Add SKReferenceNode/SKScene to another SKScene in SpriteKit

I would like to add a SKScene to my main GameScene. SKReferenceNode seems to be a good solution.

I have : - GameScene.sks (main scene) - Countdown.sks (scene to add to GameScene) - Countdown.swift (Custom class, how does to init it? SKScene ? SKReferenceNode ? SKNode)

I don't know how to add programmatically my countdown using my class Countdown.

I tried:

 let path = Bundle.main.path(forResource: "Countdown", ofType: "sks")
 let cd = SKReferenceNode (url: NSURL (fileURLWithPath: path!) as URL) as! Countdown
 cd.name = "countdown"
 self.addChild(cd)

But I have the following error :

 Could not cast value of type 'SKReferenceNode' (0x10d97ad88) to 'LYT.Countdown' (0x10a5709d0

I also tried something more simple like:

 let cd=Countdown(scene:self) 
 self.addChild(cd)

But I don't know how to init the class using the Countdown.sks file.

I know I also have the possibility to create a SKNode class, and init it 100% programmatically, but it really important for me to use the associated .sks file in order to use the Xcode scene editor.

I do that, I don't know if is the best way to do this, but works:

I've 2 file Dragon.swift and sks

I've added a "main" node like DragonNode and other node children of this

Now, the DragonNode is a custom class, set it in sks file:

The DragonNode is a normal SKSpriteNode

class DragonNode: SKSpriteNode, Fly, Fire {

    var head: SKSpriteNode!
    var body: SKSpriteNode!
    var shadow: SKSpriteNode!
    var dragonVelocity: CGFloat = 250

    required init?(coder aDecoder: NSCoder) {        
        super.init(coder: aDecoder)

        //Example other node from sks file
        body = self.childNodeWithName("Body") as! SKSpriteNode
        head = body.childNodeWithName("Head") as! SKSpriteNode
        shadow = self.childNodeWithName("Shadow") as! SKSpriteNode
        shadow.name = "shadow"
    }

    //Dragon Func
    func fireAction () {}
    func flyAction () {}
}

Inside the scene, add a SKReferenceNode:

In the SKScene code:

    let dragonReference = self.childNodeWithName("DragonReference") as! SKReferenceNode

    let dragonNode = dragonReference.getBasedChildNode() as! DragonNode
    print(dragonNode)
    //Now you can use the Dragon func
    dragonNode.flyAction()

getBasedChildNode() is an extension to find your based node (the first one)

extension SKReferenceNode {
    func getBasedChildNode () -> SKNode? {
        if let child = self.children.first?.children.first {return child}
        else {return nil}
    }
}