BottomSheetExample.swift

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        
        let btn = UIButton(type: .system)
        btn.translatesAutoresizingMaskIntoConstraints = false
        btn.setTitle("Show Sheet", for: .normal)
        btn.addTarget(self, action: #selector(tap), for: .touchUpInside)
        view.addSubview(btn)
        NSLayoutConstraint.activate([
            btn.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            btn.centerYAnchor.constraint(equalTo: view.centerYAnchor)
        ])
    }
    
    @objc func tap() {
        let bottomSheetVC = OurBottomSheetContentViewController()
        let delegate = BottomSheetTransitioningDelegate()
        bottomSheetVC.transitioningDelegate = delegate
        bottomSheetVC.modalPresentationStyle = .custom
        present(bottomSheetVC, animated: true, completion: nil)
    }
}


class OurBottomSheetContentViewController: UIViewController {
    
    let contentView = ContentView()
    
    override func loadView() {
        super.loadView()
        self.view = contentView
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .systemBackground
    }
}

class ContentView: UIView {
    let titleLabel = UILabel()
    
    let subtitleLabel = UILabel()

    let okButton = UIButton()
    
    init() {
        super.init(frame: .zero)
        
        titleLabel.translatesAutoresizingMaskIntoConstraints = false
        titleLabel.font = .systemFont(ofSize: 22, weight: .bold)
        titleLabel.text = "What's up?"
        
        subtitleLabel.translatesAutoresizingMaskIntoConstraints = false
        subtitleLabel.font = .systemFont(ofSize: 18, weight: .medium)
        subtitleLabel.text = "This is a custom bottom sheet that we made from scratch!"
        subtitleLabel.numberOfLines = 0
        subtitleLabel.textAlignment = .center
        
        okButton.translatesAutoresizingMaskIntoConstraints = false
        okButton.backgroundColor = .systemBlue
        okButton.setTitle("OK", for: .normal)
        okButton.setTitleColor(.white, for: .normal)
        okButton.layer.cornerRadius = 20
        
        addSubview(titleLabel)
        addSubview(subtitleLabel)
        addSubview(okButton)
        
        NSLayoutConstraint.activate([
            titleLabel.topAnchor.constraint(equalTo: topAnchor, constant: 20),
            titleLabel.centerXAnchor.constraint(equalTo: centerXAnchor),
            
            subtitleLabel.topAnchor.constraint(equalTo: titleLabel.bottomAnchor, constant: 10),
            subtitleLabel.leadingAnchor.constraint(equalTo: leadingAnchor, constant: 16),
            subtitleLabel.trailingAnchor.constraint(equalTo: trailingAnchor, constant: -16),

            okButton.topAnchor.constraint(equalTo: subtitleLabel.bottomAnchor, constant: 10),
            okButton.leadingAnchor.constraint(equalTo: leadingAnchor, constant: 16),
            okButton.trailingAnchor.constraint(equalTo: trailingAnchor, constant: -16),
            okButton.heightAnchor.constraint(equalToConstant: 50),
            okButton.bottomAnchor.constraint(equalTo: safeAreaLayoutGuide.bottomAnchor)
        ])
    }
    
    required init?(coder: NSCoder) {
        fatalError()
    }
}