Swift 以编程方式向视图添加约束

下面是我添加约束的简单代码：

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        let contentView = UIView()
        contentView.backgroundColor = UIColor.greenColor()
        contentView.frame.size.width = 250
        contentView.frame.size.height = 100
        contentView.center = view.center
        view.addSubview(contentView)

        let userNameLabel = UILabel()
        userNameLabel.translatesAutoresizingMaskIntoConstraints = false
        userNameLabel.text = "Username:"
        contentView.addSubview(userNameLabel)

        let passwordLabel = UILabel()
        passwordLabel.translatesAutoresizingMaskIntoConstraints = false
        passwordLabel.text = "Password:"
        contentView.addSubview(passwordLabel)

        let usernameTextField = UITextField()
        usernameTextField.translatesAutoresizingMaskIntoConstraints = false
        usernameTextField.borderStyle = .RoundedRect
        contentView.addSubview(usernameTextField)

        let passwordTextField = UITextField()
        passwordTextField.translatesAutoresizingMaskIntoConstraints = false
        passwordTextField.borderStyle = .RoundedRect
        contentView.addSubview(passwordTextField)

        let submitButton = UIButton()
        submitButton.translatesAutoresizingMaskIntoConstraints = false
        submitButton.setTitle("Submit", forState: .Normal)
        submitButton.setTitleColor(UIColor.redColor(), forState: .Normal)
        submitButton.setTitleColor(UIColor.redColor(), forState: .Highlighted)
        contentView.addSubview(submitButton)

        // For Username
        NSLayoutConstraint(item: userNameLabel, attribute: .Leading, relatedBy: .Equal, toItem: contentView, attribute: .LeadingMargin, multiplier: 1, constant: 0).active = true
        NSLayoutConstraint(item: userNameLabel, attribute: .Top, relatedBy: .Equal, toItem: contentView, attribute: .Top, multiplier: 1, constant: 10).active = true
        NSLayoutConstraint(item: usernameTextField, attribute: .Trailing, relatedBy: .Equal, toItem: contentView, attribute: .TrailingMargin, multiplier: 1.0, constant: 0.0).active = true
        NSLayoutConstraint(item: usernameTextField, attribute: .Leading, relatedBy: .Equal, toItem: userNameLabel, attribute: .Trailing, multiplier: 1.0, constant: 3).active = true
        NSLayoutConstraint(item: userNameLabel, attribute: .Baseline, relatedBy: .Equal, toItem: usernameTextField, attribute: .Baseline, multiplier: 1.0, constant: 0.0).active = true
        // For Password
        NSLayoutConstraint(item: passwordLabel, attribute: .Leading, relatedBy: .Equal, toItem: contentView, attribute: .LeadingMargin, multiplier: 1, constant: 0).active = true
        NSLayoutConstraint(item: passwordLabel, attribute: .Top, relatedBy: .Equal, toItem: userNameLabel, attribute: .Bottom, multiplier: 1, constant: 15).active = true
        NSLayoutConstraint(item: passwordTextField, attribute: .Trailing, relatedBy: .Equal, toItem: contentView, attribute: .TrailingMargin, multiplier: 1.0, constant: 0.0).active = true
        NSLayoutConstraint(item: passwordTextField, attribute: .Leading, relatedBy: .Equal, toItem: passwordLabel, attribute: .Trailing, multiplier: 1.0, constant: 3).active = true
        NSLayoutConstraint(item: passwordLabel, attribute: .Baseline, relatedBy: .Equal, toItem: passwordTextField, attribute: .Baseline, multiplier: 1.0, constant: 0.0).active = true
        // For Button
        NSLayoutConstraint(item: submitButton, attribute: .Top, relatedBy: .Equal, toItem: passwordLabel, attribute: .Bottom, multiplier: 1.0, constant: 8.0).active = true
        NSLayoutConstraint(item: submitButton, attribute: .CenterX, relatedBy: .Equal, toItem: contentView, attribute: .CenterX, multiplier: 1.0, constant: 0.0).active = true
    }

}

但是为什么看起来像这样，我希望

passWordTextField

的前导是

3

到

passWordLabel

的尾随：

我看了这一行，没有发现任何问题：

NSLayoutConstraint(item: passwordTextField, attribute: .Leading, relatedBy: .Equal, toItem: passwordLabel, attribute: .Trailing, multiplier: 1.0, constant: 3).active = true

---

任何帮助都将不胜感激。谢谢

问题在于标签和文本字段具有相同的拥抱优先级。您需要定义优先顺序。例如：

userNameLabel.setContentHuggingPriority(251, forAxis: .Horizontal)
passwordLabel.setContentHuggingPriority(251, forAxis: .Horizontal)

---

问题是label和textfield具有相同的拥抱优先级。您需要定义优先顺序。例如：

userNameLabel.setContentHuggingPriority(251, forAxis: .Horizontal)
passwordLabel.setContentHuggingPriority(251, forAxis: .Horizontal)

---

问题在于：这一行：

NSLayoutConstraint(item: passwordTextField, attribute: .Leading, relatedBy: .Equal, toItem: passwordLabel, attribute: .Trailing, multiplier: 1.0, constant: 3).active = true

应在此行之前显示：

NSLayoutConstraint(item: passwordTextField, attribute: .Trailing, relatedBy: .Equal, toItem: contentView, attribute: .TrailingMargin, multiplier: 1.0, constant: 0.0).active = true

密码的代码如下所示：

 // For Password
 NSLayoutConstraint(item: passwordLabel, attribute: .Leading, relatedBy: .Equal, toItem: contentView, attribute: .LeadingMargin, multiplier: 1, constant: 0).active = true
 NSLayoutConstraint(item: passwordLabel, attribute: .Top, relatedBy: .Equal, toItem: userNameLabel, attribute: .Bottom, multiplier: 1, constant: 15).active = true
 NSLayoutConstraint(item: passwordTextField, attribute: .Leading, relatedBy: .Equal, toItem: passwordLabel, attribute: .Trailing, multiplier: 1.0, constant: 3).active = true
 NSLayoutConstraint(item: passwordTextField, attribute: .Trailing, relatedBy: .Equal, toItem: contentView, attribute: .TrailingMargin, multiplier: 1.0, constant: 0.0).active = true
 NSLayoutConstraint(item: passwordLabel, attribute: .Baseline, relatedBy: .Equal, toItem: passwordTextField, attribute: .Baseline, multiplier: 1.0, constant: 0.0).active = true

---

问题在于：这一行：

NSLayoutConstraint(item: passwordTextField, attribute: .Leading, relatedBy: .Equal, toItem: passwordLabel, attribute: .Trailing, multiplier: 1.0, constant: 3).active = true

应在此行之前显示：

NSLayoutConstraint(item: passwordTextField, attribute: .Trailing, relatedBy: .Equal, toItem: contentView, attribute: .TrailingMargin, multiplier: 1.0, constant: 0.0).active = true

密码的代码如下所示：

 // For Password
 NSLayoutConstraint(item: passwordLabel, attribute: .Leading, relatedBy: .Equal, toItem: contentView, attribute: .LeadingMargin, multiplier: 1, constant: 0).active = true
 NSLayoutConstraint(item: passwordLabel, attribute: .Top, relatedBy: .Equal, toItem: userNameLabel, attribute: .Bottom, multiplier: 1, constant: 15).active = true
 NSLayoutConstraint(item: passwordTextField, attribute: .Leading, relatedBy: .Equal, toItem: passwordLabel, attribute: .Trailing, multiplier: 1.0, constant: 3).active = true
 NSLayoutConstraint(item: passwordTextField, attribute: .Trailing, relatedBy: .Equal, toItem: contentView, attribute: .TrailingMargin, multiplier: 1.0, constant: 0.0).active = true
 NSLayoutConstraint(item: passwordLabel, attribute: .Baseline, relatedBy: .Equal, toItem: passwordTextField, attribute: .Baseline, multiplier: 1.0, constant: 0.0).active = true

---

成功了。因为我认为UILabel会根据其中的文本自动设置宽度。但是为什么userNameLabel工作而passwordLabel不工作。但是当我更改passwordLabel上的文本时，它看起来很糟糕。不，问题不是这样。请再检查一下。好的，我知道是什么问题。让我更新一下我的答案。它是有效的，但它是随机决定哪一个优先，因为你可以从你的截图上看出。现在，标签已按优先级排列，其行为将如预期的那样。您仍然可以在标签之间添加一个等宽约束，以使文本视图在相同的x坐标处开始。这很有效。因为我认为UILabel会根据其中的文本自动设置宽度。但是为什么userNameLabel工作而passwordLabel不工作。但是当我更改passwordLabel上的文本时，它看起来很糟糕。不，问题不是这样。请再检查一下。好的，我知道是什么问题。让我更新一下我的答案。它是有效的，但它是随机决定哪一个优先，因为你可以从你的截图上看出。现在，标签已按优先级排列，其行为将如预期的那样。您仍然可以在标签之间添加等宽约束，以使文本视图从相同的x坐标开始。