Swift - Update auto layout properties of UI components programmatically - ios

Question

To design and create my UI, I always use auto layouts and do it programmatically instead of using storyboard.
In every view class of mine, I have a method called
private func setupView(frame:CGRect) {
/* START CONTAINER VIEW */
containerView = UIView()
containerView.translatesAutoresizingMaskIntoConstraints = false
addSubview(containerView)
containerView.leadingAnchor.constraint(equalTo: leadingAnchor, constant: frame.width * (13 / IPHONE8_SCREEN_WIDTH)).isActive = true
containerView.trailingAnchor.constraint(equalTo: trailingAnchor, constant: -frame.width * (13 / IPHONE8_SCREEN_WIDTH)).isActive = true
containerView.topAnchor.constraint(equalTo: topAnchor, constant: frame.height * (26 / IPHONE8_SCREEN_HEIGHT)).isActive = true
containerView.bottomAnchor.constraint(equalTo: bottomAnchor).isActive = true
containerView.backgroundColor = UIColor.white
/* END CONTAINER VIEW */
...
}
to initialize the components. Now let's say, in the method above, I initialize 10 UI components which are properly displayed when I run my code. However, depending on some variables, I have another function that is being called
private func addNextRoundInformation() {
..
nextRoundLabel = UILabel()
nextRoundLabel.translatesAutoresizingMaskIntoConstraints = false
containerView.addSubview(nextRoundLabel)
nextRoundLabel.leadingAnchor.constraint(equalTo: currentRoundLabel.leadingAnchor).isActive = true
nextRoundLabel.widthAnchor.constraint(equalTo:currentRoundLabel.widthAnchor).isActive = true
nextRoundLabel.topAnchor.constraint(equalTo: roundEndsInLabel.bottomAnchor, constant: frame.height * (19 / IPHONE8_SCREEN_HEIGHT)).isActive = true
}
which should place a new label between some others which were already initialized.
Of course, when putting the new label between some particular ones, I also update the auto layout constraints of the of the bottom label like
private func updateNumberOfWinnersLabelConstraint() {
numberOfWinnersPerRoundLabel.topAnchor.constraint(equalTo: nextRoundLabel.bottomAnchor, constant: frame.height * (19 / IPHONE8_SCREEN_HEIGHT)).isActive = true
numberOfWinnersPerRoundLabelValue.topAnchor.constraint(equalTo: nextRoundLabelValue.bottomAnchor, constant: frame.height * (19 / IPHONE8_SCREEN_HEIGHT)).isActive = true
}
The topAnchor of each label depends on the bottom anchor of the previous one.
With this approach, I can't see nextRoundLabel at all. It only appears, if I initialize it in the private func setupView(frame:CGRect) {}
Why?

Answer 1

You could do this with "Top-to-Bottom" constraints, but it would be rather complex.
You would need to essentially create a "Linked List" to track each view, the views above and below it, and its constraints.
So, to "insert" a new view after the 3rd view, you would need to:
deactivate the inter-view constraints
insert the new view into the linked list
re-create and activate the new constraints
Putting your views in a UIStackView turns that process into a single line of code:
stackView.insertArrangedSubview(newView, at: 3)
Here's a quick example:
class ViewController: UIViewController {
let testView: SampleView = SampleView()
override func viewDidLoad() {
super.viewDidLoad()
view.backgroundColor = .systemYellow
let infoLabel: UILabel = {
let v = UILabel()
v.textAlignment = .center
v.numberOfLines = 0
v.text = "Tap to Insert \"New Label\"\nafter \"Label 3\""
return v
}()
let btn: UIButton = {
let v = UIButton()
v.setTitle("Insert", for: [])
v.setTitleColor(.white, for: .normal)
v.setTitleColor(.lightGray, for: .highlighted)
v.backgroundColor = .systemBlue
return v
}()
[infoLabel, btn, testView].forEach { v in
v.translatesAutoresizingMaskIntoConstraints = false
view.addSubview(v)
}
let g = view.safeAreaLayoutGuide
NSLayoutConstraint.activate([
infoLabel.topAnchor.constraint(equalTo: g.topAnchor, constant: 40.0),
infoLabel.widthAnchor.constraint(equalTo: g.widthAnchor, multiplier: 0.75),
infoLabel.centerXAnchor.constraint(equalTo: g.centerXAnchor),
btn.topAnchor.constraint(equalTo: infoLabel.bottomAnchor, constant: 20.0),
btn.widthAnchor.constraint(equalTo: g.widthAnchor, multiplier: 0.75),
btn.centerXAnchor.constraint(equalTo: g.centerXAnchor),
testView.topAnchor.constraint(equalTo: btn.bottomAnchor, constant: 40.0),
testView.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 40.0),
testView.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -40.0),
testView.bottomAnchor.constraint(equalTo: g.bottomAnchor, constant: -40.0),
])
btn.addTarget(self, action: #selector(btnTapped(_:)), for: .touchUpInside)
}
#objc func btnTapped(_ sender: Any?) {
testView.insertNew()
}
}
class SampleView: UIView {
var containerView: UIView!
var stackView: UIStackView!
override init(frame: CGRect) {
super.init(frame: frame)
commonInit()
}
required init?(coder: NSCoder) {
super.init(coder: coder)
commonInit()
}
private func commonInit() {
backgroundColor = .red
setupView(frame: .zero)
}
private func setupView(frame:CGRect) {
/* START CONTAINER VIEW */
containerView = UIView()
containerView.translatesAutoresizingMaskIntoConstraints = false
addSubview(containerView)
stackView = UIStackView()
stackView.axis = .vertical
stackView.spacing = 8
stackView.translatesAutoresizingMaskIntoConstraints = false
containerView.addSubview(stackView)
NSLayoutConstraint.activate([
containerView.leadingAnchor.constraint(equalTo: leadingAnchor, constant: 12),
containerView.trailingAnchor.constraint(equalTo: trailingAnchor, constant: -12),
containerView.topAnchor.constraint(equalTo: topAnchor, constant: 12),
containerView.bottomAnchor.constraint(equalTo: bottomAnchor),
stackView.topAnchor.constraint(equalTo: containerView.topAnchor, constant: 8.0),
stackView.leadingAnchor.constraint(equalTo: containerView.leadingAnchor, constant: 8.0),
stackView.trailingAnchor.constraint(equalTo: containerView.trailingAnchor, constant: -8.0),
])
containerView.backgroundColor = UIColor.white
/* END CONTAINER VIEW */
// add 10 labels to the stack view
for i in 1...10 {
let v = UILabel()
v.text = "Label \(i)"
v.backgroundColor = .green
stackView.addArrangedSubview(v)
}
}
func insertNew() {
let v = UILabel()
v.text = "New Label"
v.backgroundColor = .cyan
v.translatesAutoresizingMaskIntoConstraints = false
// we're adding a label after 3rd label
stackView.insertArrangedSubview(v, at: 3)
}
}
It starts looking like this:
after tapping the "Insert" button, it looks like this:
Edit
To explain why your current approach isn't working...
Starting with this layout:
each label's Top is constrained to the previous label's Bottom (with constant: spacing).
Those constraints are indicated by the blue arrows.
You then want to "insert" Next Round Label between Round Ends In and Winners Per Round:
Your code:
adds the label
adds a constraint from the Top of Next Round Label to the Bottom of Round Ends In
adds a constraint from the Top of Winners Per Round to the Bottom of Next Round Label
but... Winners Per Round already has a .topAnchor connected to Round Ends In, so it now has two top anchors.
The conflicting constraints are shown in red:
As I said, I think your description of what you're trying to do would lend itself to using stack views, and would make "inserting" views so much easier.
But, if you need to stick with your current "Top-to-Bottom" constraints approach, you have several options.
One - remove all the labels, then re-add and re-constrain them, including the one you're inserting.
Two - track the constraints (using an array or custom object properties) so you can deactivate the conflicting constraint(s).
Three - use some code along the lines of
let theConstraint = containerView.constraints.first(where: {($0.secondItem as? UILabel) == roundEndsInLabel})
to "find" the constraint that needs to be deactivated.

Answer 2

I found a working solution:
First, I declared two helper variables
/* START HELPER VARIABLES */
var numberOfWinnersLabelTopConstraint:NSLayoutConstraint?
var numberOfWinnersLabelValueTopConstraint:NSLayoutConstraint?
/* END HELPER VARIABLES */
then I made some minor adjustments in my setupView function:
private func setupView(frame:CGRect) {
...
/* START NUMBER OF WINNERS PER ROUND LABEL */
numberOfWinnersPerRoundLabel = UILabel()
numberOfWinnersPerRoundLabel.translatesAutoresizingMaskIntoConstraints = false
containerView.addSubview(numberOfWinnersPerRoundLabel)
numberOfWinnersPerRoundLabel.leadingAnchor.constraint(equalTo: currentRoundLabel.leadingAnchor).isActive = true
numberOfWinnersPerRoundLabel.widthAnchor.constraint(equalTo:currentRoundLabel.widthAnchor).isActive = true
// numberOfWinnersPerRoundLabel.topAnchor.constraint(equalTo: roundEndsInLabel.bottomAnchor, constant: frame.height * (19 / IPHONE8_SCREEN_HEIGHT)).isActive = true // Replacing with helper variable
numberOfWinnersLabelTopConstraint = numberOfWinnersPerRoundLabel.topAnchor.constraint(equalTo: roundEndsInLabel.bottomAnchor, constant: frame.height * (19 / IPHONE8_SCREEN_HEIGHT))
numberOfWinnersLabelTopConstraint?.isActive = true
numberOfWinnersPerRoundLabel.text = NSLocalizedString(NUMBER_OF_WINNERS_PER_ROUND_TEXT, comment: "")
numberOfWinnersPerRoundLabel.textColor = .darkGray
numberOfWinnersPerRoundLabel.adjustsFontSizeToFitWidth = true
numberOfWinnersPerRoundLabel.font = .systemFont(ofSize: 12)
/* END NUMBER OF WINNERS PER ROUND LABEL */
/* START NUMBER OF WINNERS PER ROUND LABEL VALUE */
numberOfWinnersPerRoundLabelValue = UILabel()
numberOfWinnersPerRoundLabelValue.translatesAutoresizingMaskIntoConstraints = false
containerView.addSubview(numberOfWinnersPerRoundLabelValue)
numberOfWinnersPerRoundLabelValue.leadingAnchor.constraint(equalTo: currentRoundLabelValue.leadingAnchor).isActive = true
numberOfWinnersPerRoundLabelValue.widthAnchor.constraint(equalTo:currentRoundLabelValue.widthAnchor).isActive = true
// numberOfWinnersPerRoundLabelValue.topAnchor.constraint(equalTo: numberOfWinnersPerRoundLabel.topAnchor).isActive = true // replacing with helper variable
numberOfWinnersLabelValueTopConstraint = numberOfWinnersPerRoundLabelValue.topAnchor.constraint(equalTo: numberOfWinnersPerRoundLabel.topAnchor)
numberOfWinnersLabelValueTopConstraint?.isActive = true
numberOfWinnersPerRoundLabelValue.textColor = .black
/* END NUMBER OF WINNERS PER ROUND LABEL VALUE */
By introducing the helper variables, I could easily deactivate the topConstraint when adding the nextRoundLabel
private func addNextRoundInformation() {
nextRoundLabel = UILabel()
nextRoundLabel.translatesAutoresizingMaskIntoConstraints = false
containerView.addSubview(nextRoundLabel)
nextRoundLabel.leadingAnchor.constraint(equalTo: currentRoundLabel.leadingAnchor).isActive = true
nextRoundLabel.widthAnchor.constraint(equalTo:currentRoundLabel.widthAnchor).isActive = true
nextRoundLabel.topAnchor.constraint(equalTo: roundEndsInLabel.bottomAnchor, constant: frame.height * (19 / IPHONE8_SCREEN_HEIGHT)).isActive = true
nextRoundLabel.text = "Next round starts in"
nextRoundLabel.textColor = .darkGray
nextRoundLabel.font = .systemFont(ofSize: 12)
nextRoundLabelValue = UILabel()
nextRoundLabelValue.translatesAutoresizingMaskIntoConstraints = false
containerView.addSubview(nextRoundLabelValue)
nextRoundLabelValue.leadingAnchor.constraint(equalTo: currentRoundLabelValue.leadingAnchor).isActive = true
nextRoundLabelValue.widthAnchor.constraint(equalTo:currentRoundLabelValue.widthAnchor).isActive = true
nextRoundLabelValue.topAnchor.constraint(equalTo:nextRoundLabel.topAnchor).isActive = true
nextRoundLabelValue.textColor = .black
nextRoundLabelValue.text = "Next round label value"
nextRoundLabelValue.font = .systemFont(ofSize: 14)
}
private func updateNumberOfWinnersLabelConstraint() {
numberOfWinnersLabelTopConstraint?.isActive = false // Deactivate previous constraint
numberOfWinnersLabelValueTopConstraint?.isActive = false
numberOfWinnersLabelTopConstraint = numberOfWinnersPerRoundLabel.topAnchor.constraint(equalTo: nextRoundLabel.bottomAnchor, constant: frame.height * (19 / IPHONE8_SCREEN_HEIGHT))
numberOfWinnersLabelTopConstraint?.isActive = true
numberOfWinnersLabelValueTopConstraint = numberOfWinnersPerRoundLabelValue.topAnchor.constraint(equalTo: nextRoundLabelValue.bottomAnchor, constant: frame.height * (19 / IPHONE8_SCREEN_HEIGHT))
numberOfWinnersLabelValueTopConstraint?.isActive = true
}
Basically, I only had to update the topConstraints of the numberOfWinnersPerRoundLabel and numberOfWinnersPerRoundLabelValue
since everything else would be the same. No changes needed for currentRoundLabel.
I tested it and it worked!