如何让activity指示器在集合视图的底部运行，以指示正在加载更多的项目？

// 自定义Footer View
class CustomFooterView: UICollectionReusableView {
    let activityIndicatorView = UIActivityIndicatorView(style: .gray)
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        
        // 设置活动指示器的位置
        activityIndicatorView.center = CGPoint(x: frame.size.width / 2, y: frame.size.height / 2)
        
        // 添加活动指示器到Footer View
        addSubview(activityIndicatorView)
    }
    
    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

// 集合视图的数据源方法
extension YourViewController: UICollectionViewDataSource {
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        // 返回数据数量
        return yourData.count
    }
    
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        // 返回自定义的Cell
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "YourCellIdentifier", for: indexPath) as! YourCell
        
        // 配置Cell的内容
        
        return cell
    }
    
    func collectionView(_ collectionView: UICollectionView, viewForSupplementaryElementOfKind kind: String, at indexPath: IndexPath) -> UICollectionReusableView {
        // 返回自定义的Footer View
        let footerView = collectionView.dequeueReusableSupplementaryView(ofKind: UICollectionView.elementKindSectionFooter, withReuseIdentifier: "YourFooterViewIdentifier", for: indexPath) as! CustomFooterView
        
        // 根据需要显示或隐藏活动指示器
        if shouldShowActivityIndicator {
            footerView.activityIndicatorView.startAnimating()
        } else {
            footerView.activityIndicatorView.stopAnimating()
        }
        
        return footerView
    }
}