Swift slide up modal view

slideup.swift

//
//  SlideUp.swift
//
//  Created by Marlon Monroy on 1/29/19.
//  Copyright © 2019 Monroy.io. All rights reserved.
//

import UIKit
class InstantPan: UIPanGestureRecognizer {
	override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent) {
		if (self.state == .began) { return }
		super.touchesBegan(touches, with: event)
		self.state = .began
	}
	
}

 enum SlideUpState {
	case closed
	case open
	var apposite : SlideUpState {
		return self == .open ? .closed :.open
	}
}

class SlideUp {
	private let offset: CGFloat
	private var containerView: UIView
	private var state : SlideUpState = .closed
	private var animator: UIViewPropertyAnimator!
	private var progress: CGFloat = 0.0
	private let topSpacing:CGFloat = 40
	lazy var panGesture: InstantPan = {
		let pan = InstantPan()
		pan.addTarget(self, action: #selector(panned(_:)))
		return pan
	}()
	
	@objc func panned(_ sender: UIPanGestureRecognizer) {
		switch sender.state {
		case .began:
			animateTransition(to: state.apposite, and: 1)
			animator.pauseAnimation()
			progress = animator.fractionComplete
		case .changed:
			let translate = sender.translation(in: containerView)
			var fraction = -translate.y / offset
			if state == .open { fraction *= -1}
			if animator.isReversed { fraction *= -1}
			animator.fractionComplete = fraction + progress

		case .ended:
			let velocity = sender.velocity(in: containerView).y
			if velocity == 0 {
				animator.continueAnimation(withTimingParameters: nil, durationFactor: 0)
				break
			}
			shouldClose(should: velocity > 0)
			animator.continueAnimation(withTimingParameters: nil, durationFactor: 0)
		default: break
		}
	}
	func shouldClose(should:Bool) {
		switch state {
		case .open:
			if !should && !animator.isReversed { animator.isReversed = !animator.isReversed}
			if should && animator.isReversed { animator.isReversed = !animator.isReversed}
		case .closed:
			if should && !animator.isReversed { animator.isReversed = !animator.isReversed}
			if !should && animator.isReversed { animator.isReversed = !animator.isReversed}
		}
	}
	func animateTransition(to state: SlideUpState, and duration:TimeInterval) {

		let transition = UIViewPropertyAnimator(duration: duration, dampingRatio: 1) {
			switch state {
			case .open:
				self.containerView.frame.origin.y = self.topSpacing
			case .closed:
				self.containerView.frame.origin.y = self.offset
			}
		}
		
		transition.addCompletion { postion in
			switch postion {
			case .start:
				self.state = state.apposite
			case .end:
				self.state = state
			case .current: ()
			}
		
			switch self.state {
			case .open:
				self.containerView.frame.origin.y = self.topSpacing
			case .closed:
				self.containerView.frame.origin.y = self.offset
			}
			
		}
		animator = transition
		animator.startAnimation()
	}
	
	init(offset:CGFloat = 300, and containerView:UIView) {
		self.offset = offset
		self.containerView = containerView
		containerView.addGestureRecognizer(panGesture)
		containerView.frame.origin.y = offset
	}
}