import UIKit
import Foundation
import PlaygroundSupport

enum Theme {
    case light
    case dark

    var description: String {
        switch self {
        case .light: return "Light"
        case .dark: return "Dark"
        }
    }
}

extension Notification.Name {
    static let themeChanged = Notification.Name("themeChanged")
}

// Dependency injection
//final class ThemeManager {
//    private(set) var theme: Theme = .light {
//        didSet {
//            NotificationCenter.default.post(name: .themeChanged, object: nil)
//        }
//    }
//
//    func changeTheme(to theme: Theme) {
//        self.theme = theme
//    }
//}


// Singleton
final class ThemeManager {

    static let current = ThemeManager()

    private(set) var theme: Theme = .light {
        didSet {
            NotificationCenter.default.post(Notification(name: .themeChanged))
        }
    }

    private init() {}

    func changeTheme(to theme: Theme) {
        self.theme = theme
    }
}

final class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        self.applyTheme()
        NotificationCenter.default.addObserver(self, selector: #selector(self.applyTheme), name: .themeChanged, object: nil)

        DispatchQueue.main.asyncAfter(deadline: .now() + 3) {
            ThemeManager.current.changeTheme(to: .dark)
        }
    }

    @objc
    private func applyTheme() {
        let theme = ThemeManager.current.theme
        print(theme.description)
    }
}

let viewController = ViewController()
PlaygroundPage.current.liveView = viewController