https://stackoverflow.com/a/78385890/1271826

FloatingButtonDemo.swift

import UIKit

class ShadowedSwitch: UISwitch {
    override init(frame: CGRect) {
        super.init(frame: frame)
        configure()
    }

    required init?(coder: NSCoder) {
        super.init(coder: coder)
        configure()
    }
    
    func configure() {
        backgroundColor = .systemBackground.withAlphaComponent(0.95)
        layer.shadowColor = UIColor.black.cgColor
        layer.shadowOpacity = 0.5
        layer.shadowOffset = CGSize(width: 0, height: 2)
        layer.shadowRadius = 4
        setContentCompressionResistancePriority(.required, for: .vertical)
        setContentHuggingPriority(.required, for: .vertical)
    }

    override func layoutSubviews() {
        super.layoutSubviews()
        layer.cornerRadius = bounds.height / 2
    }
}

class ViewController: UIViewController {
    private lazy var floatingButton: UISwitch = {
        let button = ShadowedSwitch()
        button.translatesAutoresizingMaskIntoConstraints = false

        button.addTarget(self, action: #selector(openSheet(_:)), for: .touchUpInside)

        return button
    }()

    override func viewDidLoad() {
        super.viewDidLoad()
        configureView()
    }

    private func configureView() {
        view.backgroundColor = .systemMint
        view.addSubview(floatingButton)

        let buttonBottomConstraint = floatingButton.bottomAnchor.constraint(
            equalTo: view.layoutMarginsGuide.bottomAnchor,
            constant: -10
        )
        buttonBottomConstraint.priority = .defaultHigh

        NSLayoutConstraint.activate([
            buttonBottomConstraint,
            floatingButton.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: -10)
        ])
    }

    var childSheet: UISheetPresentationController?

    @objc
    private func openSheet(_ sender: Any) {
        if let sheet = childSheet {
            let current = sheet.selectedDetentIdentifier
            sheet.animateChanges {
                sheet.selectedDetentIdentifier = current == .hidden ? .medium : .hidden
            }
            return
        }

        let childViewController = ChildViewController()

        if let sheet = childViewController.sheetPresentationController {
            sheet.detents = [.hidden(), .small(), .medium(), .large()]
            sheet.largestUndimmedDetentIdentifier = .medium
            sheet.selectedDetentIdentifier = .medium
            sheet.prefersGrabberVisible = true
            sheet.delegate = self
            childSheet = sheet
        }

        let constraint = childViewController.view.topAnchor.constraint(equalTo: floatingButton.bottomAnchor, constant: 10)

        childViewController.isModalInPresentation = true
        childViewController.animateAlongside = { [self] _ in
            constraint.isActive = true
            view.layoutIfNeeded()
        }
        present(childViewController, animated: true)
    }
}

extension ViewController: UISheetPresentationControllerDelegate {
    func sheetPresentationControllerDidChangeSelectedDetentIdentifier(_ sheetPresentationController: UISheetPresentationController) {
        floatingButton.isOn = sheetPresentationController.selectedDetentIdentifier != .hidden
    }
}

extension UISheetPresentationController.Detent.Identifier {
    static var small: Self {
        Self("small")
    }

    static var hidden: Self {
        Self("hidden")
    }
}

extension UISheetPresentationController.Detent {
    static func small() -> UISheetPresentationController.Detent {
        .custom(identifier: .small) { $0.maximumDetentValue * 0.15 }
    }

    static func hidden() -> UISheetPresentationController.Detent {
        .custom(identifier: .hidden) { _ in 0 }
    }
}

class ChildViewController: UIViewController {
    var animateAlongside: ((any UIViewControllerTransitionCoordinatorContext) -> Void)?

    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .systemBlue
    }

    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)

        transitionCoordinator?.animate(alongsideTransition: { [self] context in
            animateAlongside?(context)
            animateAlongside = nil
        })
    }
}

Given that I changed the button to a UISwitch, the hardcoded corner radius is no longer prudent (and resulted in a weird artifact in the UI when the corner radius was set to 20 and the switch didn't happen to be 40pt tall). So I moved this corner rounding into a separate class, so it would adjust the rounding for the shadow dynamically.

This also has the virtue of abstracting the button layout code to a separate class and out of the view controller.

I also changed the identifier names for hidden and small to follow the convention of medium and large. It’s not terribly relevant, but just an FYI.