First cell starts in center of the view.

contentInsetCenter

- (CGSize)collectionViewContentSize {
    CGSize contentSize  = [super collectionViewContentSize];
    // we need to center last view

    int items   = [[self collectionView] numberOfItemsInSection:0];
    UICollectionViewLayoutAttributes* layoutAttribute = (UICollectionViewLayoutAttributes*)[super layoutAttributesForItemAtIndexPath:[NSIndexPath indexPathForItem:items-1 inSection:0]];
    
    float startX    = contentSize.width - self.collectionView.bounds.size.width + self.collectionView.bounds.size.width / 2;
    
    startX          = layoutAttribute.frame.origin.x - startX;
    
    startX          += layoutAttribute.size.width / 2;
    
    contentSize.width += startX;
    
    layoutAttribute = (UICollectionViewLayoutAttributes*)[super layoutAttributesForItemAtIndexPath:[NSIndexPath indexPathForItem:0 inSection:0]];
    
    [[self collectionView] setContentInset:UIEdgeInsetsMake(0, self.collectionView.bounds.size.width / 2 - layoutAttribute.frame.size.width / 2, 0, 0)];
    
    return contentSize;
}

This works for me (for a known size):

let halfScreen = CGSize(width: {Put here your full size - I use UIScreen.main.bounds.size.width} / 2 - 30 / 2, height: {Your height})
flowLayout.headerReferenceSize = halfScreen
flowLayout.footerReferenceSize = halfScreen