出现异常:“无法使用未准备好约束的视图层次结构设置布局"

Getting Exception: quot;Impossible to set up layout with view hierarchy unprepared for constraintquot;(出现异常:“无法使用未准备好约束的视图层次结构设置布局)

问题描述

在我的故事板应用程序中，我尝试向界面添加额外的 UITextField.但我得到了标题所说的例外.我使用 SwiftAutoLayout 库.这是我的代码:

In my storyboard application I try to add extra UITextField to interface. But I get the exception that title says. I use SwiftAutoLayout library. Here is my code:

// MARK: - IBOutlets

@IBOutlet weak var passwordTextField: UITextField!

// MARK: - Properties

let userIdTextField = UITextField()

// MARK: - Lifecycle

override func viewDidLoad() {
    super.viewDidLoad()

    self.passwordTextField.keyboardType = .NumberPad

    if getCurrentUserId() == nil {
        self.view.addSubview(userIdTextField)
        userIdTextField.setTranslatesAutoresizingMaskIntoConstraints(false)
        self.view.addConstraints([userIdTextField.al_leading == passwordTextField.al_leading,
            userIdTextField.al_trailing == passwordTextField.al_trailing,
            userIdTextField.al_top == passwordTextField.al_bottom + 8])
        userIdTextField.placeholder = "Enter User Id..."
    }
}

推荐答案

在绑定任何约束之前尝试将 passwordTextField 添加到其父视图.

Try adding passwordTextField to its superview before binding any constraints to it.

UPD(感谢 @vmeyer 和 @MacMark):请注意，在 viewDidLoad 或 viewWillAppear/viewDidAppear 但在 updateViewConstraints 或 viewWillLayoutSubviews 因为视图层次结构可能没有为此操作做好准备.

UPD (thanks @vmeyer and @MacMark): please notice that it's safer to add constraints not in viewDidLoad or viewWillAppear/viewDidAppear but in updateViewConstraints or viewWillLayoutSubviews since the view hierarchy might be unprepared for this operation.

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