Adding constraints on an existing textLabel in custom UITableViewHeaderFooterView - ios

Question

I wanted to add a simple counter of the number of objects in the table in the table header, next to its textLabel. So I created this class:
import UIKit
class CounterHeaderView: UITableViewHeaderFooterView {
static let reuseIdentifier: String = String(describing: self)
var counterLabel: UILabel
override init(reuseIdentifier: String?) {
counterLabel = UILabel()
super.init(reuseIdentifier: reuseIdentifier)
contentView.addSubview(counterLabel)
counterLabel.translatesAutoresizingMaskIntoConstraints = false
counterLabel.backgroundColor = .red
if let textLabel = self.textLabel{
counterLabel.leadingAnchor.constraint(equalTo: textLabel.trailingAnchor, constant: 6.0).isActive = true
counterLabel.topAnchor.constraint(equalTo: textLabel.topAnchor).isActive = true
counterLabel.heightAnchor.constraint(equalToConstant: 24.0).isActive = true
}
}
required init?(coder aDecoder: NSCoder) {
counterLabel = UILabel()
super.init(coder: aDecoder)
}
}
But running this results in the following error:
'Unable to activate constraint with anchors
<NSLayoutXAxisAnchor:0x60000388ae00 "UILabel:0x7fb8314710a0.leading">
and <NSLayoutXAxisAnchor:0x60000388ae80 "_UITableViewHeaderFooterViewLabel:0x7fb8314718c0.trailing">
because they have no common ancestor.
Does the constraint or its anchors reference items in different view hierarchies?
That's illegal.'
How can I add a constraint for my counterLabel based on the already existing textLabel? Isn't textLabel already a subview of ContentView?

Answer 1

You're trying to use built-in textLabel, which I'm pretty sure isn't available at the init time. Try to execute your layouting code inside layoutSubviews method, right after super call. The method could be evaluated a couple of times, so you should check if you've already layouted your view (e.g. couterLabel.superview != nil)
here's how it should looks like:
final class CounterHeaderView: UITableViewHeaderFooterView {
static let reuseIdentifier: String = String(describing: self)
let counterLabel = UILabel()
override func layoutSubviews() {
super.layoutSubviews()
if counterLabel.superview == nil {
layout()
}
}
func layout() {
contentView.addSubview(counterLabel)
counterLabel.translatesAutoresizingMaskIntoConstraints = false
counterLabel.backgroundColor = .red
if let textLabel = self.textLabel {
counterLabel.leadingAnchor.constraint(equalTo: textLabel.trailingAnchor, constant: 6.0).isActive = true
counterLabel.topAnchor.constraint(equalTo: textLabel.topAnchor).isActive = true
counterLabel.heightAnchor.constraint(equalToConstant: 24.0).isActive = true
}
}
}