如何居中对齐一个UICollectionView的 item (

- (UIEdgeInsets)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewFlowLayout*)collectionViewLayout insetForSectionAtIndex:(NSInteger)section{
    
    NSInteger numberOfItems = [collectionView numberOfItemsInSection:0];
    CGFloat combinedItemWidth = (numberOfItems * collectionViewLayout.itemSize.width) + ((numberOfItems - 1)*collectionViewLayout.minimumInteritemSpacing);
    CGFloat padding = (collectionView.frame.size.width - combinedItemWidth)/2;
    padding = padding>0 ? padding :0 ;
    return UIEdgeInsetsMake(0, padding,0, padding);
}