gistfile1.m

- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath
{
    UICollectionViewLayoutAttributes *currentItemAttributes = [super layoutAttributesForItemAtIndexPath:indexPath];
    CGFloat topInset = [self.delegate collectionView:self.collectionView layout:self insetForSectionAtIndex:indexPath.section].top;

    // First row should be aligned at the top
    if (indexPath.item < self.numColumns){
        currentItemAttributes.frame = ({
            CGRect frame = currentItemAttributes.frame;
            frame.origin.y = topInset + 0;
            frame;
        });

        return currentItemAttributes;
    }
    
    /*
     * if (firstItemInRow) Get height of highest item of previous row, add it to the origin.y of that item and assign it to the current item + topinset
     * else (secondItemInRow OR thirdItemInRow) origin.y should be the same as the first item in the row
    */
    if (indexPath.item % 3 == 0) {
        // Get the heights of the previous row's items
        CGRect frameOne = [self getFrameForItem:indexPath.item - self.numColumns inSection:indexPath.section];
        CGRect frameTwo = [self getFrameForItem:indexPath.item - (self.numColumns - 1) inSection:indexPath.section];
        CGRect frameThree = [self getFrameForItem:indexPath.item - (self.numColumns - 2) inSection:indexPath.section];

        // Add them to an array and retreive the biggest value
        NSArray *heights = @[@(frameOne.size.height), @(frameTwo.size.height), @(frameThree.size.height)];
        CGFloat highestValue = [[heights valueForKeyPath:@"@max.self"] floatValue];
        
        // Assign the highest value to the y coordinate of its frame
        currentItemAttributes.frame = ({
            CGRect frame = currentItemAttributes.frame;
            frame.origin.y = topInset + frameOne.origin.y + highestValue;
            frame;
        });
        
        firstItemInRowMaxY = currentItemAttributes.frame.origin.y;
    }
    else {
        // This item should have the same origin.y as the first item in the row
        currentItemAttributes.frame = ({
            CGRect frame = currentItemAttributes.frame;
            frame.origin.y = firstItemInRowMaxY;
            frame;
        });
    }

    return currentItemAttributes;
}