Swift cellwilldisplay和uiview动画不工作

我试图在单元格出现后设置高度常数的动画，但什么也没发生。 我在手机上尝试过同样的方法，但还是没有结果。。。你知道我错过了什么吗？ 这是我的密码：

     func collectionView(_ collectionView: UICollectionView, willDisplay cell: UICollectionViewCell, forItemAt indexPath: IndexPath) {
        let cell = cell as! barGraph
        UIView.animate(withDuration: 0.5) {
            cell.barFillHeightConst.constant = 100
        }
    }
    
   func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    
        let cell = self.dequeueReusableCell(withReuseIdentifier: cellId, for: indexPath) as! barGraph
        cell.label.text =  String(indexPath.row)
    cell.barFillHeightConst?.isActive = false
    cell.barFillHeightConst = cell.barFill.heightAnchor.constraint(equalTo: cell.bar.heightAnchor, multiplier: 0.7)
    cell.barFillHeightConst.isActive = true
    if indexPath.row % 6 != 0 {
        cell.label.textColor = .clear
        cell.dotViewConst.width?.constant = 6
        cell.dotViewConst.height?.constant = 6
        } else {
        cell.label.textColor = .lightGray
        cell.dotViewConst.width?.constant = 8
        cell.dotViewConst.height?.constant = 8
        }          
        cell.dotView.layer.cornerRadius = cell.dotViewConst.width!.constant / 2
        return cell
   }