获得例外：“无法设置具有视图层次结构的布局，无法用于约束”

问题描述

在我的storyboard应用程序中，我尝试向界面添加额外的 UITextField 。但我得到标题所说的例外。我使用库。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

尝试添加 passwordTextField 到在绑定任何约束之前的超级视图。

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

UPD （感谢@vmeyer和@MacMark）：请注意，添加约束更安全不在 viewDidLoad 或 viewWillAppear / viewDidAppear 但在或，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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