BottomSheetPresentationController.swift

import UIKit

class BottomSheetPresentationController: UIPresentationController {
    
    private let dismissThreshold: CGFloat = 0.3
    
    private let grabberView: UIView = {
        let view = UIView()
        view.frame.size = CGSize(width: 50, height: 3)
        view.backgroundColor = .lightGray
        return view
    }()
    
    private let dimmedBackgroundView: UIView = {
        let view = UIView()
        view.isUserInteractionEnabled = true
        return view
    }()
    
    private lazy var panGesture = UIPanGestureRecognizer(target: self, action: #selector(pannedPresentedView))
    
    private lazy var tapGestureToDismiss = UITapGestureRecognizer(target: self, action: #selector(tappedDimmedBackgroundView))
    
    private var transitioningDelegate: BottomSheetTransitioningDelegate? {
        presentedViewController.transitioningDelegate as? BottomSheetTransitioningDelegate
    }
    
    override var frameOfPresentedViewInContainerView: CGRect {
        guard let containerView, let presentedView else {
            return super.frameOfPresentedViewInContainerView
        }
        
        let maximumWidth = containerView.bounds.width
        
        let maximumHeight = containerView.frame.height - containerView.safeAreaInsets.top - containerView.safeAreaInsets.bottom
        
        let fittingSize = CGSize(width: maximumWidth, height: UIView.layoutFittingCompressedSize.height)
        
        let presentedViewSize = presentedView.systemLayoutSizeFitting(
            fittingSize,
            withHorizontalFittingPriority: .required,
            verticalFittingPriority: .defaultLow
        )
        
        let presentedViewHeight = presentedViewSize.height
        
        let targetHeight = presentedViewHeight == .zero ? maximumHeight : presentedViewHeight
        
        let finalHeight = min(targetHeight, maximumHeight) + containerView.safeAreaInsets.bottom
        
        let finalSize = CGSize(width: maximumWidth, height: finalHeight)
        
        let finalOrigin = CGPoint(x: .zero, y: containerView.frame.maxY - finalSize.height)
        
        return CGRect(origin: finalOrigin, size: finalSize)
    }

    
    override func presentationTransitionWillBegin() {
        super.presentationTransitionWillBegin()
        
        containerView?.addSubview(dimmedBackgroundView)
        
        dimmedBackgroundView.backgroundColor = .black.withAlphaComponent(0.5)
        
        presentedView?.addSubview(grabberView)
        
        presentedViewController.transitionCoordinator?.animate(alongsideTransition: {
            [weak self] _ in
            guard let self else { return }
            
            self.presentedView?.layer.cornerRadius = 25
        })
    }
    
    override func containerViewDidLayoutSubviews() {
        super.containerViewDidLayoutSubviews()
        
        guard let presentedView, let containerView else {
            return
        }
        
        dimmedBackgroundView.frame = containerView.bounds
        
        grabberView.frame.origin.y = 8
        grabberView.center.x = presentedView.center.x
        grabberView.layer.cornerRadius = grabberView.frame.height / 2
        
        setupInteraction()
        
        presentedView.layer.maskedCorners = [.layerMinXMinYCorner, .layerMaxXMinYCorner]
                
        presentedViewController.additionalSafeAreaInsets.top = grabberView.frame.maxY
    }
    
    override func dismissalTransitionWillBegin() {
        super.dismissalTransitionWillBegin()
        
        presentedViewController.transitionCoordinator?.animate(alongsideTransition: { [weak self] _ in
            guard let self else { return }
            
            self.presentedView?.layer.cornerRadius = .zero
            self.dimmedBackgroundView.backgroundColor = .clear
        })
    }
}

// MARK: Pan Gesture
extension BottomSheetPresentationController {
    private func setupInteraction() {
        guard let presentedView = presentedView else {
            return
        }
        
        func addGestureIfNeeded(_ gesture: UIGestureRecognizer, to view: UIView, delegate: UIGestureRecognizerDelegate? = nil) {
            if let recognizers = view.gestureRecognizers, recognizers.contains(gesture) {
                return
            }
            gesture.delegate = delegate
            view.addGestureRecognizer(gesture)
        }
                
        addGestureIfNeeded(panGesture, to: presentedView, delegate: self)
        addGestureIfNeeded(tapGestureToDismiss, to: dimmedBackgroundView)
    }
    
    
    private func dismiss(withInteraction: Bool) {
        transitioningDelegate?.transition.wantsInteractiveStart = withInteraction
        presentedViewController.dismiss(animated: true)
    }
    
    @objc private func pannedPresentedView(_ recognizer: UIPanGestureRecognizer) {
        guard let presentedView = presentedView else {
            return
        }
        
        switch recognizer.state {
        case .began:
            transitioningDelegate?.transition.wantsInteractiveStart = true
        
        case .changed:
            let translation = recognizer.translation(in: presentedView)
            didGestureTranslationChange(translation)
        case .ended:
            let translation = recognizer.translation(in: presentedView)
            let progress = translation.y / presentedView.frame.height

            if progress > dismissThreshold {
                dismiss(withInteraction: true)
                transitioningDelegate?.transition.finish()
            } else {
                UIView.animate(withDuration: 0.2) {
                    presentedView.transform = .identity
                }
                transitioningDelegate?.transition.cancel()
            }
        case .cancelled, .failed:
            break
        default:
            break
        }
    }

    private func didGestureTranslationChange(_ translation: CGPoint) {
        guard let presentedView = presentedView else {
            return
        }
        
        let translationY = max(0, translation.y)
        presentedView.transform = CGAffineTransform(translationX: 0, y: translationY)
        
        let progress = translationY / presentedView.frame.height
        transitioningDelegate?.transition.update(progress)
    }

    private func didInteractionEnd() {
        guard let presentedView = presentedView else {
            return
        }
        
        if let transitioningDelegate = transitioningDelegate {
            let dismissFraction = transitioningDelegate.transition.dismissFractionComplete
            
            if dismissFraction > dismissThreshold {
                transitioningDelegate.transition.finish()
                presentedViewController.dismiss(animated: true)
            } else {
                transitioningDelegate.transition.cancel()
                UIView.animate(withDuration: 0.3) {
                    presentedView.transform = .identity
                }
            }
        } else {
            UIView.animate(withDuration: 0.3) {
                presentedView.transform = .identity
            }
        }
    }
    
    @objc private func tappedDimmedBackgroundView() {
        dismiss(withInteraction: false)
    }
}

extension BottomSheetPresentationController: UIGestureRecognizerDelegate {
    public func gestureRecognizer(_ gestureRecognizer: UIGestureRecognizer, shouldRecognizeSimultaneouslyWith otherGestureRecognizer: UIGestureRecognizer) -> Bool {
        return true
    }
}