如何用Swift优化cell高度缓存，避免滚动卡顿

在iOS开发中，使用UITableView或UICollectionView时，cell的高度缓存是一个常见的优化点。不当的处理会导致滚动卡顿，影响用户体验。本文将详细介绍如何在Swift中优化cell高度缓存，避免滚动卡顿。

1. 使用自动布局计算高度

首先，确保你的cell布局使用自动布局（Auto Layout）进行设置。自动布局可以自动计算cell的高度，减少手动计算和设置高度的麻烦。

class MyCell: UITableViewCell {
    let label = UILabel()

    override init(style: UITableViewCell.CellStyle, reuseIdentifier: String?) {
        super.init(style: style, reuseIdentifier: reuseIdentifier)
        label.translatesAutoresizingMaskIntoConstraints = false
        contentView.addSubview(label)
        NSLayoutConstraint.activate([
            label.leadingAnchor.constraint(equalTo: contentView.leadingAnchor, constant: 16),
            label.trailingAnchor.constraint(equalTo: contentView.trailingAnchor, constant: -16),
            label.centerYAnchor.constraint(equalTo: contentView.centerYAnchor)
        ])
    }

    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

2. 使用预估高度和实际高度

在UITableView中，可以使用预估高度（estimatedHeight）和实际高度（heightForRowAt）来优化cell高度缓存。

• 预估高度：在cell加载前，使用预估高度来告诉UITableView一个大致的cell高度，这样UITableView可以提前预留空间，避免滚动卡顿。
• 实际高度：在cell加载后，根据实际内容计算cell高度，并更新UITableView。

class MyViewController: UITableViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        tableView.register(MyCell.self, forCellReuseIdentifier: "MyCell")
    }

    override func tableView(_ tableView: UITableView, estimatedHeightForRowAt indexPath: IndexPath) -> CGFloat {
        return 44 // 预估高度
    }

    override func tableView(_ tableView: UITableView, heightForRowAt indexPath: IndexPath) -> CGFloat {
        let cell = tableView.dequeueReusableCell(withIdentifier: "MyCell", for: indexPath) as! MyCell
        cell.label.text = "Hello, World!"
        return cell.contentView.systemLayoutSizeFitting(UIView.layoutFittingCompressedSize).height
    }
}

3. 使用UICollectionViewLayout

对于UICollectionView，可以使用UICollectionViewLayout来自定义cell布局和高度计算。

”`swift class MyCollectionViewLayout: UICollectionViewLayout {

override var collectionViewContentSize: CGSize {
    return CGSize(width: collectionView.bounds.width, height: 200) // 总高度
}

override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
    var attributes = [UICollectionViewLayoutAttributes]()

    for section in 0..<collectionView.numberOfSections {
        let sectionOffset = collectionView.collectionViewLayout.collectionViewContentSize.height - section * 44
        let sectionRect = CGRect(x: 0, y: sectionOffset, width: collectionView.bounds.width, height: 44)
        let attributes = UICollectionViewLayoutAttributes(forCellWith: IndexPath(item: 0, section: section))
        attributes.frame = sectionRect
        attributes.zIndex = section
        attributes.size = CGSize(width: collectionView.bounds.width, height: 44)
        attributes.center = CGPoint(x: collectionView.bounds.width / 2, y: sectionRect.midY)
        attributes.transform = CGAffineTransform(rotationAngle: CGFloat.pi / 2)
        attributes.alpha = 0.5
        attributes.contentScaleFactor = 0.5
        attributes.isHidden = true
        attributes.allowsGroupOpacity = true
        attributes.backgroundColor = .red
        attributes.layer.cornerRadius = 22
        attributes.layer.masksToBounds = true
        attributes.layer.shouldRasterize = true
        attributes.rasterizationScale = UIScreen.main.scale
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius = 5
        attributes.layer.shadowPath = UIBezierPath(rect: sectionRect).cgPath
        attributes.layer.shadowFilter = .none
        attributes.layer.shadowColor = UIColor.black.cgColor
        attributes.layer.shadowOpacity = 0.5
        attributes.layer.shadowOffset = CGSize(width: 0, height: 0)
        attributes.layer.shadowRadius =