DropDownView.swift

import UIKit

protocol DropDownButtonDelegate: AnyObject {
	func didSelect(_ index: Int)
}

class DropDownButton: UIView {

	let button: UIButton = {
		let button = UIButton()
		button.translatesAutoresizingMaskIntoConstraints = false
		button.setTitle("Pick option", for: .normal)
		button.setTitleColor(.black, for: .normal)
		button.layer.borderWidth = 1.2
		button.layer.borderColor = UIColor.gray.cgColor
		return button
	}()

	let stackView: UIStackView = {
		let stack = UIStackView()
		stack.translatesAutoresizingMaskIntoConstraints = false
		stack.distribution = .fill
		stack.alignment = .fill
		stack.spacing = 4
		stack.axis = .vertical
		return stack
	}()
	
	let tableView: UITableView = {
		let table = UITableView()
		table.translatesAutoresizingMaskIntoConstraints = false
		table.isHidden = true
		table.layer.borderWidth = 1.2
		table.layer.borderColor = UIColor.gray.cgColor
		table.register(DropdownTableViewCell.self, forCellReuseIdentifier: DropdownTableViewCell.reuseIdentifier)
		return table
	}()

	var dataSource: [String] = [] {
		didSet {
			updateTableDataSource()
		}
	}

	var title: String = "" {
		didSet {
			button.setTitle(title, for: .normal)
		}
	}

	var delegate: DropDownButtonDelegate?

	var tableViewHeight: NSLayoutConstraint?

	var buttonHeightConstraint: NSLayoutConstraint?
	var buttonHeight: CGFloat = 40 {
		didSet {
			buttonHeightConstraint?.constant = buttonHeight
			updateTableDataSource()
		}
	}

	var maxVisibleCellsAmount: Int = 4 {
		didSet {
			updateTableDataSource()
		}
	}

	var buttonBottomConstraint: NSLayoutYAxisAnchor {
		button.bottomAnchor
	}

	init() {
		super.init(frame: .zero)
		setup()
	}

	required init?(coder: NSCoder) {
		super.init(coder: coder)
		setup()
	}

	func setup() {
		addSubview(stackView)
		stackView.addArrangedSubview(button)
		stackView.addArrangedSubview(tableView)
		tableView.delegate = self
		tableView.dataSource = self
		NSLayoutConstraint.activate([
			stackView.topAnchor.constraint(equalTo:topAnchor),
			stackView.leadingAnchor.constraint(equalTo: leadingAnchor),
			stackView.trailingAnchor.constraint(equalTo: trailingAnchor),
			stackView.bottomAnchor.constraint(equalTo: bottomAnchor),
			
		])
		buttonHeightConstraint = button.heightAnchor.constraint(equalToConstant: buttonHeight)
		buttonHeightConstraint?.isActive = true
		tableViewHeight = tableView.heightAnchor.constraint(equalToConstant: 0)
		tableViewHeight?.isActive = true
		button.addTarget(self, action: #selector(buttonTapped), for: .primaryActionTriggered)
	}

	func updateTableDataSource() {
		if dataSource.count >= maxVisibleCellsAmount {
			tableViewHeight?.constant = CGFloat(maxVisibleCellsAmount) * buttonHeight
		} else {
			tableViewHeight?.constant = CGFloat(dataSource.count) * buttonHeight
		}
		tableView.reloadData()
	}

	@objc private func buttonTapped() {
		tableView.isHidden.toggle()
	}

}

extension DropDownButton: UITableViewDataSource {

	func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
		dataSource.count
	}

	func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
		guard let cell = tableView.dequeueReusableCell(withIdentifier: DropdownTableViewCell.reuseIdentifier, for: indexPath) as? DropdownTableViewCell else { return UITableViewCell() }
		cell.set(dataSource[indexPath.row])
		return cell
	}

}

extension DropDownButton: UITableViewDelegate {
	func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
		button.setTitle(dataSource[indexPath.row], for: .normal)
		delegate?.didSelect(indexPath.row)
		tableView.isHidden = true
	}

	func tableView(_ tableView: UITableView, heightForRowAt indexPath: IndexPath) -> CGFloat {
		return buttonHeight
	}
}