Custom formsheet presentation

FormsheetPresentationController.swift

class FormsheetPresentationController: UIPresentationController {
    private static let dimmingViewOpacityWhenPresented = 0.5
    private var isPresented = false

    private let dimmingView: UIView = {
        let dimmingView = UIView()
        dimmingView.backgroundColor = .black
        return dimmingView
    }()

    override var shouldRemovePresentersView: Bool {
        return false
    }

    override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {
        super.viewWillTransition(to: size, with: coordinator)

        coordinator.animate { context in
            guard let containerView = self.containerView, self.isPresented else { return }

            let targetFrame = FormsheetPresentationAnimator.targetFrame(
                in: containerView,
                preferredContentSize: self.presentedViewController.preferredContentSize
            )

            self.presentedView?.frame = targetFrame
        }
    }

    override func containerViewWillLayoutSubviews() {
        dimmingView.frame = containerView?.bounds ?? .zero
    }

    override func presentationTransitionWillBegin() {
        dimmingView.alpha = 0
        containerView?.addSubview(dimmingView)

        if let transitionCoordinator = presentingViewController.transitionCoordinator {
            transitionCoordinator.animate { context in
                self.dimmingView.alpha = Self.dimmingViewOpacityWhenPresented
            }
        } else {
            self.dimmingView.alpha = Self.dimmingViewOpacityWhenPresented
        }
    }

    override func presentationTransitionDidEnd(_ completed: Bool) {
        if completed {
            isPresented = true
        } else {
            dimmingView.removeFromSuperview()
        }
    }

    override func dismissalTransitionWillBegin() {
        presentingViewController.transitionCoordinator?.animate { context in
            self.dimmingView.alpha = 0
        }
    }

    override func dismissalTransitionDidEnd(_ completed: Bool) {
        if completed {
            dimmingView.removeFromSuperview()
            isPresented = false
        }
    }
}

class FormsheetTransitioningDelegate: NSObject, UIViewControllerTransitioningDelegate {
    func animationController(forPresented presented: UIViewController, presenting: UIViewController, source: UIViewController) -> UIViewControllerAnimatedTransitioning? {
        return FormsheetPresentationAnimator()
    }

    func animationController(forDismissed dismissed: UIViewController) -> UIViewControllerAnimatedTransitioning? {
        return FormsheetPresentationAnimator()
    }

    func presentationController(forPresented presented: UIViewController, presenting: UIViewController?, source: UIViewController) -> UIPresentationController? {
        return FormsheetPresentationController(presentedViewController: presented, presenting: source)
    }
}

class FormsheetPresentationAnimator: NSObject, UIViewControllerAnimatedTransitioning {
    func transitionDuration(using transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval {
        return (transitionContext?.isAnimated ?? true) ? 0.5 : 0
    }

    func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
        let destination = transitionContext.viewController(forKey: .to)

        if destination?.isBeingPresented ?? false {
            animatePresentation(transitionContext)
        } else {
            animateDismissal(transitionContext)
        }
    }

    private func animatePresentation(_ transitionContext: UIViewControllerContextTransitioning) {
        let duration = transitionDuration(using: transitionContext)
        let containerView = transitionContext.containerView
        let destinationView = transitionContext.view(forKey: .to)!
        let destinationController = transitionContext.viewController(forKey: .to)!
        let preferredContentSize = destinationController.preferredContentSize

        containerView.addSubview(destinationView)
        destinationView.frame = Self.initialFrame(
            in: containerView,
            preferredContentSize: preferredContentSize
        )

        UIView.animate(
            withDuration: duration,
            delay: 0,
            options: [.curveEaseInOut],
            animations: {
                destinationView.frame = Self.targetFrame(
                    in: containerView,
                    preferredContentSize: preferredContentSize
                )
            },
            completion: { _ in
                transitionContext.completeTransition(true)
            })
    }

    private func animateDismissal(_ transitionContext: UIViewControllerContextTransitioning) {
        let duration = transitionDuration(using: transitionContext)
        let containerView = transitionContext.containerView
        let sourceView = transitionContext.view(forKey: .from)!
        let sourceController = transitionContext.viewController(forKey: .from)!
        let preferredContentSize = sourceController.preferredContentSize

        UIView.animate(
            withDuration: duration,
            delay: 0,
            options: [.curveEaseInOut],
            animations: {
                sourceView.frame = Self.initialFrame(
                    in: containerView,
                    preferredContentSize: preferredContentSize
                )
            },
            completion: { _ in
                transitionContext.completeTransition(true)
            })
    }

    fileprivate static func initialFrame(in containerView: UIView, preferredContentSize: CGSize) -> CGRect {
        assert(preferredContentSize.width > 0 && preferredContentSize.height > 0)

        return CGRect(
            origin: CGPoint(
                x: containerView.bounds.midX - preferredContentSize.width * 0.5,
                y: containerView.bounds.maxY
            ),
            size: preferredContentSize
        )
    }

    fileprivate static func targetFrame(in containerView: UIView, preferredContentSize: CGSize) -> CGRect {
        assert(preferredContentSize.width > 0 && preferredContentSize.height > 0)

        return CGRect(
            origin: CGPoint(
                x: containerView.bounds.midX - preferredContentSize.width * 0.5,
                y: containerView.bounds.midY - preferredContentSize.height * 0.5
            ),
            size: preferredContentSize
        )
    }
}


// USAGE:

// Store somewhere for the duration of transition, i.e in presenting class
let formsheetTransitioningDelegate = FormsheetTransitioningDelegate()

// Present with custom modal transition
let viewController = UIViewController()
viewController.view.backgroundColor = .systemMint

let navigationController = UINavigationController(rootViewController: viewController)
navigationController.transitioningDelegate = formsheetTransitioningDelegate
navigationController.modalPresentationStyle = .custom
navigationController.preferredContentSize = CGSize(width: 320, height: 240)
navigationController.view.layer.cornerRadius = 8

present(navigationController, animated: true)