SpriteSceneNavigationDelegate.swift

/// Make the ViewController conform to this protocol 
///
protocol SceneKitViewController {
    var sceneKitView: SceneKitView! { get }
}

/// Make the ViewController conform to this protocol too
///
protocol SpriteSceneNavigationDelegate {
    func present(_ scene: BaseSpriteScene)
    func transition(to scene: BaseSpriteScene, completion: (() -> Void)?)
    func willTransition(from srcScene: BaseSpriteScene?, to dstScene: BaseSpriteScene)
    func didTransition(from srcScene: BaseSpriteScene?, to dstScene: BaseSpriteScene)  
}

/// All SpriteKit scenes must subclass this. To transition to another scene,
/// call `navigationDelegate.transtion(to:completion:)
///
class BaseSpriteKitScene: SKScene {
    weak var navigationDelegate: SpriteSceneNavigationDelegate?
}

/// Default implementation of the transition methods for an object that
/// conforms to both `SpriteSceneNavigationDelegate` and `SceneKitViewController`
///
extension SpriteSceneNavigationDelegate where Self: SceneKitViewController {
    
    /// Presents a sprite scene immediately.
    func present(_ scene: BaseSpriteScene) {
        sceneKitView.overlaySKScene = scene
        scene.navigationDelegate = self
    }

    /// Transitions into a new sprite scene gradually.
    func transition(to scene: BaseSpriteScene, completion: (() -> Void)? = nil) {
        let source = sceneKitView?.overlaySKScene as? BaseSpriteScene

        willTransition(from: source, to: scene)
        sceneKitView.overlaySKScene?.run(.fadeAlpha(to: 0, duration: 0.25)) { [unowned self] in
            scene.alpha = 0
            self.sceneKitView.overlaySKScene = scene
            scene.run(.fadeAlpha(to: 1, duration: 0.25)) { [unowned self] in
                self.didTransition(from: source, to: scene)
                completion?()
            }
        }
    }
}