UITableView中sendSubviewToBack失效，如何调整单元格层级？

我想要调整UITableView的视图层级，已经使用sendSubviewToBack和bringSubviewToFront方法，且先创建RedCell，需要确保RedCell位于BlueCell下方（RedCell中的overBgView不应遮挡BlueCell），但实际结果不符合预期：RedCell的黄色overBgView覆盖在了BlueCell上方。

相关代码 #

import UIKit

class ViewController: UIViewController, UITableViewDataSource, UITableViewDelegate {
    func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return 2
    }
    
    func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        print("<<< \(indexPath)")
        
        if indexPath.row == 0 {
            let cell = tableView.dequeueReusableCell(withIdentifier: RedCell.identifier, for: indexPath) as! RedCell
            // RedCell
            tableView.sendSubviewToBack(cell)
            return cell
        } else {
            let cell = tableView.dequeueReusableCell(withIdentifier: BlueCell.identifier, for: indexPath) as! BlueCell
            // BlueCell
            tableView.bringSubviewToFront(cell)
            return cell
        }
    }
    
    private lazy var tableView: UITableView = {
        let table = UITableView(frame: .zero, style: .plain)
        table.estimatedRowHeight = 80
        table.showsVerticalScrollIndicator = false
        table.separatorStyle = .none
        table.delegate = self
        table.dataSource = self
        table.register(RedCell.self, forCellReuseIdentifier: RedCell.identifier)
        table.register(BlueCell.self, forCellReuseIdentifier: BlueCell.identifier)
        return table
    }()
    
    private lazy var label: UILabel = {
        let label = UILabel()
        label.text = "------split----the views below are not table cell"
        label.textColor = .black
        return label
    }()
    
    private lazy var view1: UIViewOne = {
        let view = UIViewOne()
        return view
    }()
    
    private lazy var view2: UIView = {
        let view = UIView()
        view.backgroundColor = .blue
        return view
    }()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        tableView.register(RedCell.self, forCellReuseIdentifier: "redCell")
        tableView.register(BlueCell.self, forCellReuseIdentifier: "blueCell")
        setupUI()
    }
    
    public init() {
        super.init(nibName: nil, bundle: nil)
    }
    
    required init?(coder: NSCoder) {
        super.init(coder: coder)
    }
    
    private func setupUI() {
        view.addSubview(tableView)
        view.addSubview(label)
        view.addSubview(view1)
        view.addSubview(view2)
        
        tableView.snp.makeConstraints { make in
            make.top.equalToSuperview().inset(200)
            make.left.right.equalToSuperview()
            make.height.equalTo(120)
        }
        
        label.snp.makeConstraints { make in
            make.top.equalTo(tableView.snp.bottom).offset(30)
            make.centerX.equalToSuperview()
            make.height.equalTo(20)
        }
        
        view1.snp.makeConstraints { make in
            make.top.equalTo(label.snp.bottom).offset(30)
            make.left.right.equalToSuperview()
            make.height.equalTo(44)
        }
        view2.snp.makeConstraints { make in
            make.top.equalTo(view1.snp.bottom)
            make.left.right.equalToSuperview()
            make.height.equalTo(44)
        }
    }
}

class RedCell: UITableViewCell {
    static let identifier = "RedCell"
    
    private lazy var overBgView: UIView = {
        let view = UIView()
        view.backgroundColor = .yellow
        return view
    }()
    
    override init(style: UITableViewCell.CellStyle, reuseIdentifier: String?) {
        super.init(style: style, reuseIdentifier: reuseIdentifier)
        backgroundColor = .red
        setupUI()
    }

    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    private func setupUI() {
        contentView.addSubview(overBgView)
//        contentView.sendSubviewToBack(overBgView)
        self.superview?.sendSubviewToBack(overBgView)
        
        overBgView.snp.makeConstraints { make in
            make.top.left.right.equalToSuperview()
            make.height.equalTo(60)
        }
    }
}

class BlueCell: UITableViewCell {
    static let identifier = "BlueCell"
    
    override init(style: UITableViewCell.CellStyle, reuseIdentifier: String?) {
        super.init(style: style, reuseIdentifier: reuseIdentifier)
        backgroundColor = .blue
        setupUI()
    }

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

class UIViewOne: UIView {
    private lazy var oversizeView: UIView = {
        let view = UIView()
        view.backgroundColor = .yellow
        return view
    }()
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        backgroundColor = .red
        addSubview(oversizeView)
        oversizeView.snp.makeConstraints { make in
            make.top.left.right.equalToSuperview()
            make.height.equalTo(60)
        }
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

核心问题1：Cell初始化时操作父视图层级无效 #

在RedCell的setupUI方法中，self.superview?.sendSubviewToBack(overBgView)完全无效——Cell初始化阶段还未被添加到UITableView，此时superview为nil，代码不会执行。而且overBgView是添加在contentView上的，你却试图通过Cell的父视图调整它的层级，逻辑错误。

核心问题2：UITableView的Cell层级由系统自动管理，手动调整会被覆盖 #

UITableView会根据滚动、复用逻辑自动维护Cell的层级关系，在cellForRowAt中调用sendSubviewToBack/bringSubviewToFront的操作会被系统后续的布局逻辑覆盖，无法保证持久生效。

修复步骤 #

1. 修正RedCell中overBgView的层级
   将overBgView放在contentView的最底层，直接调用contentView的层级调整方法即可：

   private func setupUI() {
       contentView.addSubview(overBgView)
       contentView.sendSubviewToBack(overBgView) // 替换原无效代码
       
       overBgView.snp.makeConstraints { make in
           make.top.left.right.equalToSuperview()
           make.height.equalTo(60)
       }
   }
   

2. 通过layer.zPosition强制指定Cell层级
   利用图层的zPosition属性控制Cell的显示层级，数值越大越靠上，这个设置不会被UITableView的系统逻辑覆盖：
   修改cellForRowAt方法：

   func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
       print("<<< \(indexPath)")
       
       if indexPath.row == 0 {
           let cell = tableView.dequeueReusableCell(withIdentifier: RedCell.identifier, for: indexPath) as! RedCell
           cell.layer.zPosition = 0 // RedCell层级更低
           return cell
       } else {
           let cell = tableView.dequeueReusableCell(withIdentifier: BlueCell.identifier, for: indexPath) as! BlueCell
           cell.layer.zPosition = 1 // BlueCell层级更高
           return cell
       }
   }
   

3. 移除重复的Cell注册代码
   viewDidLoad中重复注册Cell的代码无意义，直接删除：

   // 移除以下两行
   // tableView.register(RedCell.self, forCellReuseIdentifier: "redCell")
   // tableView.register(BlueCell.self, forCellReuseIdentifier: "blueCell")
   

   因为在tableView的懒加载初始化代码中已经完成了Cell注册。

最终效果 #

修复后，BlueCell会稳定显示在RedCell上方，RedCell的黄色overBgView不会遮挡BlueCell，符合预期需求。

内容的提问来源于stack exchange，提问作者malcopolocei