NavigationHelpers.swift

import UIKit

enum StoryboardIdentifier: String {
    case splashViewController
    case mainMenuViewController
    case levelSelectViewController
    case optionsViewController
    case gameViewController
}

fileprivate extension UIStoryboard {
    func instantiateAppViewController(withIdentifier identifier: StoryboardIdentifier) -> UIViewController {
        return instantiateViewController(withIdentifier: identifier.rawValue)
    }
}

fileprivate class NavigationAssistant {
    private static let mainStoryboard = UIStoryboard(name: "Main", bundle: nil)
    
    private static var splashViewController: SplashViewController {
        return mainStoryboard.instantiateAppViewController(withIdentifier: .splashViewController) as! SplashViewController
    }
    
    private static var mainMenuViewController: MainMenuViewController {
        return mainStoryboard.instantiateAppViewController(withIdentifier: .mainMenuViewController) as! MainMenuViewController
    }
    
    private static var levelSelectViewController: LevelSelectViewController {
        return mainStoryboard.instantiateAppViewController(withIdentifier: .levelSelectViewController) as! LevelSelectViewController
    }
    
    private static var optionsViewController: OptionsViewController {
        return mainStoryboard.instantiateAppViewController(withIdentifier: .optionsViewController) as! OptionsViewController
    }
    
    private static var gameViewController: GameViewController {
        return mainStoryboard.instantiateAppViewController(withIdentifier: .gameViewController) as! GameViewController
    }
    
    fileprivate static func viewController(forIdentifier identifier: StoryboardIdentifier) -> UIViewController {
        switch identifier {
        case .splashViewController:
            return splashViewController
        case .mainMenuViewController:
            return mainMenuViewController
        case .levelSelectViewController:
            return levelSelectViewController
        case .optionsViewController:
            return optionsViewController
        case .gameViewController:
            return gameViewController
        }
    }
}

extension UIViewController {
    func crossDissolve(toIdentifier identifier: StoryboardIdentifier) {
        let viewController = NavigationAssistant.viewController(forIdentifier: identifier)
        viewController.modalPresentationStyle = .fullScreen
        viewController.modalTransitionStyle = .crossDissolve
        show(viewController, sender: self)
    }
}