Constraints not working when device is rotated - ios

Question

I have this UIViewController:
import UIKit
class ViewController: UIViewController {
var object: DraggableView?
override func viewDidLoad() {
super.viewDidLoad()
// Create the object
object = DraggableView(parent: self)
}
override func viewWillAppear(_ animated: Bool) {
super.viewWillAppear(animated)
// Add subview
object?.setup()
}
}
And I have this class to add the view in this VC:
import UIKit
class DraggableView {
var parent: UIViewController!
let pieceOfViewToShow: CGFloat = 30.0
init(parent: UIViewController) {
self.parent = parent
}
func setup() {
let view = UIView(frame: parent.view.frame)
view.backgroundColor = UIColor.red
parent.view.addSubview(view)
view.translatesAutoresizingMaskIntoConstraints = false
view.leadingAnchor.constraint(equalTo: parent.view.safeAreaLayoutGuide.leadingAnchor).isActive = true
view.trailingAnchor.constraint(equalTo: parent.view.safeAreaLayoutGuide.trailingAnchor).isActive = true
view.heightAnchor.constraint(equalTo: parent.view.safeAreaLayoutGuide.heightAnchor).isActive = true
// I need to show only a piece of the view at bottom, so:
view.topAnchor.constraint(equalTo: parent.view.safeAreaLayoutGuide.topAnchor, constant: parent.view.frame.height - pieceOfViewToShow).isActive = true
}
}
Problem
Everything is correct but when the device rotates it loses the constraint and the added view is lost.
I think the problem is in the next line that is not able to update the correct height [parent.view.frame.height] when the device is rotated.
view.topAnchor.constraint(equalTo: parent.view.safeAreaLayoutGuide.topAnchor, constant: parent.view.frame.height - pieceOfViewToShow).isActive = true
How could I make to update this constant when rotating?
I'm using Swift 3.

Answer 1

You can try using traitCollectionDidChange callback on the UIView to update the constraint when a rotation changes, for that to work you'll need to make DraggableView a subclass of the UIView:
import UIKit
class DraggableView: UIView {
var parent: UIViewController!
let pieceOfViewToShow: CGFloat = 30.0
// keep the constraint around to have access to it
var topConstraint: NSLayoutConstraint?
init(parent: UIViewController) {
super.init(frame: parent.view.frame)
self.parent = parent
}
required init?(coder aDecoder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
func setup() {
self.backgroundColor = UIColor.red
parent.view.addSubview(self)
self.translatesAutoresizingMaskIntoConstraints = false
self.leadingAnchor.constraint(equalTo: parent.view.safeAreaLayoutGuide.leadingAnchor).isActive = true
self.trailingAnchor.constraint(equalTo: parent.view.safeAreaLayoutGuide.trailingAnchor).isActive = true
self.heightAnchor.constraint(equalTo: parent.view.safeAreaLayoutGuide.heightAnchor).isActive = true
// keep a reference to the constraint
topConstraint = self.topAnchor.constraint(equalTo: parent.view.safeAreaLayoutGuide.topAnchor, constant: parent.view.frame.height - pieceOfViewToShow)
topConstraint?.isActive = true
}
override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
super.traitCollectionDidChange(previousTraitCollection)
// update the constraints constant
topConstraint?.constant = parent.view.frame.height - pieceOfViewToShow
}
}