使用 swift 的代码中的约束 - 无法激活

Question

我正在尝试理解自动布局约束。在 Ray W 教程挑战之后（没有解决方案或讨论），布局应如下所示：

在 IB 中执行此操作非常简单 - 创建一个具有宽度和高度的黄色视图，垂直居中并水平固定到边距；然后在该视图中创建带有简单约束的标签、字段和按钮。

我的第一个问题是：如果标签具有固有的内容大小，并且其他所有内容都固定在黄色视图上，那么为什么我需要定义固定的宽度和高度？为什么它不从其子视图的内在内容推断宽度和高度？

继续并尝试在代码中重新创建此布局给了我一个错误：

由于未捕获的异常“NSGenericException”而终止应用程序， 原因：'无法使用项目 <UILabel: 0x7a961d60; frame = (0 0; 0 0); text = 'Password:'; userInteractionEnabled = NO; layer = <_UILabelLayer: 0x7a961e40>> 和激活约束 <UIView: 0x7a96b6f0; frame = (0 0; 0 0); layer = <CALayer: 0x7a96b880>> 因为他们没有共同的祖先。是否 不同视图层次结构中的约束引用项？那是 非法。'

第二个问题：这些层是什么，我所有的视图都在一个层次结构中 - 超级视图包含黄色视图包含文本标签和字段。

痛苦万分后，我尝试完全重新创建 IB 中的约束，但这只会添加以下错误： 无法同时满足约束。

(
    "<NSLayoutConstraint:0x7ac57370 H:[UIView:0x7a96b6f0(0)]>",
    "<NSLayoutConstraint:0x7ac57400 H:|-(8)-[UILabel:0x7a96bb50'Username:']   (Names: '|':UIView:0x7a96b6f0 )>",
    "<NSLayoutConstraint:0x7ac57430 UILabel:0x7a96bb50'Username:'.trailing == UITextField:0x7a961020.leading + 8>",
    "<NSLayoutConstraint:0x7ac57520 UITextField:0x7a961020.trailing == UIView:0x7a96b6f0.trailing - 8>" )

最后一个问题：我如何知道视图 0x7aetc 是什么视图？我的代码中的这个约束在哪里？其余的看起来还可以（？）。

我一定是在某些基本层面上做错了什么。

这是我的代码：

import UIKit

class ViewController: UIViewController {

    let centerView = UIView()
    let usernameLabel = UILabel()
    let passwordLabel = UILabel()
    let usernameField = UITextField()
    let passwordField = UITextField()
    let submitButton = UIButton()

    override func viewDidLoad() {
        super.viewDidLoad()

        centerView.backgroundColor = UIColor.yellowColor()
        usernameLabel.text = "Username:"
        passwordLabel.text = "Password:"
        usernameField.borderStyle = .RoundedRect
        passwordField.borderStyle = .RoundedRect
        submitButton.setTitle("Submit!", forState: .Normal)
        submitButton.setTitleColor(UIColor.blackColor(), forState: .Normal)
        submitButton.setTitleColor(UIColor.blueColor(), forState: .Highlighted)

        view.addSubview(centerView)
        self.centerView.addSubview(usernameField)
        self.centerView.addSubview(passwordField)
        self.centerView.addSubview(usernameLabel)
        self.centerView.addSubview(submitButton)

        let constraintCenterViewHeight = NSLayoutConstraint(item: centerView, attribute: .Height, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1.0, constant: 0)
        let constraintCenterViewWidth = NSLayoutConstraint(item: centerView, attribute: .Width, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1.0, constant: 0)

        let constraintCenterViewCenterX = NSLayoutConstraint(item: centerView, attribute: .CenterX, relatedBy: .Equal, toItem: view, attribute: .CenterX, multiplier: 1.0, constant: 0)
        let constraintCenterViewCenterY = NSLayoutConstraint(item: centerView, attribute: .CenterY, relatedBy: .Equal, toItem: view, attribute: .CenterY, multiplier: 1.0, constant: 0)

        let constraintUsernameLabelHLeading = NSLayoutConstraint(item: usernameLabel, attribute: .Leading, relatedBy: .Equal, toItem: centerView, attribute: .Leading, multiplier: 1.0, constant: 8)
        let constraintUsernameLabelHTrailing = NSLayoutConstraint(item: usernameLabel, attribute: .Trailing, relatedBy: .Equal, toItem: usernameField, attribute: .Leading, multiplier: 1.0, constant: 8)
        let constraintUsernameLabelAlignBottom = NSLayoutConstraint(item: usernameLabel, attribute: .Bottom, relatedBy: .Equal, toItem: usernameField, attribute: .Bottom, multiplier: 1.0, constant: 0)

        let constraintUsernameFieldVTop = NSLayoutConstraint(item: usernameField, attribute: .Top, relatedBy: .Equal, toItem: centerView, attribute: .Top, multiplier: 1.0, constant: 8)
        let constraintUsernameFieldHTrailing = NSLayoutConstraint(item: usernameField, attribute: .Trailing, relatedBy: .Equal, toItem: centerView, attribute: .Trailing, multiplier: 1.0, constant: -8)
        let constraintUsernameFieldVBottom = NSLayoutConstraint(item: usernameField, attribute: .Bottom, relatedBy: .Equal, toItem: passwordField, attribute: .Top, multiplier: 1.0, constant: 8)

        let constraintPasswordLabelHLeading = NSLayoutConstraint(item: passwordLabel, attribute: .Leading, relatedBy: .Equal, toItem: centerView, attribute: .Leading, multiplier: 1.0, constant: 8)
        let constraintPasswordLabelHTrailing = NSLayoutConstraint(item: passwordLabel, attribute: .Trailing, relatedBy: .Equal, toItem: passwordField, attribute: .Leading, multiplier: 1.0, constant: 8)
        let constraintPasswordLabelAlignBottom = NSLayoutConstraint(item: passwordLabel, attribute: .Bottom, relatedBy: .Equal, toItem: passwordField, attribute: .Bottom, multiplier: 1.0, constant: 0)

        let constraintPasswordFieldHTrailing = NSLayoutConstraint(item: passwordField, attribute: .Trailing, relatedBy: .Equal, toItem: centerView, attribute: .Trailing, multiplier: 1.0, constant: -8)

        centerView.setTranslatesAutoresizingMaskIntoConstraints(false)
        usernameLabel.setTranslatesAutoresizingMaskIntoConstraints(false)
        usernameField.setTranslatesAutoresizingMaskIntoConstraints(false)
        passwordField.setTranslatesAutoresizingMaskIntoConstraints(false)
        passwordLabel.setTranslatesAutoresizingMaskIntoConstraints(false)
       // submitButton.setTranslatesAutoresizingMaskIntoConstraints(false)

        NSLayoutConstraint.activateConstraints([constraintCenterViewHeight, constraintCenterViewWidth, constraintCenterViewCenterX, constraintCenterViewCenterY, constraintUsernameLabelHLeading,
            constraintUsernameLabelHTrailing, constraintUsernameLabelAlignBottom, constraintUsernameFieldVTop, constraintUsernameFieldHTrailing, constraintUsernameFieldVBottom, constraintPasswordLabelHLeading, constraintPasswordLabelHTrailing, constraintPasswordLabelAlignBottom, constraintPasswordFieldHTrailing])
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }


}

Answer 1

我的第一个问题是：如果标签具有固有的内容大小， 其他所有内容都固定在黄色视图上，那我为什么需要 定义一个固定的宽度和高度？为什么它不能推断宽度 和子视图的内在内容的高度？

username 和 password 字段没有宽度。它们通过固定在centerView 左右边缘的标签上来获得宽度。如果您添加了约束来为这些字段指定宽度，那么centerView 可以根据其内容确定其宽度。

---

继续并尝试在代码中重新创建此布局给了我一个 错误：由于未捕获的异常“NSGenericException”而终止应用程序， 原因：'无法使用项目 > 和 > 激活约束，因为它们 没有共同的祖先。约束是否引用中的项目 不同的视图层次结构？这是非法的。'

您的第一个问题是您没有将passwordLabel 添加为子视图：

self.centerView.addSubview(passwordLabel)

这就是给你错误的原因。 passwordLabel 不在视图层次结构中，因此自动布局不知道如何使用 centerView 对其进行约束。

您的第二个问题是您将centerView 的width 和height 设置为0。尝试更大的值，例如 100 和 300：

let constraintCenterViewHeight = NSLayoutConstraint(item: centerView, attribute: .Height,
    relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1.0,
    constant: 100)

let constraintCenterViewWidth = NSLayoutConstraint(item: centerView, attribute: .Width,
    relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1.0,
    constant: 300)

您的第三个问题是您的某些常量需要为负数：

// changed to -8
let constraintUsernameLabelHTrailing = NSLayoutConstraint(item: usernameLabel,
    attribute: .Trailing, relatedBy: .Equal, toItem: usernameField,
    attribute: .Leading, multiplier: 1.0, constant: -8)

// changed to -8
let constraintUsernameFieldVBottom = NSLayoutConstraint(item: usernameField,
    attribute: .Bottom, relatedBy: .Equal, toItem: passwordField,
    attribute: .Top, multiplier: 1.0, constant: -8)

// changed to -8
let constraintPasswordLabelHTrailing = NSLayoutConstraint(item: passwordLabel,
    attribute: .Trailing, relatedBy: .Equal, toItem: passwordField,
    attribute: .Leading, multiplier: 1.0, constant: -8)

Answer 2

我遇到了同样的错误，但发生的原因不同：

我收到错误是因为我引用了我的 superview 在我将视图添加到它的 superview 之前。 view.addSubview(stackView) 本身没有什么不同。两者都会产生相同的错误——因为视图与其父视图没有定义的关系。

在下面的 my 代码中，B1、B2 行发生在我将视图添加到超级视图之前（在 AA 行）。

这是我的代码：

class ViewController: UIViewController {

    var label = UILabel()
    var button = UIButton()
    var stackView = UIStackView()

    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = UIColor.cyan
        navigationItem.title = "VCTitle"
        setupUI()
    }

    private func setupUI(){

        label.backgroundColor = UIColor.blue
        label.heightAnchor.constraint(equalToConstant: 50).isActive = true
        label.widthAnchor.constraint(equalToConstant: 80).isActive = true

        button.backgroundColor = UIColor.purple
        button.heightAnchor.constraint(equalToConstant: 30).isActive = true
        button.widthAnchor.constraint(equalToConstant: 10).isActive = true

        setupStackView()

        stackView.addArrangedSubview(label)
        stackView.addArrangedSubview(button)
        stackView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(stackView) // AA


    }

    private func setupStackView(){

        stackView.axis = UILayoutConstraintAxis.vertical
        stackView.distribution = UIStackViewDistribution.equalSpacing
        stackView.alignment = UIStackViewAlignment.center
        stackView.spacing = 15

        stackView.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true // B1
        stackView.centerYAnchor.constraint(equalTo: view.centerYAnchor).isActive = true // B2

    }


}

正确的做法是将 B1、B2 行移动到 AA 行之后的任意位置。

class ViewController: UIViewController {

    var label = UILabel()
    var button = UIButton()
    var stackView = UIStackView()

    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = UIColor.cyan
        navigationItem.title = "VCTitle"
        setupUI()
    }

    private func setupUI(){

        label.backgroundColor = UIColor.blue
        label.heightAnchor.constraint(equalToConstant: 50).isActive = true
        label.widthAnchor.constraint(equalToConstant: 80).isActive = true

        button.backgroundColor = UIColor.purple
        button.heightAnchor.constraint(equalToConstant: 30).isActive = true
        button.widthAnchor.constraint(equalToConstant: 10).isActive = true

        setupStackView()

        stackView.addArrangedSubview(label)
        stackView.addArrangedSubview(button)
        stackView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(stackView) //AA

        // The 2 lines below are now in the right place.
        stackView.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true // B1
        stackView.centerYAnchor.constraint(equalTo: view.centerYAnchor).isActive = true // B2


    }

    private func setupStackView(){

        stackView.axis = UILayoutConstraintAxis.vertical
        stackView.distribution = UIStackViewDistribution.equalSpacing
        stackView.alignment = UIStackViewAlignment.center
        stackView.spacing = 15

    }
}