使用Swift 3在SceneKit中绘制SpriteKit对象

Question

我正在尝试在SceneKit中绘制一个SpriteKit按钮。

似乎一切正常，但对象离开了屏幕。

代码：

import UIKit
import QuartzCore
import SceneKit
import SpriteKit

class GameViewController: UIViewController , SCNSceneRendererDelegate {

var walkAnimButton: SKSpriteNode!
var overlay: SKScene!

override func viewDidLoad() {
    super.viewDidLoad()

...
    self.setupHUD()
}


func setupHUD () {

    let screenSize: CGSize = UIScreen.main.bounds.size
    let sceneView = self.view as! SCNView

    /* Create overlay SKScene for 3D scene */
    overlay = SKScene.init(size: sceneView.bounds.size)
    overlay.scaleMode = SKSceneScaleMode.aspectFill
    sceneView.overlaySKScene = overlay

    /* Create button for controlling walk animation */
    let walkAnimButton: SKSpriteNode = SKSpriteNode.init(imageNamed: "walking")
    walkAnimButton.anchorPoint = CGPoint(x: 0.5, y: 0.5)
    walkAnimButton.size = CGSize(width: 32, height: 32)
    walkAnimButton.position = CGPoint(x: screenSize.width-walkAnimButton.size.width, y: screenSize.height-walkAnimButton.size.height)
    walkAnimButton.name = "WalkAnimationButton"
    overlay.addChild(walkAnimButton)
}
}

可能是什么错误？

overlay.scaleMode = SKSceneScaleMode.aspectFill

提前致谢。