UIView internal constraints - ios

Question

I have created a simple UIView that contains a red box (UIImage) at its centre. When all the constraints are constants the code works fine. However, if I replace the height constraint with one that makes the box half the height of the view then the box disappears.
I assume that this is either because I am doing it wrong (obviously) or I need to do something more to force the constraint to realise the UIView height is greater than zero.
How do I set the redBox height constraint so that it is always half the height of the BoxView?
import UIKit
class BoxView: UIView {
public var redBox: UIImageView
public override init(frame: CGRect) {
redBox = UIImageView(frame: .zero)
redBox.backgroundColor = .red
super.init(frame: frame)
self.backgroundColor = .yellow
addSubview(redBox)
redBox.translatesAutoresizingMaskIntoConstraints = false
let margins = layoutMarginsGuide
redBox.centerXAnchor.constraint(equalTo: self.centerXAnchor).isActive = true
redBox.centerYAnchor.constraint(equalTo: self.centerYAnchor).isActive = true
redBox.widthAnchor.constraint(equalToConstant: 100).isActive = true
//redBox.heightAnchor.constraint(equalToConstant: 100).isActive = true
redBox.heightAnchor.constraint(equalTo: self.heightAnchor, constant: 0.5)
}
required init?(coder aDecoder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
}
class ViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
view = BoxView()
}
}

Answer 1

Replace
redBox.heightAnchor.constraint(equalTo: self.heightAnchor, constant: 0.5)
with
redBox.heightAnchor.constraint(equalTo: self.heightAnchor, multiplier: 0.5).isActive = true
NSLayoutConstraint.activate([
redBox.centerXAnchor.constraint(equalTo: self.centerXAnchor),
redBox.centerYAnchor.constraint(equalTo: self.centerYAnchor),
redBox.widthAnchor.constraint(equalToConstant: 100),
redBox.heightAnchor.constraint(equalTo: self.heightAnchor, multiplier: 0.5)
])
In your current code first you miss the .isActive = true which has the same effect as if the line doesn't exist , and if specified this will make the box height equal to the view's height + constant ( = 0.5 )
box height = view height * multiplier + constant
and since default multiplier = 1 and you set constant = 0.5 this will be
box height = view height * 1.0 + 0.5
But instead you need
box height = view height * 0.5 + 0 // omit consatnt in constraint and it will be zero
class BoxView: UIView {
public var redBox: UIImageView
public override init(frame: CGRect) {
super.init(frame: frame)
redBox = UIImageView(frame: .zero)
redBox.backgroundColor = .red
self.backgroundColor = .yellow
addSubview(redBox)
redBox.translatesAutoresizingMaskIntoConstraints = false
let margins = layoutMarginsGuide
NSLayoutConstraint.activate([
redBox.centerXAnchor.constraint(equalTo: self.centerXAnchor),
redBox.centerYAnchor.constraint(equalTo: self.centerYAnchor),
redBox.widthAnchor.constraint(equalToConstant: 100),
redBox.heightAnchor.constraint(equalTo: self.heightAnchor, multiplier: 0.5)
])
}
required init?(coder aDecoder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
}