Using Swift playground UIKit, Constraints are not working

Question

ChoiysApple OP

Created Apr ’21

Replies 1

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Views 1.2k

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I'm working on Swift Playground right now. I declared UI objects in loadView()

Code Block Swiftclass ViewController: UIViewController {
   
  override func loadView() {
    let view = UIView(frame: viewRect)
    view.backgroundColor = UIColor.white
     
    let titleLabel = UILabel()
    titleLabel.translatesAutoresizingMaskIntoConstraints = false
    titleLabel.text = "title"
    titleLabel.backgroundColor = .gray
     
     
    view.addSubview(titleLabel)
    NSLayoutConstraint.activate([
      titleLabel.widthAnchor.constraint(equalToConstant: 200),
      titleLabel.topAnchor.constraint(equalTo: view.topAnchor, constant: 100),
      titleLabel.centerXAnchor.constraint(equalTo: view.centerXAnchor)
    ])
     
    let subLabel = UILabel()
    subLabel.translatesAutoresizingMaskIntoConstraints = false
    subLabel.text = "title"
    subLabel.backgroundColor = .red
     
     
    view.addSubview(subLabel)
    NSLayoutConstraint.activate([
      subLabel.widthAnchor.constraint(equalToConstant: 100),
      subLabel.topAnchor.constraint(equalTo: titleLabel.bottomAnchor, constant: 100),
      subLabel.centerXAnchor.constraint(equalTo: view.centerXAnchor)
    ])
     
    self.view = view
  }
}

in this case, Everything works fine.

But I tried to declare those labels outside of loadView() like this:

Code Block Swiftclass ViewController: UIViewController {
   
  var titleLabel = UILabel()
  var subLabel = UILabel()
   
  override func loadView() {
    let view = UIView(frame: viewRect)
    view.backgroundColor = UIColor.white
    titleLabel = UILabel()
    titleLabel.translatesAutoresizingMaskIntoConstraints = false
    titleLabel.text = "title"
    titleLabel.backgroundColor = .gray
     
     
    view.addSubview(titleLabel)
    NSLayoutConstraint.activate([
      titleLabel.widthAnchor.constraint(equalToConstant: 200),
      titleLabel.topAnchor.constraint(equalTo: view.topAnchor, constant: 100),
      titleLabel.centerXAnchor.constraint(equalTo: view.centerXAnchor)
    ])
     
     
    subLabel.translatesAutoresizingMaskIntoConstraints = false
    subLabel.text = "title"
    subLabel.backgroundColor = .red
     
     
    view.addSubview(subLabel)
    NSLayoutConstraint.activate([
      subLabel.widthAnchor.constraint(equalToConstant: 100),
      subLabel.topAnchor.constraint(equalTo: titleLabel.bottomAnchor, constant: 100),
      subLabel.centerXAnchor.constraint(equalTo: view.centerXAnchor)
    ])
     
    self.view = view
  }
}

When I run this code (which labels are declared outside of loadView()), Constraints are not working except for width/height constraints.
Also, when i run the code only with 1 label, Everything works fine even label is declared outside of loadView()

Is there any solution to fix this problem?

Boost

Answer 1

OOPer OP

Apr ’21

An interesting issue.
It seems as if iOS (or Playground?) is removing some constraints on the properties after loadView().

Is there any solution to fix this problem?

Setting up subviews in viewDidLoad() would fix the issue:

Code Block class ViewController2: UIViewController {
    
    var titleLabel: UILabel!
    var subLabel: UILabel!
    
    override func loadView() {
        let view = UIView(frame: viewRect)
        view.backgroundColor = UIColor.white
        
        self.view = view
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        titleLabel = UILabel()
        titleLabel.translatesAutoresizingMaskIntoConstraints = false
        titleLabel.text = "title"
        titleLabel.backgroundColor = .gray
        
        view.addSubview(titleLabel)
        NSLayoutConstraint.activate([
            titleLabel.widthAnchor.constraint(equalToConstant: 200),
            titleLabel.topAnchor.constraint(equalTo: view.topAnchor, constant: 100),
            titleLabel.centerXAnchor.constraint(equalTo: view.centerXAnchor)
        ])
        
        subLabel = UILabel()
        subLabel.translatesAutoresizingMaskIntoConstraints = false
        subLabel.text = "title"
        subLabel.backgroundColor = .red
        
        view.addSubview(subLabel)
        NSLayoutConstraint.activate([
            subLabel.widthAnchor.constraint(equalToConstant: 100),
            subLabel.topAnchor.constraint(equalTo: titleLabel.bottomAnchor, constant: 100),
            subLabel.centerXAnchor.constraint(equalTo: view.centerXAnchor)
        ])
    }
}

But I do not understand what's happening behind and am not sure if this can be the solution for you.

0