SheetImagePicker/SheetImagePicker/SheetContainerViewController.swift

//
//  SheetContainerViewController.swift
//  SheetImagePicker
//
//  Created by Shadowfacts on 9/23/19.
//  Copyright © 2019 Shadowfacts. All rights reserved.
//

import UIKit

public class SheetContainerViewController: UIViewController {

    let content: UIViewController
    
    public var detents: [Detent] = [.bottom, .middle, .top] {
        didSet {
            sortDetents()
        }
    }
    var sortedDetentOffsets: [CGFloat] = []

    var topConstraint: NSLayoutConstraint!
    lazy var initialConstant: CGFloat = view.bounds.height / 2
    
    public var minimumDetentJumpVelocity: CGFloat = 500
    public var maximumStretchDistance: CGFloat = 15
    
    public init(content: UIViewController) {
        self.content = content
        
        super.init(nibName: nil, bundle: nil)
    }
    
    required public init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    override public func viewDidLoad() {
        super.viewDidLoad()
        
        addChild(content)
        content.didMove(toParent: self)
        view.addSubview(content.view)
        topConstraint = content.view.topAnchor.constraint(equalTo: view.topAnchor, constant: initialConstant)
        NSLayoutConstraint.activate([
            content.view.leadingAnchor.constraint(equalTo: view.leadingAnchor),
            content.view.trailingAnchor.constraint(equalTo: view.trailingAnchor),
            topConstraint,
            content.view.bottomAnchor.constraint(equalTo: view.bottomAnchor)
        ])
        
        let panGesture = UIPanGestureRecognizer(target: self, action: #selector(panGestureRecognized(_:)))
        content.view.addGestureRecognizer(panGesture)
    }
    
    public override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        
        sortDetents()
    }
    
    private func sortDetents() {
        sortedDetentOffsets = detents.map {
            $0.offset(in: view)
        }.sorted()
    }
    
    @objc func panGestureRecognized(_ recognizer: UIPanGestureRecognizer) {
        switch recognizer.state {
        case .began:
            initialConstant = topConstraint.constant
            
        case .changed:
            let translation = recognizer.translation(in: content.view)
            var realOffset = initialConstant + translation.y
            if realOffset < sortedDetentOffsets.first! {
                func clamp(_ value: CGFloat, from: CGFloat, to: CGFloat) -> CGFloat {
                    if value < from {
                        return from
                    } else if value > to {
                        return to
                    } else {
                        return value
                    }
                }
                func smoothstep(value: CGFloat, from: CGFloat, to: CGFloat) -> CGFloat {
                    let x = clamp((value - from) / (to - from), from: 0, to: 1)
                    // 3x^2 - 2x^3
                    return 3 * pow(x, 2) - 2 * pow(x, 3)
                }
                let topOffset = sortedDetentOffsets.first!
                let smoothed = smoothstep(value: realOffset, from: topOffset, to: 0)
                realOffset = topOffset - smoothed * maximumStretchDistance
                
            }
            topConstraint.constant = realOffset
            
        case .ended:
            let velocity = recognizer.velocity(in: view)

            let springToDetent: CGFloat
            if abs(velocity.y) > minimumDetentJumpVelocity,
                let offsetInVelocityDirection = nearestDetentOffset(currentOffset: topConstraint.constant, direction: velocity.y) {
                springToDetent = offsetInVelocityDirection
            } else if let nearestOffset = nearestDetentOffset(currentOffset: topConstraint.constant) {
                springToDetent = nearestOffset
            } else {
                return
            }
            
            let springDistance = abs(topConstraint.constant - springToDetent)
            self.topConstraint.constant = springToDetent
            let springVelocity = velocity.y / springDistance
            
            UIView.animate(withDuration: 0.35, delay: 0, usingSpringWithDamping: 0.6, initialSpringVelocity: springVelocity, animations: {
                self.view.layoutIfNeeded()
            })
        default:
            return
        }
    }
    
    func nearestDetentOffset(currentOffset: CGFloat) -> CGFloat? {
        return sortedDetentOffsets.min(by: { abs($0 - currentOffset) < abs($1 - currentOffset) })
    }
    
    func nearestDetentOffset(currentOffset: CGFloat, direction: CGFloat) -> CGFloat? {
        if direction < 0 {
            return sortedDetentOffsets.last(where: { $0 < currentOffset })
        } else {
            return sortedDetentOffsets.first(where: { $0 > currentOffset })
        }
    }
    
}