ModalVC.swift

import Foundation
import UIKit

class ModalScreenVC: UIViewController {
    
    // MARK: - Outlets
    
    /// A subview filling this controller's view with desired top margin
    @IBOutlet private var contentView: UIView!
    
    // MARK: - Properties
    
    private var contentViewInitialYOrigin: CGFloat = 0
    
    // MARK: - Lifecycle
    
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        presentContent()
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        modalPresentationStyle = .overFullScreen
        modalTransitionStyle = .crossDissolve
        addGesture()
    }
    
    override func dismiss(animated flag: Bool, completion: (() -> Void)? = nil) {
        dismissContent()
    }
    
    // MARK: - Actions
    
    @objc private func onGestureCalled(_ gesture: UIPanGestureRecognizer) {
        handleGesture(gesture)
    }
    
    @IBAction private func didTapClose(_ sender: UIButton) {
        dismiss(animated: false)
    }
}

private extension ModalScreenVC {
    
    func presentContent() {
        guard let presentingVC = presentingViewController else { return }
        
        view.backgroundColor = UIColor.init(red: 0, green: 0, blue: 0, alpha: 0.6)
        contentView.transform = CGAffineTransform(translationX: 0, y: UIScreen.main.bounds.height)
        presentingVC.view.layer.cornerRadius = 10
        
        UIView.animate(withDuration: 0.3, delay: 0, usingSpringWithDamping: 1, initialSpringVelocity: 0.3, options: [.curveEaseOut], animations: {
            presentingVC.view.transform = CGAffineTransform(scaleX: 0.9, y: 0.9)
            presentingVC.view.alpha = 0.4
            self.contentView.transform = .identity
        }) { _ in
            self.contentViewInitialYOrigin = self.contentView.frame.origin.y
        }
    }
    
    func addGesture() {
        let panGesture = UIPanGestureRecognizer(target: self, action: #selector(onGestureCalled(_:)))
        contentView.addGestureRecognizer(panGesture)
    }
    
    func handleGesture(_ gesture: UIPanGestureRecognizer) {
        switch gesture.state {
        case .changed:
            contentView.frame.origin.y = contentViewInitialYOrigin + (gesture.translation(in: view).y / 2)
        case .cancelled, .ended, .failed:
            if gesture.translation(in: view).y > UIScreen.main.bounds.height / 4 {
                dismiss(animated: false)
            } else {
                UIView.animate(withDuration: 0.3, delay: 0, usingSpringWithDamping: 1, initialSpringVelocity: 0.3, options: [.curveEaseOut], animations: {
                    self.contentView.frame.origin.y = self.contentViewInitialYOrigin
                }, completion: nil)
            }
        default: break
        }
    }
    
    func dismissContent() {
        guard let presentingVC = presentingViewController else { super.dismiss(animated: false, completion: nil); return }
        
        UIView.animate(withDuration: 0.3, animations: {
            self.contentView.transform = CGAffineTransform(translationX: 0, y: UIScreen.main.bounds.height * 1.5)
            presentingVC.view.transform = .identity
            presentingVC.view.alpha = 1
            self.view.backgroundColor = .clear
        }) { _ in
            super.dismiss(animated: false, completion: { presentingVC.setNeedsStatusBarAppearanceUpdate() })
        }
    }
}