UICollectionViewFlowLayout子类不起作用

这是我的UICollectionViewFlowLayout子类：

@implementation MyCollectionViewFlowLayout

- (id)init
{
    self = [super init];
    if (self) {
        [self setup];
    }

    return self;
}

- (void)setup
{
    self.itemSize = CGSizeMake(320, 320);
    self.sectionInset = UIEdgeInsetsMake(0, 0, 0, 0);
    self.minimumInteritemSpacing = 0;
    self.minimumLineSpacing = 0;
    self.scrollDirection = UICollectionViewScrollDirectionVertical;
}


- (void)prepareLayout {
    [super prepareLayout];

}

- (CGSize)collectionViewContentSize {
    return self.itemSize;
}

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect {
    NSArray* allAttributesInRect = [super layoutAttributesForElementsInRect:rect];

    return allAttributesInRect;
}


- (UICollectionViewLayoutAttributes*)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath {
    UICollectionViewLayoutAttributes* attributes = [super layoutAttributesForItemAtIndexPath:indexPath];

    return attributes;
}

@end

当我执行以下操作时，收藏夹视图将不会滚动：

MyCollectionViewFlowLayout* flowLayout = [[MyCollectionViewFlowLayout alloc] init];

但是，如果我这样做：

UICollectionViewFlowLayout* flowLayout = [[UICollectionViewFlowLayout alloc] init];
flowLayout.itemSize = CGSizeMake(320, 320);
flowLayout.sectionInset = UIEdgeInsetsMake(0, 0, 0, 0);
flowLayout.minimumInteritemSpacing = 0;
flowLayout.minimumLineSpacing = 0;
flowLayout.scrollDirection = UICollectionViewScrollDirectionVertical;

然后，收藏夹视图滚动良好。我在UICollectionViewFlowLayout子类中做错了什么？