Bottom sheet modal view controller with swift

import UIKit

class ViewController: UIViewController {
    
    // Defined UI views
    lazy var titleLabel: UILabel = {
        let label = UILabel()
        label.text = "百字令·偶忆"
        label.font = .boldSystemFont(ofSize: 32)
        return label
    }()

    
    lazy var textView: UITextView = {
        let textView = UITextView(frame: .zero)
        textView.font = UIFont.systemFont(ofSize: 22)
        textView.isEditable = false
        textView.text = "横街南巷，记钿车小小，翠帘徐揭。绿酒分曹人散后，心事低徊潜说。莲子湖头，枇杷花下，绾就同心结。明珠未斛，朔风千里催别。\n同是沦落天涯，青青柳色，争忍先攀折。红浪香温围夜玉，堕我怀中明月。暮雨空归，秋河不动，虬箭丁丁咽。十年一梦，鬓丝今已如雪。 "
        return textView
    }()
    
    lazy var registerButton: UIButton = {
        let button = UIButton()
        button.setTitle("Get Started", for: .normal)
        button.setTitleColor(.white, for: .normal)
        button.backgroundColor = view.tintColor
        button.layer.cornerRadius = 8
        button.clipsToBounds = true
        return button
    }()
    
    lazy var containerStackView: UIStackView = {
        let spacer = UIView()
        let stackView = UIStackView(arrangedSubviews: [titleLabel, textView, spacer, registerButton])
        stackView.axis = .vertical
        stackView.spacing = 16.0
        return stackView
    }()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        setupView()
        setupConstraints()
        // 3. Add action
        registerButton.addTarget(self, action: #selector(presentModalController), for: .touchUpInside)

    }
    
    func setupView() {
        // cosmetics
        view.backgroundColor = .systemBackground
    }
    
    // Add subviews and set constraints
    func setupConstraints() {
        view.addSubview(containerStackView)
        containerStackView.translatesAutoresizingMaskIntoConstraints = false
        
        let safeArea = view.safeAreaLayoutGuide
        // Call .activate method to enable the defined constraints
        NSLayoutConstraint.activate([
            // Set containerStackView edges to superview with 24 spacing
            containerStackView.topAnchor.constraint(equalTo: safeArea.topAnchor, constant: 24),
            containerStackView.bottomAnchor.constraint(equalTo: safeArea.bottomAnchor, constant: -24),
            containerStackView.leadingAnchor.constraint(equalTo: safeArea.leadingAnchor, constant: 24),
            containerStackView.trailingAnchor.constraint(equalTo: safeArea.trailingAnchor, constant: -24),
            // Set button height
            registerButton.heightAnchor.constraint(equalToConstant: 50)
        ])
    }
    
    // To be updated
    @objc func presentModalController() {
        let vc = CustomModalViewController()
        vc.modalPresentationStyle = .overCurrentContext
        // Keep animated value as false
        // Custom Modal presentation animation will be handled in VC itself
        self.present(vc, animated: false)
    } 
}

import UIKit

class CustomModalViewController: UIViewController {
    
    // define lazy views
    lazy var titleLabel: UILabel = {
        let label = UILabel()
        label.text = "Get Started"
        label.font = .boldSystemFont(ofSize: 20)
        return label
    }()
    
    lazy var notesLabel: UILabel = {
        let label = UILabel()
        label.text = "横街南巷，记钿车小小，翠帘徐揭。绿酒分曹人散后，心事低徊潜说。莲子湖头，枇杷花下，绾就同心结。明珠未斛，朔风千里催别。\n同是沦落天涯，青青柳色，争忍先攀折。红浪香温围夜玉，堕我怀中明月。暮雨空归，秋河不动，虬箭丁丁咽。十年一梦，鬓丝今已如雪。 "
        label.font = .systemFont(ofSize: 16)
        label.textColor = .darkGray
        label.numberOfLines = 0
        return label
    }()
    
    lazy var contentStackView: UIStackView = {
        let spacer = UIView()
        let stackView = UIStackView(arrangedSubviews: [titleLabel, notesLabel, spacer])
        stackView.axis = .vertical
        stackView.spacing = 12.0
        return stackView
    }()
    
    lazy var containerView: UIView = {
        let view = UIView()
        view.backgroundColor = .white
        view.layer.cornerRadius = 16
        view.clipsToBounds = true
        return view
    }()
    
    let maxDimmedAlpha: CGFloat = 0.6
    lazy var dimmedView: UIView = {
        let view = UIView()
        view.backgroundColor = .black
        view.alpha = maxDimmedAlpha
        return view
    }()
    
    let defaultHeight: CGFloat = 300
    let dismissibleHeight: CGFloat = 200
    let maximumContainerHeight: CGFloat = UIScreen.main.bounds.height - 64
    // keep updated with new height
    var currentContainerHeight: CGFloat = 300
    
    // Dynamic container constraint
    var containerViewHeightConstraint: NSLayoutConstraint?
    var containerViewBottomConstraint: NSLayoutConstraint?
    
    override func viewDidLoad() {
        super.viewDidLoad()
        setupView()
        setupConstraints()
        
        // tap gesture on dimmed view to dismiss
        let tapGesture = UITapGestureRecognizer(target: self, action: #selector(self.handleCloseAction))
        dimmedView.addGestureRecognizer(tapGesture)
        
        setupPanGesture()
    }
    
    @objc func handleCloseAction() {
        animateDismissView()
    }
    
    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        animateShowDimmedView()
        animatePresentContainer()
    }
    
    func setupView() {
        view.backgroundColor = .clear
    }
    
    func setupConstraints() {
        // Add subviews
        view.addSubview(dimmedView)
        view.addSubview(containerView)
        dimmedView.translatesAutoresizingMaskIntoConstraints = false
        containerView.translatesAutoresizingMaskIntoConstraints = false
        
        containerView.addSubview(contentStackView)
        contentStackView.translatesAutoresizingMaskIntoConstraints = false
        
        // Set static constraints
        NSLayoutConstraint.activate([
            // set dimmedView edges to superview
            dimmedView.topAnchor.constraint(equalTo: view.topAnchor),
            dimmedView.bottomAnchor.constraint(equalTo: view.bottomAnchor),
            dimmedView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
            dimmedView.trailingAnchor.constraint(equalTo: view.trailingAnchor),
            // set container static constraint (trailing & leading)
            containerView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
            containerView.trailingAnchor.constraint(equalTo: view.trailingAnchor),
            // content stackView
            contentStackView.topAnchor.constraint(equalTo: containerView.topAnchor, constant: 32),
            contentStackView.bottomAnchor.constraint(equalTo: containerView.bottomAnchor, constant: -20),
            contentStackView.leadingAnchor.constraint(equalTo: containerView.leadingAnchor, constant: 20),
            contentStackView.trailingAnchor.constraint(equalTo: containerView.trailingAnchor, constant: -20),
        ])
        
        // Set dynamic constraints
        // First, set container to default height
        // after panning, the height can expand
        containerViewHeightConstraint = containerView.heightAnchor.constraint(equalToConstant: defaultHeight)
        
        // By setting the height to default height, the container will be hide below the bottom anchor view
        // Later, will bring it up by set it to 0
        // set the constant to default height to bring it down again
        containerViewBottomConstraint = containerView.bottomAnchor.constraint(equalTo: view.bottomAnchor, constant: defaultHeight)
        // Activate constraints
        containerViewHeightConstraint?.isActive = true
        containerViewBottomConstraint?.isActive = true
    
    }
    
    
    @objc func handlePanGesture(gesture: UIPanGestureRecognizer) {
        let translation = gesture.translation(in: view)
        // drag to top will be minus value and vice versa
        print("Pan gesture y offset: \(translation.y)")
        
        // get drag direction
        let isDraggingDown = translation.y > 0
        print("Dragging direction: \(isDraggingDown ? "going down" : "going up")")
        
        // New height is based on value of dragging plus current container height
        let newHeight = currentContainerHeight - translation.y
        
        // Handle based on gesture state
        switch gesture.state {
        case .changed:
            // This state will occur when user is dragging
            if newHeight < maximumContainerHeight {
                // Keep updating the height constraint
                containerViewHeightConstraint?.constant = newHeight
                // refresh layout
                view.layoutIfNeeded()
            }
        case .ended:
            // This happens when user stop drag,
            // so we will get the last height of container
            // Condition 1: If new height is below min, dismiss controller
            if newHeight < dismissibleHeight {
                self.animateDismissView()
            }
            else if newHeight < defaultHeight {
                // Condition 2: If new height is below default, animate back to default
                animateContainerHeight(defaultHeight)
            }
            else if newHeight < maximumContainerHeight && isDraggingDown {
                // Condition 3: If new height is below max and going down, set to default height
                animateContainerHeight(defaultHeight)
            }
            else if newHeight > defaultHeight && !isDraggingDown {
                // Condition 4: If new height is below max and going up, set to max height at top
                animateContainerHeight(maximumContainerHeight)
            }
        default:
            break
        }
        
    }
    
    func setupPanGesture() {
        // add pan gesture recognizer to the view controller's view (the whole screen)
        let panGesture = UIPanGestureRecognizer(target: self, action: #selector(self.handlePanGesture(gesture:)))
        // change to false to immediately listen on gesture movement
        panGesture.delaysTouchesBegan = false
        panGesture.delaysTouchesEnded = false
        view.addGestureRecognizer(panGesture)
    }
    
    func animateContainerHeight(_ height: CGFloat) {
        UIView.animate(withDuration: 0.4) {
            // Update container height
            self.containerViewHeightConstraint?.constant = height
            // Call this to trigger refresh constraint
            self.view.layoutIfNeeded()
        }
        // Save current height
        currentContainerHeight = height
    }
    
    func animatePresentContainer() {
        // Update bottom constraint in animation block
        UIView.animate(withDuration: 0.3) {
            self.containerViewBottomConstraint?.constant = 0
            // Call this to trigger refresh constraint
            self.view.layoutIfNeeded()
        }
    }
    
    func animateShowDimmedView() {
        dimmedView.alpha = 0
        UIView.animate(withDuration: 0.4) {
            self.dimmedView.alpha = self.maxDimmedAlpha
        }
    }
    
    func animateDismissView() {
        // hide blur view
        dimmedView.alpha = maxDimmedAlpha
        UIView.animate(withDuration: 0.4) {
            self.dimmedView.alpha = 0
        } completion: { _ in
            // once done, dismiss without animation
            self.dismiss(animated: false)
        }
        // hide main view by updating bottom constraint in animation block
        UIView.animate(withDuration: 0.3) {
            self.containerViewBottomConstraint?.constant = self.defaultHeight
            // call this to trigger refresh constraint
            self.view.layoutIfNeeded()
        }
    }
    
}