ios - 使用 Swift 向 ViewController 添加约束

Question

我想将container View 添加到主视图(rootViewController.view)，但下面给出了异常(exception)。我知道约束，但无法找出原因。

import UIKit

class rootViewController : UIViewController {

    init() {
        super.init(nibName: nil, bundle: nil)
        setupLoginView()
    }

    func setupLoginView() {

        // User ID label
        let userIDLabel:UILabel = UILabel()
        userIDLabel.text = "User ID"

        // Password label
        let passwordLabel:UILabel = UILabel()
        passwordLabel.text = "Password"

        // User ID text
        let userIDText:UITextField = UITextField()

        // Password text
        let passwordText:UITextField = UITextField()

        // Login button
        let loginBtn:UIButton = UIButton()
        loginBtn.setTitle("Login", for: .normal)

        // Container view
        let container:UIView = UIView()

        container.addSubview(userIDLabel)
        container.addSubview(userIDText)
        container.addSubview(passwordLabel)
        container.addSubview(passwordText)
        container.addSubview(loginBtn)

        view.addSubview(container)

        // Add constraints
        let heightConstraint = NSLayoutConstraint(item: container, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1, constant: 300)
        let widthConstraint = NSLayoutConstraint(item: container, attribute: .width, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1, constant: 200)
        let centerXConstraint = NSLayoutConstraint(item: container, attribute: .centerX, relatedBy: .equal, toItem: view, attribute: .centerX, multiplier: 1, constant: 0)
        let centerYConstraint = NSLayoutConstraint(item: container, attribute: .centerY, relatedBy: .equal, toItem: view, attribute: .centerY, multiplier: 1, constant: 0)

        container.addConstraint(heightConstraint)
        container.addConstraint(widthConstraint)
        container.addConstraint(centerXConstraint)
        container.addConstraint(centerYConstraint)

    }

}

给出以下异常，并提示“约束是否引用了 View 子树之外的内容？这是非法的”

'NSGenericException', reason: 'Unable to install constraint on view. Does the constraint reference something from outside the subtree of the view? That's illegal. constraint: NSLayoutConstraint:0x170089830 UIView:0x12de138c0.centerX == UIView:0x12de0e650.centerX (active)>

Answer 1

将添加中心约束的代码替换为以下行:

view.addConstraint(centerXConstraint)
view.addConstraint(centerYConstraint)

您不能在 container 上添加对其父 View 的引用的约束。

并将 setupLoginView() 移动到 viewDidLoad 而不是 init()。

不要忘记删除控制台中的警告以将所有创建的 View (容器、标签、按钮和 TextView )的 translatesAutoresizingMaskIntoConstraints 设置为 false。