Accordion-style view controllers

import UIKit

class PlaygroundViewController: UIViewController {
    
    let firstViewController = UIViewController()
    let secondViewController = UIViewController()
    let thirdViewController = UIViewController()

    lazy var viewControllersToAdd = [firstViewController, secondViewController, thirdViewController]
    
    let heightOfView: CGFloat = 300
    let viewOverlap: CGFloat = 200
    
    let stackView = UIStackView()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = UIColor.white
        firstViewController.view.backgroundColor = .red
        secondViewController.view.backgroundColor = .blue
        thirdViewController.view.backgroundColor = .green

        stackView.translatesAutoresizingMaskIntoConstraints = false
        stackView.axis = .vertical
        stackView.spacing = -viewOverlap
        
        viewControllersToAdd.forEach { (controller: UIViewController) in
            if let childView = controller.view {
                
                stackView.addArrangedSubview(childView)
                NSLayoutConstraint.activate([
                    childView.heightAnchor.constraint(equalToConstant: heightOfView),
                    childView.widthAnchor.constraint(equalTo: stackView.widthAnchor)
                    ])
                
                let gestureRecognizer = UITapGestureRecognizer(target: self, action: #selector(didTapChildView(sender:)))
                childView.addGestureRecognizer(gestureRecognizer)
                childView.isUserInteractionEnabled = true
            }
            addChild(controller)
            controller.didMove(toParent: self)
        }
        
        view.addSubview(stackView)
        
        NSLayoutConstraint.activate([
            stackView.rightAnchor.constraint(equalTo: view.rightAnchor),
            stackView.leftAnchor.constraint(equalTo: view.leftAnchor),
            stackView.topAnchor.constraint(equalTo: view.topAnchor),
            ])
    }
    
    @objc func didTapChildView(sender: UITapGestureRecognizer) {
        
        if let targetView = sender.view {
            UIView.animate(withDuration: 0.3, animations: {
                let currentSpacing = self.stackView.customSpacing(after: targetView)
                if currentSpacing == 0 {
                    // targetView is already expanded, collapse it
                    self.stackView.setCustomSpacing(-self.viewOverlap, after: targetView)
                } else {
                    // expand view
                    self.stackView.setCustomSpacing(0, after: targetView)
                }
            })
            
            
        }
    }
}