added sample app that initializes ASCollectionView cells using view controllers, wip though

examples/CollectionViewWithViewControllerCells/Sample/MosaicCollectionViewLayout.m

@@ -0,0 +1,230 @@
+//
+//  MosaicCollectionViewLayout.m
+//  Sample
+//
+//  Created by McCallum, Levi on 11/22/15.
+//  Copyright (c) 2015 Facebook. All rights reserved.
+//
+
+#import "MosaicCollectionViewLayout.h"
+
+@implementation MosaicCollectionViewLayout {
+  NSMutableArray *_columnHeights;
+  NSMutableArray *_itemAttributes;
+  NSMutableDictionary *_headerAttributes;
+  NSMutableArray *_allAttributes;
+}
+
+- (instancetype)init
+{
+  self = [super init];
+  if (self != nil) {
+    self.numberOfColumns = 3;
+    self.columnSpacing = 10.0;
+    self.sectionInset = UIEdgeInsetsMake(10.0, 10.0, 10.0, 10.0);
+    self.interItemSpacing = UIEdgeInsetsMake(10.0, 0, 10.0, 0);
+  }
+  return self;
+}
+
+- (void)prepareLayout
+{
+  _itemAttributes = [NSMutableArray array];
+  _columnHeights = [NSMutableArray array];
+  _allAttributes = [NSMutableArray array];
+  _headerAttributes = [NSMutableDictionary dictionary];
+  
+  CGFloat top = 0;
+  
+  NSInteger numberOfSections = [self.collectionView numberOfSections];
+  for (NSUInteger section = 0; section < numberOfSections; section++) {
+    NSInteger numberOfItems = [self.collectionView numberOfItemsInSection:section];
+    
+    top += _sectionInset.top;
+    
+    if (_headerHeight > 0) {
+      CGSize headerSize = [self _headerSizeForSection:section];
+      UICollectionViewLayoutAttributes *attributes = [UICollectionViewLayoutAttributes
+                                                      layoutAttributesForSupplementaryViewOfKind:UICollectionElementKindSectionHeader
+                                                      withIndexPath:[NSIndexPath indexPathForItem:0 inSection:section]];
+      attributes.frame = CGRectMake(_sectionInset.left, top, headerSize.width, headerSize.height);
+      _headerAttributes[@(section)] = attributes;
+      [_allAttributes addObject:attributes];
+      top = CGRectGetMaxY(attributes.frame);
+    }
+    
+    [_columnHeights addObject:[NSMutableArray array]];
+    for (NSUInteger idx = 0; idx < self.numberOfColumns; idx++) {
+      [_columnHeights[section] addObject:@(top)];
+    }
+    
+    CGFloat columnWidth = [self _columnWidthForSection:section];
+    [_itemAttributes addObject:[NSMutableArray array]];
+    for (NSUInteger idx = 0; idx < numberOfItems; idx++) {
+      NSUInteger columnIndex = [self _shortestColumnIndexInSection:section];
+      NSIndexPath *indexPath = [NSIndexPath indexPathForItem:idx inSection:section];
+      
+      CGSize itemSize = [self _itemSizeAtIndexPath:indexPath];
+      CGFloat xOffset = _sectionInset.left + (columnWidth + _columnSpacing) * columnIndex;
+      CGFloat yOffset = [_columnHeights[section][columnIndex] floatValue];
+      
+      UICollectionViewLayoutAttributes *attributes = [UICollectionViewLayoutAttributes
+                                                      layoutAttributesForCellWithIndexPath:indexPath];
+      attributes.frame = CGRectMake(xOffset, yOffset, itemSize.width, itemSize.height);
+      
+      _columnHeights[section][columnIndex] = @(CGRectGetMaxY(attributes.frame) + _interItemSpacing.bottom);
+      
+      [_itemAttributes[section] addObject:attributes];
+      [_allAttributes addObject:attributes];
+    }
+    
+    NSUInteger columnIndex = [self _tallestColumnIndexInSection:section];
+    top = [_columnHeights[section][columnIndex] floatValue] - _interItemSpacing.bottom + _sectionInset.bottom;
+    
+    for (NSUInteger idx = 0; idx < [_columnHeights[section] count]; idx++) {
+      _columnHeights[section][idx] = @(top);
+    }
+  }
+}
+
+- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
+{
+  NSMutableArray *includedAttributes = [NSMutableArray array];
+  // Slow search for small batches
+  for (UICollectionViewLayoutAttributes *attributes in _allAttributes) {
+    if (CGRectIntersectsRect(attributes.frame, rect)) {
+      [includedAttributes addObject:attributes];
+    }
+  }
+  return includedAttributes;
+}
+
+- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath
+{
+  if (indexPath.section >= _itemAttributes.count) {
+    return nil;
+  } else if (indexPath.item >= [_itemAttributes[indexPath.section] count]) {
+    return nil;
+  }
+  return _itemAttributes[indexPath.section][indexPath.item];
+}
+
+- (UICollectionViewLayoutAttributes *)layoutAttributesForSupplementaryViewOfKind:(NSString *)elementKind atIndexPath:(NSIndexPath *)indexPath
+{
+  if ([elementKind isEqualToString:UICollectionElementKindSectionHeader]) {
+    return _headerAttributes[@(indexPath.section)];
+  }
+  return nil;
+}
+
+- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds
+{
+  if (!CGRectEqualToRect(self.collectionView.bounds, newBounds)) {
+    return YES;
+  }
+  return NO;
+}
+
+- (CGFloat)_widthForSection:(NSUInteger)section
+{
+  return self.collectionView.bounds.size.width - _sectionInset.left - _sectionInset.right;
+}
+
+- (CGFloat)_columnWidthForSection:(NSUInteger)section
+{
+  return ([self _widthForSection:section] - ((_numberOfColumns - 1) * _columnSpacing)) / _numberOfColumns;
+}
+
+- (CGSize)_itemSizeAtIndexPath:(NSIndexPath *)indexPath
+{
+  CGSize size = CGSizeMake([self _columnWidthForSection:indexPath.section], 0);
+  CGSize originalSize = [[self _delegate] collectionView:self.collectionView layout:self originalItemSizeAtIndexPath:indexPath];
+  if (originalSize.height > 0 && originalSize.width > 0) {
+    size.height = originalSize.height / originalSize.width * size.width;
+  }
+  return size;
+}
+
+- (CGSize)_headerSizeForSection:(NSUInteger)section
+{
+  return CGSizeMake([self _widthForSection:section], _headerHeight);
+}
+
+- (CGSize)collectionViewContentSize
+{
+  CGFloat height = [[[_columnHeights lastObject] firstObject] floatValue];
+  return CGSizeMake(self.collectionView.bounds.size.width, height);
+}
+
+- (NSUInteger)_tallestColumnIndexInSection:(NSUInteger)section
+{
+  __block NSUInteger index = 0;
+  __block CGFloat tallestHeight = 0;
+  [_columnHeights[section] enumerateObjectsUsingBlock:^(NSNumber *height, NSUInteger idx, BOOL *stop) {
+    if (height.floatValue > tallestHeight) {
+      index = idx;
+      tallestHeight = height.floatValue;
+    }
+  }];
+  return index;
+}
+
+- (NSUInteger)_shortestColumnIndexInSection:(NSUInteger)section
+{
+  __block NSUInteger index = 0;
+  __block CGFloat shortestHeight = CGFLOAT_MAX;
+  [_columnHeights[section] enumerateObjectsUsingBlock:^(NSNumber *height, NSUInteger idx, BOOL *stop) {
+    if (height.floatValue < shortestHeight) {
+      index = idx;
+      shortestHeight = height.floatValue;
+    }
+  }];
+  return index;
+}
+
+- (id<MosaicCollectionViewLayoutDelegate>)_delegate
+{
+  return (id<MosaicCollectionViewLayoutDelegate>)self.collectionView.delegate;
+}
+
+@end
+
+@implementation MosaicCollectionViewLayoutInspector
+
+- (ASSizeRange)collectionView:(ASCollectionView *)collectionView constrainedSizeForNodeAtIndexPath:(NSIndexPath *)indexPath
+{
+  MosaicCollectionViewLayout *layout = (MosaicCollectionViewLayout *)[collectionView collectionViewLayout];
+  return ASSizeRangeMake(CGSizeZero, [layout _itemSizeAtIndexPath:indexPath]);
+}
+
+- (ASSizeRange)collectionView:(ASCollectionView *)collectionView constrainedSizeForSupplementaryNodeOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath
+{
+  MosaicCollectionViewLayout *layout = (MosaicCollectionViewLayout *)[collectionView collectionViewLayout];
+  return ASSizeRangeMake(CGSizeZero, [layout _headerSizeForSection:indexPath.section]);
+}
+
+/**
+ * Asks the inspector for the number of supplementary sections in the collection view for the given kind.
+ */
+- (NSUInteger)collectionView:(ASCollectionView *)collectionView numberOfSectionsForSupplementaryNodeOfKind:(NSString *)kind
+{
+  if ([kind isEqualToString:UICollectionElementKindSectionHeader]) {
+    return [[collectionView asyncDataSource] numberOfSectionsInCollectionView:collectionView];
+  } else {
+    return 0;
+  }
+}
+
+/**
+ * Asks the inspector for the number of supplementary views for the given kind in the specified section.
+ */
+- (NSUInteger)collectionView:(ASCollectionView *)collectionView supplementaryNodesOfKind:(NSString *)kind inSection:(NSUInteger)section
+{
+  if ([kind isEqualToString:UICollectionElementKindSectionHeader]) {
+    return 1;
+  } else {
+    return 0;
+  }
+}
+
+@end