IGListKit Support II: Electric Boogaloo (#2942)

* Reimplement IGListKit support in a cleaner way

* Rename and fix some stuff

* Fix supplementaries more

* Update docs

* Update test

* Cleanup minor things

* Tweak it

* Indentation

* Remove irrelevant changes

* Break out cell into its own file

* Fix indentation

* Address feedback

AsyncDisplayKit/Details/_ASCollectionViewCell.m

@@ -0,0 +1,74 @@
+//
+//  _ASCollectionViewCell.m
+//  AsyncDisplayKit
+//
+//  Created by Adlai Holler on 1/30/17.
+//  Copyright © 2017 Facebook. All rights reserved.
+//
+
+#import "_ASCollectionViewCell.h"
+#import "ASCellNode+Internal.h"
+#import <AsyncDisplayKit/AsyncDisplayKit.h>
+
+@implementation _ASCollectionViewCell
+
+- (void)setNode:(ASCellNode *)node
+{
+  ASDisplayNodeAssertMainThread();
+  node.layoutAttributes = _layoutAttributes;
+  _node = node;
+  self.backgroundColor = node.backgroundColor;
+  self.clipsToBounds = node.clipsToBounds;
+  [node __setSelectedFromUIKit:self.selected];
+  [node __setHighlightedFromUIKit:self.highlighted];
+}
+
+- (void)setSelected:(BOOL)selected
+{
+  [super setSelected:selected];
+  [_node __setSelectedFromUIKit:selected];
+}
+
+- (void)setHighlighted:(BOOL)highlighted
+{
+  [super setHighlighted:highlighted];
+  [_node __setHighlightedFromUIKit:highlighted];
+}
+
+- (void)setLayoutAttributes:(UICollectionViewLayoutAttributes *)layoutAttributes
+{
+  _layoutAttributes = layoutAttributes;
+  _node.layoutAttributes = layoutAttributes;
+}
+
+- (void)prepareForReuse
+{
+  self.layoutAttributes = nil;
+
+  // Need to clear node pointer before UIKit calls setSelected:NO / setHighlighted:NO on its cells
+  self.node = nil;
+  [super prepareForReuse];
+}
+
+/**
+ * In the initial case, this is called by UICollectionView during cell dequeueing, before
+ *   we get a chance to assign a node to it, so we must be sure to set these layout attributes
+ *   on our node when one is next assigned to us in @c setNode: . Since there may be cases when we _do_ already
+ *   have our node assigned e.g. during a layout update for existing cells, we also attempt
+ *   to update it now.
+ */
+- (void)applyLayoutAttributes:(UICollectionViewLayoutAttributes *)layoutAttributes
+{
+  self.layoutAttributes = layoutAttributes;
+}
+
+/**
+ * Keep our node filling our content view.
+ */
+- (void)layoutSubviews
+{
+  [super layoutSubviews];
+  self.node.frame = self.contentView.bounds;
+}
+
+@end