Adding Constraint Programmatically Does Not Work

Question

I'm setting constraints to image view programmatically like so:

        imgScrollView = UIScrollView(frame: CGRect(x: 0, y: 0, width: 100, height: 100))
        imgScrollView.showsHorizontalScrollIndicator = false
        imgScrollView.showsVerticalScrollIndicator = false
        imgScrollView.bouncesZoom = false
        imgScrollView.bounces = false

        view.addSubview(imgScrollView)

        imgScrollView.translatesAutoresizingMaskIntoConstraints = false
        imgScrollView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor, constant: 0).isActive = true
        imgScrollView.bottomAnchor.constraint(equalTo: toolBar.topAnchor, constant: -100).isActive = true
        imgScrollView.leftAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leftAnchor, constant: 0).isActive = true
        imgScrollView.rightAnchor.constraint(equalTo: view.safeAreaLayoutGuide.rightAnchor, constant: 0).isActive = true



        // add image view to scrollview
        imgView = UIImageView(frame: CGRect(x: 0, y: 0,width: 100, height: 100))

        imgScrollView.addSubview(imgView)
        imgView.translatesAutoresizingMaskIntoConstraints = false

        imgView.topAnchor.constraint(equalTo: imgScrollView.contentLayoutGuide.topAnchor, constant: 0).isActive = true
        imgView.bottomAnchor.constraint(equalTo: imgScrollView.contentLayoutGuide.bottomAnchor, constant: 0).isActive = true
        imgView.leftAnchor.constraint(equalTo: imgScrollView.contentLayoutGuide.leftAnchor, constant: 0).isActive = true
        imgView.rightAnchor.constraint(equalTo: imgScrollView.contentLayoutGuide.rightAnchor, constant: 0).isActive = true


        imgView.widthAnchor.constraint(equalTo: imgScrollView.widthAnchor, multiplier: 1).isActive = true

Now later on with button tap I'm adding additional constraint

imgView.heightAnchor.constraint(equalTo: imgScrollView.heightAnchor, multiplier: 1).isActive = true

However the constraint is not being added. Why is it happening?

Answer 1

Your implementation logic working for me.

• Left image show implementation without height constraint.
• Right image show button pressed with height constraint.

In function viewDidLoad

override func viewDidLoad() {
  super.viewDidLoad()
  view.backgroundColor = .white
  scrollView.isScrollEnabled = true
  scrollView.bounces = false
  scrollView.alwaysBounceVertical = true
  imageView.image = UIImage(color: .lightGray)

  navigationItem.rightBarButtonItem = barButtonItem
  view.addSubview(scrollView)
  scrollView.addSubview(imageView)

  scrollView.translatesAutoresizingMaskIntoConstraints = false
  imageView.translatesAutoresizingMaskIntoConstraints = false
  NSLayoutConstraint.activate([
    scrollView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor),
    scrollView.leftAnchor.constraint(equalTo: view.leftAnchor),
    scrollView.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor),
    scrollView.rightAnchor.constraint(equalTo: view.rightAnchor)
  ])
  NSLayoutConstraint.activate([
    imageView.topAnchor.constraint(equalTo: scrollView.contentLayoutGuide.topAnchor),
    imageView.leftAnchor.constraint(equalTo: scrollView.contentLayoutGuide.leftAnchor),
    imageView.bottomAnchor.constraint(equalTo: scrollView.contentLayoutGuide.bottomAnchor),
    imageView.rightAnchor.constraint(equalTo: scrollView.contentLayoutGuide.rightAnchor),
    imageView.widthAnchor.constraint(equalTo: scrollView.widthAnchor)
  ])
}

In function buttonPressed

@objc func buttonPressed() {
  navigationItem.rightBarButtonItem = nil
  UIView.animate(withDuration: 1) {
    NSLayoutConstraint.activate([
      self.imageView.heightAnchor.constraint(equalTo: self.scrollView.heightAnchor)
    ])
    self.view.setNeedsLayout()
    self.view.layoutIfNeeded()
  }
}

UIImage util Create UIImage with solid color in Swift

fileprivate extension UIImage {
  convenience init?(color: UIColor, size: CGSize = CGSize(width: 1, height: 1)) {
    let rect = CGRect(origin: .zero, size: size)
    UIGraphicsBeginImageContextWithOptions(rect.size, false, 0.0)
    color.setFill()
    UIRectFill(rect)
    let image = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()
    guard let cgImage = image?.cgImage else { return nil }
    self.init(cgImage: cgImage)
  }
}