How do you vertically align the UICollectionViewCells in a UICollectionView?

Question

I have cells of varying height on a horizontally scrolling UICollectionView that appears to evenly distribute the cells vertically leaving empty space in between the cells and I would like to top align them and have the variable empty space at the bottom of each column.

Answer 1

I extended my UICollectionViewFlowLayout provided to my UICollectionView. The following overriding worked for me.

#import "TopAlignedCollectionViewFlowLayout.h"

const NSInteger kVerticalSpacing = 10;

@implementation TopAlignedCollectionViewFlowLayout

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
{
    NSArray* attributesToReturn = [super layoutAttributesForElementsInRect:rect];
    for (UICollectionViewLayoutAttributes* attributes in attributesToReturn) {
        if (nil == attributes.representedElementKind) {
            NSIndexPath* indexPath = attributes.indexPath;
            attributes.frame = [self layoutAttributesForItemAtIndexPath:indexPath].frame;
        }
    }
    return attributesToReturn;
}

- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath
{
    UICollectionViewLayoutAttributes* currentItemAttributes =
    [super layoutAttributesForItemAtIndexPath:indexPath];
    UIEdgeInsets sectionInset = [(UICollectionViewFlowLayout *)self.collectionView.collectionViewLayout sectionInset];
    if (indexPath.item == 0) { 
        CGRect frame = currentItemAttributes.frame;
        frame.origin.y = sectionInset.top;
        currentItemAttributes.frame = frame;

        return currentItemAttributes;
    }
    NSIndexPath* previousIndexPath = [NSIndexPath indexPathForItem:indexPath.item-1 inSection:indexPath.section];
    CGRect previousFrame = [self layoutAttributesForItemAtIndexPath:previousIndexPath].frame;
    CGFloat previousFrameRightPoint = previousFrame.origin.y + previousFrame.size.height + kVerticalSpacing;
    CGRect currentFrame = currentItemAttributes.frame;
    CGRect strecthedCurrentFrame = CGRectMake(currentFrame.origin.x,
                                              0,
                                              currentFrame.size.width,
                                              self.collectionView.frame.size.height
                                              );
    if (!CGRectIntersectsRect(previousFrame, strecthedCurrentFrame)) {
        CGRect frame = currentItemAttributes.frame;
        frame.origin.y = frame.origin.y = sectionInset.top;
        currentItemAttributes.frame = frame;
        return currentItemAttributes;
    }
    CGRect frame = currentItemAttributes.frame;
    frame.origin.y = previousFrameRightPoint;
    currentItemAttributes.frame = frame;
    return currentItemAttributes;
}

@end