override func viewDidLoad() {
    super.viewDidLoad()
    let skView = self.view as! SKView

    let scene = GameScene(size: skView.bounds.size)
    scene.scaleMode = .aspectFit

    skView.presentScene(scene)
}

import SpriteKit

class GameScene: SKScene {
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }

    override init(size: CGSize) {
        super.init(size: size)
        // add all the code of the game here...
    }

}

override func didMove(to view: SKView) {
    <#code#>
}