打开新控制器后App崩溃求助：EXC_BAD_ACCESS错误排查

问题描述 #

App启动后点击打开新控制器时崩溃，错误信息：

Thread 1: EXC_BAD_ACCESS (code=2, address=0x1d27fc844)
崩溃触发点在NSLayoutConstraint.activate(constants)代码行。

问题代码 #

var dimmedBaclroundView : UIView {
    let view = UIView()
    
    view.translatesAutoresizingMaskIntoConstraints = false
    view.backgroundColor = UIColor.black.withAlphaComponent(0.3)
    return view
}

lazy var CView = CustomView { [weak self] in
    guard let self = self else {return}
    self.dismiss(animated: true, completion:  nil)
    
}

init() {
    super.init(nibName: nil, bundle: nil)
    modalPresentationStyle = .overCurrentContext
    modalTransitionStyle = .crossDissolve
    
}

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



override func viewDidLoad() {
    super.viewDidLoad()

    view.addSubview(dimmedBaclroundView)
    view.addSubview(CView)
    
    
    var constants = [
        dimmedBaclroundView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
        dimmedBaclroundView.trailingAnchor.constraint(equalTo: view.trailingAnchor),
        dimmedBaclroundView.topAnchor.constraint(equalTo: view.topAnchor),
        dimmedBaclroundView.bottomAnchor.constraint(equalTo: view.bottomAnchor)
    ]
    
    constants.append(contentsOf: [
        CView.leadingAnchor.constraint(equalTo: view.readableContentGuide.leadingAnchor),
        CView.trailingAnchor.constraint(equalTo: view.readableContentGuide.trailingAnchor),
        CView.centerYAnchor.constraint(equalTo: view.centerYAnchor, constant: 10),
        CView.heightAnchor.constraint(equalTo: view.heightAnchor, constant: 0.3)
    ])
    
    NSLayoutConstraint.activate(constants) // 崩溃位置
}

问题原因 #

核心错误是dimmedBaclroundView被定义为计算属性：

• 每次访问该属性，都会创建一个全新的UIView实例
• view.addSubview(dimmedBaclroundView)添加的是第一个实例，而后续创建约束时访问dimmedBaclroundView生成的是第二个未加入视图层级的实例
• 激活约束时，系统无法找到这个孤立视图的有效上下文，直接触发EXC_BAD_ACCESS内存错误

修复方案 #

1. 将dimmedBaclroundView改为懒加载存储属性，确保全局只有一个实例：

lazy var dimmedBaclroundView: UIView = {
    let view = UIView()
    view.translatesAutoresizingMaskIntoConstraints = false
    view.backgroundColor = UIColor.black.withAlphaComponent(0.3)
    return view
}()

2. 修正拼写错误：dimmedBaclroundView建议改为dimmedBackgroundView（少了字母k），提升代码可读性
3. 修正高度约束逻辑：如果是要设置为父视图高度的30%，应该用multiplier而非constant（constant是固定偏移值）：

CView.heightAnchor.constraint(equalTo: view.heightAnchor, multiplier: 0.3)

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