将粒子发射器放在UIButton前面

Question

这是一个非常小的场景，有白雪皑皑的背景以及居中的播放按钮。

如何让雪落在FRONT（可选择在我的按钮的前面和后面）？我似乎无法将UIButton与雪发射器混合搭配。如果我不需要，我宁愿不将按钮变成SKNode（除非那是'正确的'方式）......

class WelcomeViewController: UIViewController {
@IBOutlet weak var playButton: UIButton!
@IBOutlet weak var snowView: SKView!

override func viewDidLoad() {
    super.viewDidLoad()

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

        if let emitter = SKEmitterNode(fileNamed: "snow") {
            emitter.position = CGPointMake(view.center.x, view.bounds.maxY + 20)
            emitter.particlePositionRange = CGVector(dx: view.bounds.width + 20, dy: 10)
            emitter.zPosition = 2
            scene.addChild(emitter)
        }

        playButton.layer.cornerRadius = playButton.frame.size.height / 2
        playButton.bringSubviewToFront(snowView)

        /* Set the scale mode to scale to fit the window */
        scene.scaleMode = .AspectFill
        snowView.presentScene(scene)
    }
}