（Swift SpriteKit）只允许SKScene出现一次

Question

此代码驻留在viewDidLoad函数中。第一次被称为它是好的。但是后来我从同一个故事板文件中提出了一个子视图，当时让skView = gameView为！ SKView＆＃39;再次被调用并抛出致命错误：发现无...我如何阻止它被其他时间调用？我无法想到我能做到的任何检查。

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)
            }
        }
}