublayout in layout.sublayouts) {
-    
+      queue.push({sublayout, absolutePosition + sublayout.position});
-    UICollectionViewLayoutAttributes *attrs;
-    if (supplementaryElementKind == nil) {
-      attrs = [UICollectionViewLayoutAttributes layoutAttributesForCellWithIndexPath:indexPath];
-    } else {
-      attrs = [UICollectionViewLayoutAttributes layoutAttributesForSupplementaryViewOfKind:supplementaryElementKind withIndexPath:indexPath];
     }
-    
-    attrs.frame = sublayout.frame;
-    [table setObject:attrs forKey:element];
   }
 
   return [self initWithContext:context contentSize:layout.size elementToLayoutAttributesTable:table];

Source/Layout/ASLayout.mm

@@ -258,7 +258,7 @@ static std::atomic_bool static_retainsSublayoutLayoutElements = ATOMIC_VAR_INIT(
     } else if (sublayoutsCount > 0){
       std::vector<Context> sublayoutContexts;
       for (ASLayout *sublayout in sublayouts) {
-        sublayoutContexts.push_back({sublayout, context.absolutePosition + sublayout.position});
+        sublayoutContexts.push_back({sublayout, absolutePosition + sublayout.position});
       }
       queue.insert(queue.cbegin(), sublayoutContexts.begin(), sublayoutContexts.end());
     }

Source/Private/_ASCollectionGalleryLayoutInfo.h

@@ -0,0 +1,30 @@
+//
+//  _ASCollectionGalleryLayoutInfo.h
+//  Texture
+//
+//  Copyright (c) 2017-present, Pinterest, Inc.  All rights reserved.
+//  Licensed under the Apache License, Version 2.0 (the "License");
+//  you may not use this file except in compliance with the License.
+//  You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+
+#import <UIKit/UIKit.h>
+
+@interface _ASCollectionGalleryLayoutInfo : NSObject
+
+// Read-only properties
+@property (nonatomic, assign, readonly) CGSize itemSize;
+@property (nonatomic, assign, readonly) CGFloat minimumLineSpacing;
+@property (nonatomic, assign, readonly) CGFloat minimumInteritemSpacing;
+@property (nonatomic, assign, readonly) UIEdgeInsets sectionInset;
+
+- (instancetype)initWithItemSize:(CGSize)itemSize
+              minimumLineSpacing:(CGFloat)minimumLineSpacing
+         minimumInteritemSpacing:(CGFloat)minimumInteritemSpacing
+                    sectionInset:(UIEdgeInsets)sectionInset NS_DESIGNATED_INITIALIZER;
+
+- (instancetype)init __unavailable;
+
+@end

Source/Private/_ASCollectionGalleryLayoutInfo.m

@@ -0,0 +1,72 @@
+//
+//  _ASCollectionGalleryLayoutInfo.m
+//  Texture
+//
+//  Copyright (c) 2017-present, Pinterest, Inc.  All rights reserved.
+//  Licensed under the Apache License, Version 2.0 (the "License");
+//  you may not use this file except in compliance with the License.
+//  You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+
+#import <AsyncDisplayKit/_ASCollectionGalleryLayoutInfo.h>
+#import <AsyncDisplayKit/ASHashing.h>
+
+@implementation _ASCollectionGalleryLayoutInfo
+
+- (instancetype)initWithItemSize:(CGSize)itemSize
+              minimumLineSpacing:(CGFloat)minimumLineSpacing
+         minimumInteritemSpacing:(CGFloat)minimumInteritemSpacing
+                    sectionInset:(UIEdgeInsets)sectionInset
+{
+  self = [super init];
+  if (self) {
+    _itemSize = itemSize;
+    _minimumLineSpacing = minimumLineSpacing;
+    _minimumInteritemSpacing = minimumInteritemSpacing;
+    _sectionInset = sectionInset;
+  }
+  return self;
+}
+
+- (BOOL)isEqualToInfo:(_ASCollectionGalleryLayoutInfo *)info
+{
+  if (info == nil) {
+    return NO;
+  }
+
+  return CGSizeEqualToSize(_itemSize, info.itemSize)
+  && _minimumLineSpacing == info.minimumLineSpacing
+  && _minimumInteritemSpacing == info.minimumInteritemSpacing
+  && UIEdgeInsetsEqualToEdgeInsets(_sectionInset, info.sectionInset);
+}
+
+- (BOOL)isEqual:(id)other
+{
+  if (self == other) {
+    return YES;
+  }
+  if (! [other isKindOfClass:[_ASCollectionGalleryLayoutInfo class]]) {
+    return NO;
+  }
+  return [self isEqualToInfo:other];
+}
+
+- (NSUInteger)hash
+{
+  struct {
+    CGSize itemSize;
+    CGFloat minimumLineSpacing;
+    CGFloat minimumInteritemSpacing;
+    UIEdgeInsets sectionInset;
+  } data = {
+    _itemSize,
+    _minimumLineSpacing,
+    _minimumInteritemSpacing,
+    _sectionInset,
+  };
+  return ASHashBytes(&data, sizeof(data));
+}
+
+@end

examples/ASCollectionView/Sample/ViewController.m

@@ -23,7 +23,7 @@
 
 #define ASYNC_COLLECTION_LAYOUT 0
 
-@interface ViewController () <ASCollectionDataSource, ASCollectionDelegateFlowLayout, ASCollectionGalleryLayoutSizeProviding>
+@interface ViewController () <ASCollectionDataSource, ASCollectionDelegateFlowLayout, ASCollectionGalleryLayoutPropertiesProviding>
 
 @property (nonatomic, strong) ASCollectionNode *collectionNode;
 @property (nonatomic, strong) NSArray *data;
@@ -48,7 +48,7 @@
 
 #if ASYNC_COLLECTION_LAYOUT
   ASCollectionGalleryLayoutDelegate *layoutDelegate = [[ASCollectionGalleryLayoutDelegate alloc] initWithScrollableDirections:ASScrollDirectionVerticalDirections];
-  layoutDelegate.sizeProvider = self;
+  layoutDelegate.propertiesProvider = self;
   self.collectionNode = [[ASCollectionNode alloc] initWithLayoutDelegate:layoutDelegate layoutFacilitator:nil];
 #else
   UICollectionViewFlowLayout *layout = [[UICollectionViewFlowLayout alloc] init];
@@ -108,7 +108,7 @@
   [self.collectionNode reloadData];
 }
 
-#pragma mark - ASCollectionGalleryLayoutSizeProviding
+#pragma mark - ASCollectionGalleryLayoutPropertiesProviding
 
 - (CGSize)sizeForElements:(ASElementMap *)elements
 {