BottomSheet with variable size fitting collapsed view

BottomSheetExample.swift

//: A UIKit based Playground for presenting user interface
// based on: https://stackoverflow.com/questions/37967555/how-can-i-mimic-the-bottom-sheet-from-the-maps-app
// This example includes an autoresizing collapsed-view. 
// Clicking on "testing" will add another label, updating the collapsed-view's height.
// Click and drag the red section (BottomSheet) to update its position.

import UIKit
import PlaygroundSupport

let screenHeight: CGFloat = 400
let screenWidth: CGFloat = 200

class BottomSheet: UIViewController {
    
    let topPadding: CGFloat = 50
    var bottomPadding: CGFloat {
        screenHeight - collapsedView!.frame.height
    }
    
    private var collapsedView: UIView?
    
    public var collapsedViewHeight: CGFloat {
        collapsedView!.frame.height
    }
    
    public func addCollapsedView(_ innerView: UIView) {
        collapsedView = innerView
        
        innerView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(innerView)
        
        innerView.leadingAnchor.constraint(equalTo: view.leadingAnchor).isActive = true
        innerView.topAnchor.constraint(equalTo: view.topAnchor).isActive = true
        innerView.trailingAnchor.constraint(equalTo: view.trailingAnchor).isActive = true
        innerView.bottomAnchor.constraint(lessThanOrEqualTo: view.bottomAnchor).isActive = true
        adjustCollapsedViewInitialHeight()
    }
    
    private func adjustCollapsedViewInitialHeight() {
        view.layoutIfNeeded()
        let bottomPadding = screenHeight - collapsedViewHeight
        let bottomSheetHeight = view.frame.height
        let bottomSheetWidth = view.frame.width
        
        UIView.animate(withDuration: 0.2, delay: 0.0, options: [.allowUserInteraction], animations: {
            self.view.frame = CGRect(x: 0,
                                     y: bottomPadding,
                                     width: bottomSheetWidth,
                                     height: bottomSheetHeight)
        })
    }
    
    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        adjustCollapsedViewInitialHeight()
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .green
        
        let gesture = UIPanGestureRecognizer(target: self,
                                             action: #selector(Self.panGesture))
        view.addGestureRecognizer(gesture)
    }
    
    @objc func panGesture(_ recognizer: UIPanGestureRecognizer) {
        let translation = recognizer.translation(in: self.view)
        let velocity = recognizer.velocity(in: self.view)
        let y = self.view.frame.minY
        if ( y + translation.y >= topPadding) && (y + translation.y <= bottomPadding ) {
            self.view.frame = CGRect(x: 0, y: y + translation.y, width: view.frame.width, height: view.frame.height)
            recognizer.setTranslation(CGPoint.zero, in: self.view)
        }
        
        if recognizer.state == .ended {
            var duration =  velocity.y < 0 ? Double((y - topPadding) / -velocity.y) : Double((bottomPadding - y) / velocity.y )
            
            duration = duration > 1.3 ? 1 : duration
            
            UIView.animate(withDuration: duration, delay: 0.0, options: [.allowUserInteraction], animations: {
                if  velocity.y >= 0 {
                    self.view.frame = CGRect(x: 0, y: self.bottomPadding, width: self.view.frame.width, height: self.view.frame.height)
                } else {
                    self.view.frame = CGRect(x: 0, y: self.topPadding, width: self.view.frame.width, height: self.view.frame.height)
                }
                
            }, completion: nil)
        }
    }
}

class ViewController: UIViewController {
    var stack = UIStackView()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .gray
    }
    
    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        addBottomSheetView()
    }
    
    func addBottomSheetView() {
        // 1- Init bottomSheetVC
        let bottomSheetVC = BottomSheet()
        bottomSheetVC.addCollapsedView(makeStackView())
        
        // 2- Add bottomSheetVC as a child view
        self.addChild(bottomSheetVC)
        self.view.addSubview(bottomSheetVC.view)
        bottomSheetVC.didMove(toParent: self)
    }
    
    func makeStackView() -> UIStackView {
        stack.backgroundColor = .red
        stack.axis = .vertical
        stack.addArrangedSubview(makeLabel())
        stack.addArrangedSubview(makeLabel())
        return stack
    }
    
    func makeLabel() -> UILabel {
        let label = UILabel()
        label.text = "testing"
        label.isUserInteractionEnabled = true
        let gesture = UITapGestureRecognizer(target: self, action: #selector(componentTapped))
        label.addGestureRecognizer(gesture)
        return label
    }
    
    @objc func componentTapped() {
        stack.addArrangedSubview(makeLabel())
    }
}

let vc = ViewController()
vc.view.frame = CGRect(x: 0, y: 0, width: screenWidth, height: screenHeight)

PlaygroundPage.current.needsIndefiniteExecution = true
PlaygroundPage.current.liveView = vc

Example: