检测距离集合视图中心最近的集合视图单元

怀疑我在下面做了一些根本错误的事情...我有一个水平集合视图，拖动后我想将最近的单元格捕捉到中心。但我的结果是不可预测的......我在这里做错了什么？

    func scrollViewDidEndDragging(scrollView: UIScrollView, willDecelerate decelerate: Bool) {
    // Find collectionview cell nearest to the center of collectionView
    // Arbitrarily start with the last cell (as a default)
    var closestCell : UICollectionViewCell = collectionView.visibleCells()[0];
    for cell in collectionView!.visibleCells() as [UICollectionViewCell] {
        let closestCellDelta = abs(closestCell.center.x - collectionView.bounds.size.width/2.0)
        let cellDelta = abs(cell.center.x - collectionView.bounds.size.width/2.0)
        if (cellDelta < closestCellDelta){
            closestCell = cell
        }
    }
    let indexPath = collectionView.indexPathForCell(closestCell)
    collectionView.scrollToItemAtIndexPath(indexPath!, atScrollPosition: UICollectionViewScrollPosition.CenteredHorizontally, animated: true)
}

事实证明，我的原始代码缺少对 collecitonview 内容偏移量的说明。此外，我已将居中移动到scrollViewDidEndDecelerating 回调中。我修改了原始代码并将其包含在下面。

    func scrollViewDidEndDecelerating(scrollView: UIScrollView) {
    // Find collectionview cell nearest to the center of collectionView
    // Arbitrarily start with the last cell (as a default)
    var closestCell : UICollectionViewCell = collectionView.visibleCells()[0];
    for cell in collectionView!.visibleCells() as [UICollectionViewCell] {
        let closestCellDelta = abs(closestCell.center.x - collectionView.bounds.size.width/2.0 - collectionView.contentOffset.x)
        let cellDelta = abs(cell.center.x - collectionView.bounds.size.width/2.0 - collectionView.contentOffset.x)
        if (cellDelta < closestCellDelta){
            closestCell = cell
        }
    }
    let indexPath = collectionView.indexPathForCell(closestCell)
    collectionView.scrollToItemAtIndexPath(indexPath!, atScrollPosition: UICollectionViewScrollPosition.CenteredHorizontally, animated: true)
}