Simple Swift example of adding Child View Controllers

ViewController.swift

class ViewController: UIViewController {

	let infoLabel = UILabel()
	let stack = UIStackView()
	
	override func viewDidLoad() {
		super.viewDidLoad()
		view.backgroundColor = .systemYellow

		infoLabel.textAlignment = .center
		infoLabel.text = "Tap anywhere to add children..."
		
		stack.axis = .vertical
		stack.distribution = .fillEqually
		stack.spacing = 20.0

		infoLabel.translatesAutoresizingMaskIntoConstraints = false
		view.addSubview(infoLabel)

		stack.translatesAutoresizingMaskIntoConstraints = false
		view.addSubview(stack)
		
		let g = view.safeAreaLayoutGuide
		NSLayoutConstraint.activate([
			
			infoLabel.topAnchor.constraint(equalTo: g.topAnchor, constant: 20.0),
			infoLabel.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 20.0),
			infoLabel.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -20.0),

			stack.topAnchor.constraint(equalTo: infoLabel.bottomAnchor, constant: 20.0),
			stack.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 20.0),
			stack.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -20.0),
			stack.bottomAnchor.constraint(equalTo: g.bottomAnchor, constant: -20.0),

		])
	}

	override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
		
		// we only want to add the child controllers once
		if self.children.count > 0 { return() }
		
		let colors: [UIColor] = [
			.systemRed, .systemGreen, .systemBlue,
		]
		for (i, c) in colors.enumerated() {
			let vc = SimpleChildVC()
			addChild(vc)
			vc.text = "Child \(i)"
			vc.view.backgroundColor = c
			stack.addArrangedSubview(vc.view)
			vc.didMove(toParent: self)
		}

		infoLabel.text = "Three child view controllers added!"
		
	}
	
}

class SimpleChildVC: UIViewController {
	public var text: String = "" {
		didSet {
			label.text = text
		}
	}
	private let label = UILabel()
	override func viewDidLoad() {
		super.viewDidLoad()
		label.backgroundColor = UIColor(white: 0.95, alpha: 1.0)
		label.textAlignment = .center
		label.translatesAutoresizingMaskIntoConstraints = false
		view.addSubview(label)
		let g = view.safeAreaLayoutGuide
		NSLayoutConstraint.activate([
			label.widthAnchor.constraint(equalTo: g.widthAnchor, multiplier: 0.8),
			label.heightAnchor.constraint(equalTo: g.heightAnchor, multiplier: 0.6),
			label.centerXAnchor.constraint(equalTo: g.centerXAnchor),
			label.centerYAnchor.constraint(equalTo: g.centerYAnchor),
		])
	}
}