为什么在添加一项后所有 UI 都消失了？

Question

我正在学习教程并且有这样的代码示例

import UIKit

class ViewController: UIViewController {

    var cluesLabel: UILabel!
    var answersLabel: UILabel!
    var currentAnswer: UITextField!
    var scoreLabel: UILabel!
    var letterButtons = [UIButton]()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
    }

    override func loadView() {
        view = UIView()
        view.backgroundColor = .white

        scoreLabel = UILabel()
        scoreLabel.translatesAutoresizingMaskIntoConstraints = false
        scoreLabel.textAlignment = .right
        scoreLabel.text = "Score: 0"
        view.addSubview(scoreLabel)
        
        cluesLabel = UILabel()
        cluesLabel.translatesAutoresizingMaskIntoConstraints = false
        cluesLabel.font = UIFont.systemFont(ofSize: 24)
        cluesLabel.text = "CLUES"
        cluesLabel.numberOfLines = 0
        cluesLabel.textAlignment = .center
        view.addSubview(cluesLabel)

        answersLabel = UILabel()
        answersLabel.translatesAutoresizingMaskIntoConstraints = false
        answersLabel.font = UIFont.systemFont(ofSize: 24)
        answersLabel.text = "ANSWERS"
        answersLabel.numberOfLines = 0
        answersLabel.textAlignment = .center
        view.addSubview(answersLabel)
        
        currentAnswer = UITextField()
        answersLabel.translatesAutoresizingMaskIntoConstraints = false
        currentAnswer.text = "Hi there!"
        currentAnswer.isUserInteractionEnabled = false
        currentAnswer.textAlignment = .center
        currentAnswer.font = UIFont.systemFont(ofSize: 44)
        view.addSubview(currentAnswer)
        
        NSLayoutConstraint.activate([
            scoreLabel.topAnchor.constraint(equalTo: view.layoutMarginsGuide.topAnchor),
            scoreLabel.trailingAnchor.constraint(equalTo: view.layoutMarginsGuide.trailingAnchor),

            // pin the top of the clues label to the bottom of the score label
            cluesLabel.topAnchor.constraint(equalTo: scoreLabel.bottomAnchor),

            // pin the leading edge of the clues label to the leading edge of our layout margins, adding 100 for some space
            cluesLabel.leadingAnchor.constraint(equalTo: view.layoutMarginsGuide.leadingAnchor),

            // make the clues label 60% of the width of our layout margins, minus 100
            cluesLabel.widthAnchor.constraint(equalTo: view.layoutMarginsGuide.widthAnchor, multiplier: 0.6),

            // also pin the top of the answers label to the bottom of the score label
            answersLabel.topAnchor.constraint(equalTo: scoreLabel.bottomAnchor),

            // make the answers label stick to the trailing edge of our layout margins, minus 100
            answersLabel.trailingAnchor.constraint(equalTo: view.layoutMarginsGuide.trailingAnchor),

            // make the answers label take up 40% of the available space, minus 100
            answersLabel.widthAnchor.constraint(equalTo: view.layoutMarginsGuide.widthAnchor, multiplier: 0.4),

            // make the answers label match the height of the clues label
            //answersLabel.heightAnchor.constraint(equalTo: cluesLabel.heightAnchor),
            
            
            currentAnswer.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            currentAnswer.widthAnchor.constraint(equalTo: view.widthAnchor, multiplier: 0.5),
            currentAnswer.topAnchor.constraint(equalTo: cluesLabel.bottomAnchor, constant: 20),
        ])
        
        cluesLabel.backgroundColor = .red
        answersLabel.backgroundColor = .blue
        currentAnswer.backgroundColor = .link
    }
}

当我尝试运行应用程序时，我在屏幕上看不到任何东西，但是，如果我删除这三行

...
            currentAnswer.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            currentAnswer.widthAnchor.constraint(equalTo: view.widthAnchor, multiplier: 0.5),
            currentAnswer.topAnchor.constraint(equalTo: cluesLabel.bottomAnchor, constant: 20),
...

一切正常。这里有什么技巧？

Answer 1

改变

answersLabel.translatesAutoresizingMaskIntoConstraints = false

至

currentAnswer.translatesAutoresizingMaskIntoConstraints = false

之后

currentAnswer = UITextField()

Answer 2

你没说过currentAnswer.translatesAutoresizingMaskIntoConstraints = false。因此 currentAnswer 必须 永远不会 受到任何限制。

注意 Xcode 控制台，当应用程序运行时，它应该会向您大声喊出此信息。

为什么在添加一项后所有 UI 都消失了？

Why all the UI vanished after one more item was added?

uikit

swift