TraitView.swift

class TraitView: UIView {

	let c: [UIColor] = [.red, .green, .blue]
	var i = 0
	
	override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
		print("trait changed \(i)")
		super.traitCollectionDidChange(previousTraitCollection)
		layer.backgroundColor = c[i % 3].cgColor
		i = i + 1
	}
	
}

class PresentTestViewController: UIViewController {
	
	override func viewDidLoad() {
		super.viewDidLoad()
		
		let btn = UIButton()
		btn.setTitle("Tap Me", for: .normal)
		btn.backgroundColor = .lightGray
		
		btn.translatesAutoresizingMaskIntoConstraints = false
		
		view.addSubview(btn)
		
		let g = view.safeAreaLayoutGuide
		
		NSLayoutConstraint.activate([
			btn.topAnchor.constraint(equalTo: g.topAnchor, constant: 40.0),
			btn.widthAnchor.constraint(equalToConstant: 200.0),
			btn.centerXAnchor.constraint(equalTo: g.centerXAnchor, constant: 0.0),
		])
		
		btn.addTarget(self, action: #selector(didTap(_:)), for: .touchUpInside)
		
	}
	
	@objc func didTap(_ sender: Any?) -> Void {
		let vc = TraitTestViewController()
		present(vc, animated: true, completion: nil)
	}
	
}

class TraitTestViewController: UIViewController, UITableViewDataSource, UITableViewDelegate {
	
	var tableView = UITableView()
		
	override func viewDidLoad() {
		super.viewDidLoad()
		
		view.backgroundColor = .systemTeal
		
		tableView.dataSource = self
		tableView.delegate = self

		tableView.translatesAutoresizingMaskIntoConstraints = false
		
		view.addSubview(tableView)
		
		let fv = TraitView(frame: CGRect(x: 0, y: 0, width: 200, height: 60))
		tableView.tableFooterView = fv
		
		let g = view.safeAreaLayoutGuide
		
		NSLayoutConstraint.activate([
			tableView.topAnchor.constraint(equalTo: g.topAnchor, constant: 100.0),
			tableView.bottomAnchor.constraint(equalTo: g.bottomAnchor, constant: -40.0),
			tableView.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 40.0),
			tableView.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -40.0),
		])
		
		tableView.register(UITableViewCell.self, forCellReuseIdentifier: "bCell")
		
	}
	
	func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
		return 30
	}
	
	func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
		let c = tableView.dequeueReusableCell(withIdentifier: "bCell", for: indexPath)
		c.textLabel?.text = "\(indexPath)"
		return c
	}
	
}