I am moving through tutorial and there is such code example

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
    }
}

When I try to run the app I see nothing on the screen, however, if I delete these three lines

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

everything is working. What is a trick here?

CodePudding user response：

Change

answersLabel.translatesAutoresizingMaskIntoConstraints = false

to

currentAnswer.translatesAutoresizingMaskIntoConstraints = false

after

currentAnswer = UITextField()

CodePudding user response：

You never said currentAnswer.translatesAutoresizingMaskIntoConstraints = false. Therefore currentAnswer must never be given or touched by any constraints.

Be observant of the Xcode console, which should be shouting this information quite loudly at you when the app runs.