How can I change the scroll direction in UICollectionView?

Swift 4 and 4.2

if let layout = collectionViewObj.collectionViewLayout as? UICollectionViewFlowLayout {
        layout.scrollDirection = .vertical  // .horizontal
    }

UICollectionViewFlowLayout *flowLayout = [[UICollectionViewFlowLayout alloc] init];
[flowLayout setScrollDirection:UICollectionViewScrollDirectionVertical];

---

Note too that it seems to be fine to call this in prepareForLayout in your flow layout...

@interface LayoutHorizontalThings : UICollectionViewFlowLayout
@end

@implementation LayoutHorizontalBooks
-(void)prepareLayout
    {
    [super prepareLayout];

    self.scrollDirection = UICollectionViewScrollDirectionHorizontal;

    self.minimumInteritemSpacing = 0;
    self.minimumLineSpacing = 0;
    self.itemSize = CGSizeMake(110,130);
    self.sectionInset = UIEdgeInsetsMake(0, 0, 0, 0);
    }

Set the scrollDirection of the collection view's collectionViewLayout.

Docs are here.