ios - (Swift SpriteKit) 只允许 SKScene 呈现一次

Question

此代码位于 viewDidLoad 函数中。第一次调用它就很好。但是然后我在同一个 Storyboard 文件中在其顶部呈现一个 subview ，并且当'let skView = gameView as! SKView' 再次被调用并抛出 fatal error :Found nil...我如何阻止它在其他时间被调用？除了到位之外，我想不出任何可以进行的检查。

if let scene = GameScene(fileNamed:"GameScene") {
        // Configure the view.

        let skView = gameView as! SKView
        skView.showsFPS = true
        skView.showsNodeCount = true

        /* Sprite Kit applies additional optimizations to improve rendering performance */
        skView.ignoresSiblingOrder = true

        /* Set the scale mode to scale to fit the window */
        scene.scaleMode = .AspectFill

        skView.presentScene(scene)

    }

Answer 1

override func viewWillLayoutSubviews() {
        super.viewWillLayoutSubviews()
        let skView = self.view as! SKView
        if skView.scene == nil {
            if let scene = GameScene(fileNamed:"GameScene") {
                skView.presentScene(scene)
            }
        }
}