Setting width constraint to a UIView with a function call doesn't work anymore. iOS ( Xcode 12.0.1 )

Question

I had the following code in Swift to fill a status bar within its container, in relation to the completion of a quiz percentage by changing its width dynamically and it worked fine in 2018:

func updateUI() {
    questionCounter.text = "\(Texts.questionCounter) \(questionNumber + 1)"
    progressBar.frame.size.width = (containerOfBar.frame.size.width / CGFloat(allQuestions.list.count)) * CGFloat(questionNumber)
}

The instantiation of the elements have been made by closures in this way:

private let containerOfBar: UIView = {
    let view = UIView()
    view.translatesAutoresizingMaskIntoConstraints = false
    view.backgroundColor = .white
    view.layer.cornerRadius = 8
    view.layer.borderColor = UIColor.white.cgColor
    view.layer.borderWidth = 2
    
    return view
}()

private let progressBar: UIView = {
    let bar = UIView()
    bar.backgroundColor = .blue
    bar.translatesAutoresizingMaskIntoConstraints = false
    
    return bar
}()

The auto-layout graphic constraints for the container and the bar, have been set in the following code only without a storyboard.

The bar itself:

progressBar.leadingAnchor.constraint(equalTo: containerOfBar.leadingAnchor, constant: 2),
progressBar.topAnchor.constraint(equalTo: containerOfBar.topAnchor, constant: 2),
progressBar.bottomAnchor.constraint(equalTo: containerOfBar.bottomAnchor, constant: 2),

The container of the bar:

containerOfBar.centerXAnchor.constraint(equalTo: optionsViewContainer.centerXAnchor),
containerOfBar.topAnchor.constraint(equalTo: optionsView[enter image description here][1].bottomAnchor, constant: self.view.frame.size.height/42),
containerOfBar.bottomAnchor.constraint(equalTo: optionsViewContainer.bottomAnchor, constant: -self.view.frame.size.height/42), 
containerOfBar.widthAnchor.constraint(equalTo: optionsViewContainer.widthAnchor, multiplier: 0.3),

In the link, there is the image of the completion bar drawn by code. Can't understand why the frame.width property doesn't work anymore, maybe a change in constraints workflow logic that I am missing... I tried also to use the code of the function separately, but it seems like frame.width is not dynamically usable anymore.

Any suggestions?

Answer 1

You are mixing constraints with explicit frame settings, which won't give you the desired results. Each time auto-layout updates the screen, it will reset your progressBar.frame.size.width back to its constraint value -- in this case, it will be Zero because you didn't give it one.

A better approach is to set a Width Anchor on the progressBar. Make it equal to the Width Anchor of containerOfBar, with a multiplier of the percent of progress, and a constant of -4 (so you have 2-pts on each side).

Here's an example. It uses a questionCounter of 10 ... each time you tap the screen, it will increment the "current question number" and update the progress bar:

class ProgViewController: UIViewController {
    
    private let containerOfBar: UIView = {
        let view = UIView()
        view.translatesAutoresizingMaskIntoConstraints = false
        view.backgroundColor = .white
        view.layer.cornerRadius = 8
        view.layer.borderColor = UIColor.white.cgColor
        view.layer.borderWidth = 2
        
        return view
    }()
    
    
    private let progressBar: UIView = {
        let bar = UIView()
        bar.backgroundColor = .blue
        bar.translatesAutoresizingMaskIntoConstraints = false
        
        return bar
    }()

    private let questionCounter: UILabel = {
        let v = UILabel()
        v.backgroundColor = .cyan
        v.translatesAutoresizingMaskIntoConstraints = false
        return v
    }()
    
    var numberOfQuestions = 10
    var questionNumber = 0
    
    // width constraint of progressBar
    var progressBarWidthConstraint: NSLayoutConstraint!
    
    override func viewDidLoad() {
        super.viewDidLoad()

        view.backgroundColor = .systemYellow
        
        containerOfBar.addSubview(progressBar)
        view.addSubview(containerOfBar)
        view.addSubview(questionCounter)

        // create width constraint of progressBar
        //  start at 0% (multiplier: 0)
        //  this will be changed by updateUI()
        progressBarWidthConstraint = progressBar.widthAnchor.constraint(equalTo: containerOfBar.widthAnchor, multiplier: 0, constant: -4)
        progressBarWidthConstraint.priority = .defaultHigh
        
        NSLayoutConstraint.activate([
            
            progressBarWidthConstraint,
            
            progressBar.leadingAnchor.constraint(equalTo: containerOfBar.leadingAnchor, constant: 2),
            progressBar.topAnchor.constraint(equalTo: containerOfBar.topAnchor, constant: 2),
            progressBar.bottomAnchor.constraint(equalTo: containerOfBar.bottomAnchor, constant: -2),
            
            //The container of the bar:
            
            containerOfBar.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            containerOfBar.topAnchor.constraint(equalTo: view.topAnchor, constant: 100),
            containerOfBar.heightAnchor.constraint(equalToConstant: 50),
            containerOfBar.widthAnchor.constraint(equalTo: view.widthAnchor, multiplier: 0.9),
        
            // label under the container
            questionCounter.topAnchor.constraint(equalTo: containerOfBar.bottomAnchor, constant: 8.0),
            questionCounter.leadingAnchor.constraint(equalTo: containerOfBar.leadingAnchor),
            questionCounter.trailingAnchor.constraint(equalTo: containerOfBar.trailingAnchor),
            
        ])

        // every time we tap on the screen, we'll increment the question number
        let tap = UITapGestureRecognizer(target: self, action: #selector(self.nextQuestion(_:)))
        view.addGestureRecognizer(tap)
        
        updateUI()
    }
    
    @objc func nextQuestion(_ g: UITapGestureRecognizer) -> Void {
        // increment the question number
        questionNumber += 1
        // don't exceed number of questions
        questionNumber = min(numberOfQuestions - 1, questionNumber)
        updateUI()
    }
    
    func updateUI() {
        
        questionCounter.text = "Question: \(questionNumber + 1) of \(numberOfQuestions) total questions."
        
        // get percent completion
        //  for example, if we're on question 4 of 10,
        //  percent will be 0.4
        let percent: CGFloat = CGFloat(questionNumber + 1) / CGFloat(numberOfQuestions)
        
        // we can't change the multiplier directly, so
        // deactivate the width constraint
        progressBarWidthConstraint.isActive = false
        
        // re-create it with current percentage of width
        progressBarWidthConstraint = progressBar.widthAnchor.constraint(equalTo: containerOfBar.widthAnchor, multiplier: percent, constant: -4)
        
        // activate it
        progressBarWidthConstraint.isActive = true
        
        // don't mix frame settings with auto-layout constraints
        //progressBar.frame.size.width = (containerOfBar.frame.size.width / CGFloat(allQuestions.list.count)) * CGFloat(questionNumber)
        
    }
    
}

It will look like this: