PresentationController.swift

import UIKit

class PresentationController: UIPresentationController {
    private var chrome: UIView = UIView()
    var interactor: Interactor? = nil

    override var frameOfPresentedViewInContainerView: CGRect {
        guard let containerView = containerView,
            let presentedView = presentedView else { return .zero }

        let inset: CGFloat = 0
        
        var safeAreaFrame: CGRect = .zero
        // Make sure to account for the safe area insets
        if #available(iOS 11.0, *) {
            safeAreaFrame = containerView.bounds
                .inset(by: containerView.safeAreaInsets)
        } else {
            safeAreaFrame = containerView.bounds
        }

        let targetWidth = safeAreaFrame.width - (2 * inset)
        let fittingSize = CGSize(
            width: targetWidth,
            height: UIView.layoutFittingCompressedSize.height
        )
        let targetHeight = presentedView.systemLayoutSizeFitting(
            fittingSize, withHorizontalFittingPriority: .required,
            verticalFittingPriority: .defaultLow).height

        var frame = safeAreaFrame
        frame.origin.x += inset
        frame.origin.y += UIScreen.main.bounds.height - targetHeight - inset
        frame.size.width = targetWidth
        frame.size.height = targetHeight
        return frame
    }
    
    override init(presentedViewController: UIViewController, presenting presentingViewController: UIViewController?) {
        super.init(presentedViewController: presentedViewController, presenting: presentingViewController)
        chrome.backgroundColor = UIColor(white: 0.0, alpha: 0.5)
        chrome.alpha = 0.0
        chrome.addGestureRecognizer(
            UITapGestureRecognizer(target: self, action: #selector(tapGesture(sender:)))
        )
        presentedViewController.view.addGestureRecognizer(UIPanGestureRecognizer(target: self, action: #selector(panGesture(_:))))
    }
    
    @objc
    private func panGesture(_ sender: UIPanGestureRecognizer) {
        let percentThreshold: CGFloat = 0.3
        let translation = sender.translation(in: sender.view)
        let verticalMovement = translation.y / sender.view!.bounds.height
        let downwardMovement = fmaxf(Float(verticalMovement), 0.0)
        let downwardMovementPercent = fminf(downwardMovement, 1.0)
        let progress = CGFloat(downwardMovementPercent)
        
        guard let interactor = interactor else { return }
        
        switch sender.state {
        case .began:
            presentingViewController.dismiss(animated: true, completion: nil)
        case .changed:
            interactor.shouldFinish = progress > percentThreshold
            interactor.update(progress)
        case .cancelled:
            interactor.hasStarted = false
            interactor.cancel()
        case .ended:
            interactor.hasStarted = false
            interactor.shouldFinish ? interactor.finish() : interactor.cancel()
        default:
            break
        }
    }
    
    override func containerViewWillLayoutSubviews() {
        chrome.frame = containerView!.bounds
        presentedView?.center = .zero
        presentedView?.roundCorners([.topLeft, .topRight], radius: 21)
        presentedView?.frame = frameOfPresentedViewInContainerView
    }
    
    override func presentationTransitionWillBegin() {
        containerView?.insertSubview(chrome, at: 0)
        presentedViewController
            .transitionCoordinator?
            .animate(alongsideTransition: { (_) in
                self.chrome.alpha = 1.0
            }, completion: nil)
    }
    
    override func dismissalTransitionWillBegin() {
        presentedViewController
            .transitionCoordinator?
            .animate(alongsideTransition: { (_) in
                self.chrome.alpha = 0.0
            }, completion: nil)
    }
    
    override func dismissalTransitionDidEnd(_ completed: Bool) {
        if completed {
            chrome.removeFromSuperview()
        }
    }
    
    @objc private func tapGesture(sender: UIGestureRecognizer) {
        presentedViewController.dismiss(animated: true, completion: nil)
    }
}