//
//  BottomSheetAnimationViewController.swift
//

import UIKit

// MARK: BottomSheetView

final class BottomSheetView: UIView {
    private let handleViewHeight: CGFloat = 8

    private lazy var handleView: UIView = {
        let view = UIView()
        view.translatesAutoresizingMaskIntoConstraints = false
        view.backgroundColor = .tertiarySystemBackground
        view.layer.cornerRadius = handleViewHeight / 2
        view.clipsToBounds = true
        return view
    }()

    init() {
        super.init(frame: .zero)

        addSubview(handleView)

        NSLayoutConstraint.activate([
            handleView.centerXAnchor.constraint(equalTo: centerXAnchor),
            handleView.centerYAnchor.constraint(equalTo: topAnchor, constant: handleViewHeight),
            handleView.heightAnchor.constraint(equalToConstant: handleViewHeight),
            handleView.widthAnchor.constraint(equalToConstant: handleViewHeight * 10)
        ])

        backgroundColor = .secondarySystemBackground
        layer.cornerRadius = 16
        layer.maskedCorners = [
            .layerMinXMinYCorner,
            .layerMaxXMinYCorner
        ]
        clipsToBounds = true
    }

    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

// MARK: BottomSheetAnimationViewController

class BottomSheetAnimationViewController: UIViewController {
    private lazy var bottomSheetView: UIView = {
        let bottomSheetView = BottomSheetView()
        bottomSheetView.translatesAutoresizingMaskIntoConstraints = false
        return bottomSheetView
    }()

    private lazy var panGestureRecognizer = UIPanGestureRecognizer(
        target: self, action: #selector(onPan)
    )

    private let animator: BottomSheetAnimator = BottomSheetAnimator()

    override func loadView() {
        let view = UIView()

        view.addSubview(bottomSheetView)

        let bottomConstraint = bottomSheetView.bottomAnchor.constraint(
            equalTo: view.bottomAnchor
        )

        animator.bottomConstraint = bottomConstraint

        NSLayoutConstraint.activate([
            bottomSheetView.heightAnchor.constraint(
                equalTo: view.heightAnchor, multiplier: 0.66
            ),
            bottomSheetView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
            bottomSheetView.trailingAnchor.constraint(equalTo: view.trailingAnchor),
            bottomConstraint,
        ])

        view.backgroundColor = .systemBackground

        self.view = view
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        bottomSheetView.addGestureRecognizer(panGestureRecognizer)
    }

    @objc
    private func onPan(_ panGestureRecognizer: UIPanGestureRecognizer) {
        guard let view = panGestureRecognizer.view else {
            return
        }

        let translation = panGestureRecognizer.translation(in: view.superview)

        switch panGestureRecognizer.state {
            case .possible:
                break
            case .failed:
                break
            case .began:
                animator.start(animating: view)
            case .changed:
                animator.move(view, basedOn: translation)
            default: // .ended, .cancelled, @unknown
                let velocity = panGestureRecognizer.velocity(in: view.superview)
                animator.stop(animating: view, with: velocity)
        }
    }
}

// MARK: BottomSheetAnimator

class BottomSheetAnimator {
    var bottomConstraint: NSLayoutConstraint?

    // Capture the sheet's initial height.
    private var initialSheetHeight: CGFloat = .zero

    private var offsetAnimator: UIViewPropertyAnimator?

    private func makeOffsetAnimator(
        animating view: UIView,
        to offset: CGFloat
    ) -> UIViewPropertyAnimator {
        let propertyAnimator = UIViewPropertyAnimator(
            duration: 0.25, // Low values makes the scrubbing snap between values at the start.
            dampingRatio: 1
        )

        propertyAnimator.addAnimations {
            self.bottomConstraint?.constant = offset

            view.superview?.layoutIfNeeded()
        }

        propertyAnimator.addCompletion { position in
            self.bottomConstraint?.constant = position == .end ? offset : 0
        }

        return propertyAnimator
    }
}

extension BottomSheetAnimator {
    func start(animating view: UIView) {
        initialSheetHeight = view.frame.height

        offsetAnimator = makeOffsetAnimator(
            animating: view, to: initialSheetHeight
        )
    }

    func move(_ view: UIView, basedOn translation: CGPoint) {
        let fractionComplete = min(max(translation.y, 0) / initialSheetHeight, 1)

        offsetAnimator?.fractionComplete = fractionComplete
    }

    func stop(animating view: UIView, with velocity: CGPoint) {
        let fractionComplete = offsetAnimator?.fractionComplete ?? 0

        offsetAnimator?.isReversed = fractionComplete < 0.5

        offsetAnimator?.continueAnimation(
            withTimingParameters: nil,
            durationFactor: 1
        )
    }
}