ios - iOS奇怪的“无法使用锚激活约束”错误

Question

我目前正在开发的 ios 应用程序从登录屏幕开始，然后通过登录按钮显示下一个视图控制器。转到下一个 VC 后，我收到错误消息：

Terminating app due to uncaught exception 'NSGenericException', 
reason: 'Unable to activate constraint with anchors 
<NSLayoutYAxisAnchor:0x600000d0af40 "UIStackView:0x7fe87df14160.top"> and
<NSLayoutYAxisAnchor:0x600000d0ad40 "UILayoutGuide:0x600002102300'UIViewSafeAreaLayoutGuide'.bottom"> 
because they have no common ancestor.  
Does the constraint or its anchors reference items
in different view hierarchies?  That's illegal.'

以下是显示的控制器的相关 VC 代码：

let promptLabel: UILabel = {
    let label = UILabel()
    
    label.translatesAutoresizingMaskIntoConstraints = false
    label.text = "Enter Your Location`s \nAddress"
    label.font = .boldSystemFont(ofSize: 23)
    label.numberOfLines = 0
    label.preferredMaxLayoutWidth = label.frame.width
    label.textAlignment = .center
    
    return label
}()

let streetAddressField: UITextField = {
    let textField = UITextField()

    textField.translatesAutoresizingMaskIntoConstraints = false
    textField.placeholder = "street address" 
    textField.borderStyle = .roundedRect
    textField.textAlignment = .left

    return textField
}()

let cityField: UITextField = {
    let textField = UITextField()

    textField.translatesAutoresizingMaskIntoConstraints = false
    textField.placeholder = "city"
    textField.borderStyle = .roundedRect
    textField.textAlignment = .left

    return textField
}()

let stateField: UITextField = {
    let textField = UITextField()

    textField.translatesAutoresizingMaskIntoConstraints = false
    textField.placeholder = "state"
    textField.borderStyle = .roundedRect
    textField.textAlignment = .left

    return textField
}()

let saveButton: UIButton = {
    let button = UIButton(type: .system)
    button.setTitle("Add Location", for: .normal)
    button.translatesAutoresizingMaskIntoConstraints = false
    
    return button
}()

var currentUser: User?

override func viewDidLoad() {
    super.viewDidLoad()
    
    view.backgroundColor = .white
    
    let views: [UIView] = [streetAddressField, cityField, stateField]
    
    let stackView = UIStackView(arrangedSubviews: views)
    
    stackView.axis = .vertical
    stackView.spacing = 17
    stackView.translatesAutoresizingMaskIntoConstraints = false
    
    view.addSubview(promptLabel)
    view.addSubview(stackView)
    view.addSubview(saveButton)
            
    promptLabel.centerXAnchor.constraint(equalTo: view.safeAreaLayoutGuide.centerXAnchor).isActive = true

    promptLabel.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor, constant: 30).isActive = true
    
    promptLabel.heightAnchor.constraint(lessThanOrEqualToConstant: 70).isActive = true
    
    stackView.topAnchor.constraint(equalTo: promptLabel.bottomAnchor, constant: 40).isActive = true
    
    stackView.centerXAnchor.constraint(equalTo: view.safeAreaLayoutGuide.centerXAnchor).isActive = true
    
    stackView.widthAnchor.constraint(equalToConstant: 300).isActive = true
    
    saveButton.centerXAnchor.constraint(equalTo: view.safeAreaLayoutGuide.centerXAnchor).isActive = true
    
    saveButton.topAnchor.constraint(equalTo: stackView.bottomAnchor, constant: 30).isActive = true
    
    saveButton.addTarget(self, action: #selector(addLocationTouchUpInside), for: .touchUpInside)
}

奇怪的是，这个错误有一段时间没有出现。在不同的 iphone 模拟器上进行测试时，错误需要运行很多次才能显示出来。错误也显示@main在AppDelegate.swift.

我知道这个问题有多个重复项。然而，他们都没有将他们的子视图添加到我在这里所做的超级视图中。

更新：我已经更新到 xcode 13 并且不再看到这个错误。不幸的是，它在模拟器和我的个人 iphone 上再次抬起了丑陋的头。

UPDATE2：设置了许多断点之后，这个错误似乎甚至在viewWillAppear()被调用之前就出现了。self.navigationController.pushViewController()当我们看到错误时，在调用后立即返回我以前的 VC 。（self这里当然指的是之前的VC）

标签： iosswiftxcode