Constraints in UITableViewCell not being calculated - ios

Question

I have the following cell with constraints being setup programmatically:
class RadioButtonCell: UITableViewCell {
static let identifier = "RadioButtonCell"
let radioButton = RadioButton()
let labelTitle = UILabel()
private var didUpdateConstraints = false
override init(style: UITableViewCellStyle, reuseIdentifier: String?) {
super.init(style: style, reuseIdentifier: reuseIdentifier)
setupSubViews()
}
required init?(coder aDecoder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
private func setupSubViews() {
radioButton.translatesAutoresizingMaskIntoConstraints = false
labelTitle.translatesAutoresizingMaskIntoConstraints = false
contentView.addSubview(radioButton)
contentView.addSubview(labelTitle)
}
override func updateConstraints() {
super.updateConstraints()
if !didUpdateConstraints {
radioButton.anchor(leading: contentView.leadingAnchor, padding: UIEdgeInsets(top: 0, left: Constants.UI.defaultMarginX2, bottom: 0, right: 0))
radioButton.centerYAnchor.constraint(equalTo: contentView.centerYAnchor).isActive = true
labelTitle.anchor(leading: radioButton.trailingAnchor, padding: UIEdgeInsets(top: 0, left: Constants.UI.defaultMarginX2, bottom: 0, right: 0))
labelTitle.centerYAnchor.constraint(equalTo: contentView.centerYAnchor).isActive = true
didUpdateConstraints = true
}
}
}
The anchor method is just a helper method to add constraints. The constraints are setup correctly (no issues with Autolayout).
Then in the cellForRowAtIndexPath method I create the cell like so:
guard let cell = tableView.dequeueReusableCell(withIdentifier: RadioButtonCell.identifier, for: indexPath) as? RadioButtonCell else { return UITableViewCell() }
radtioButtonController.addButton(cell.radioButton)
cell.labelTitle.text = "test"
return cell
This creates me the following layout in the tableView (which is obviously wrong):
If I move the setup of the constraints to the setupSubViews() method, the layout is correct:
class RadioButtonCell: UITableViewCell {
static let identifier = "RadioButtonCell"
let radioButton = RadioButton()
let labelTitle = UILabel()
private var didUpdateConstraints = false
override init(style: UITableViewCellStyle, reuseIdentifier: String?) {
super.init(style: style, reuseIdentifier: reuseIdentifier)
setupSubViews()
}
required init?(coder aDecoder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
private func setupSubViews() {
radioButton.translatesAutoresizingMaskIntoConstraints = false
labelTitle.translatesAutoresizingMaskIntoConstraints = false
contentView.addSubview(radioButton)
contentView.addSubview(labelTitle)
radioButton.anchor(leading: contentView.leadingAnchor, padding: UIEdgeInsets(top: 0, left: Constants.UI.defaultMarginX2, bottom: 0, right: 0))
radioButton.centerYAnchor.constraint(equalTo: contentView.centerYAnchor).isActive = true
labelTitle.anchor(leading: radioButton.trailingAnchor, padding: UIEdgeInsets(top: 0, left: Constants.UI.defaultMarginX2, bottom: 0, right: 0))
labelTitle.centerYAnchor.constraint(equalTo: contentView.centerYAnchor).isActive = true
}
}
Why is this like that? I thought we should setup constraint in the updateConstraints method...
Thank you for an answer :)
Edit I found out, that it works with updateConstraints when I call cell.updateConstraintsIfNeeded() or cell.setNeedsUpdateConstraints() in cellForRowAtIndexPath. Why do we need to tell the cell to calculate the constraints again? We do not need to do that when adding the constraints with IB...

Answer 1

Well, in your first example you haven't just added the constraints to your UI elements in the init. You just set up your sub views.
override init(style: UITableViewCellStyle, reuseIdentifier: String?) {
super.init(style: style, reuseIdentifier: reuseIdentifier)
setupSubViews()
// No explicit constraint setup happened
}
To setup your constraints you've overridden the override func updateConstraints() { ... } method. Let's take a look at the official documentation from Apple.
In short, your override will be in effect when you notify the system that you need constraints' update. As a result you need to explicitly inform the system by invoking setNeedsUpdateConstraints() or updateConstraintsIfNeeded().
Let's look at your second example. You embedded the constraints setup inside your private func setupSubViews(){ ... }. So at the time this function gets called, your constraints are ready to be applied. No system call needed.