KTCenterFlowLayout.m

//
//  KTCenterFlowLayout.m
//
//  Created by Kyle Truscott on 10/9/14.
//  Copyright (c) 2014 keighl. All rights reserved.
//

#import "KTCenterFlowLayout.h"

@interface KTCenterFlowLayout ()
@end

@implementation KTCenterFlowLayout

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
{
  NSMutableArray *superAttributes = [[NSMutableArray alloc] initWithArray:[super layoutAttributesForElementsInRect:rect] copyItems:YES];
  
  NSMutableDictionary *rowCollections = [NSMutableDictionary new];

  id <UICollectionViewDelegateFlowLayout> flowDelegate = (id<UICollectionViewDelegateFlowLayout>) [[self collectionView] delegate];
  BOOL delegateSupportsInteritemSpacing = [flowDelegate respondsToSelector:@selector(collectionView:layout:minimumInteritemSpacingForSectionAtIndex:)];
  
  // Collect attributes by their midY coordinate.. i.e. rows!
  for (UICollectionViewLayoutAttributes *itemAttributes in superAttributes)
  {
    // Normalize the midY to others in the row
    // with variable cell heights the midYs can be ever so slightly
    // different.
    CGFloat midYRound = roundf(CGRectGetMidY(itemAttributes.frame));
    CGFloat midYPlus = midYRound + 1;
    CGFloat midYMinus = midYRound - 1;
    NSNumber *key;
    
    if (rowCollections[@(midYPlus)])
      key = @(midYPlus);
    
    if (rowCollections[@(midYMinus)])
      key = @(midYMinus);
    
    if (!key)
      key = @(midYRound);
    
    if (!rowCollections[key])
      rowCollections[key] = [NSMutableArray new];
    
    [(NSMutableArray *) rowCollections[key] addObject:itemAttributes];
  }

  CGFloat collectionViewWidth = CGRectGetWidth(self.collectionView.bounds) - self.collectionView.contentInset.left - self.collectionView.contentInset.right;
  
  // Adjust the items in each row
  [rowCollections enumerateKeysAndObjectsUsingBlock:^(id key, NSArray *itemAttributesCollection, BOOL *stop) {
    
    NSInteger itemsInRow = [itemAttributesCollection count];

    // x-x-x-x ... sum up the interim space
    CGFloat interitemSpacing = [self minimumInteritemSpacing];
    
    // Check for delegate support
    if (delegateSupportsInteritemSpacing && itemsInRow > 0)
    {
      NSInteger section = [[itemAttributesCollection[0] indexPath] section];
      interitemSpacing = [flowDelegate collectionView:self.collectionView
                                               layout:self
             minimumInteritemSpacingForSectionAtIndex:section];
    }
    
    CGFloat aggregateInteritemSpacing = interitemSpacing * (itemsInRow -1);

    // Sum the width of all elements in the row
    CGFloat aggregateItemWidths = 0.f;
    for (UICollectionViewLayoutAttributes *itemAttributes in itemAttributesCollection)
      aggregateItemWidths += CGRectGetWidth(itemAttributes.frame);

    // Build an alignment rect
    // |==|--------|==|
    CGFloat alignmentWidth = aggregateItemWidths + aggregateInteritemSpacing;
    CGFloat alignmentXOffset = (collectionViewWidth - alignmentWidth) / 2.f;

    // Adjust each item's position to be centered
    CGRect previousFrame = CGRectZero;
    for (UICollectionViewLayoutAttributes *itemAttributes in itemAttributesCollection)
    {
      CGRect itemFrame = itemAttributes.frame;

      if (CGRectEqualToRect(previousFrame, CGRectZero))
        itemFrame.origin.x = alignmentXOffset;
      else
        itemFrame.origin.x = CGRectGetMaxX(previousFrame) + interitemSpacing;

      itemAttributes.frame = itemFrame;
      previousFrame = itemFrame;
    }
  }];

  return superAttributes;
}

- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath {
    UICollectionViewLayoutAttributes *superAttributes = [super layoutAttributesForItemAtIndexPath:indexPath];
    
    // Rect that encompasses all items on indexPath's line.
    CGRect lineRect = CGRectMake(0.0, CGRectGetMinY(superAttributes.frame), CGRectGetWidth(self.collectionView.frame), CGRectGetHeight(superAttributes.frame));
    
    NSArray *lineAttributes = [self layoutAttributesForElementsInRect:lineRect];
    
    for (UICollectionViewLayoutAttributes *itemAttributes in lineAttributes) {
        if (itemAttributes.indexPath == indexPath) {
            return itemAttributes;
        }
    }
    
    return nil;
}

@end