CAEmitterLayer doesn't work in MacOS with Swift - swift

Question

I just simply create a storyboard program, and change the main ViewController code to the code below which works well in others' example. but it appears nothing on the view? seeking help.
What I did:
create an project with storyboard, and I got 'AppDelegate.swift' and 'ViewController.swift'
paste 'Ball_green.png' to my project and see it under the file tree of my project.
paste the code below in 'ViewController.swift'
import Cocoa
class ViewController: NSViewController {
override func viewDidLoad() {
super.viewDidLoad()
// Do view setup here.
self.view.wantsLayer = true
self.addLayer()
}
func addLayer(){
let rootLayer = self.view.layer
let snowEmitter = CAEmitterLayer()
snowEmitter.drawsAsynchronously = true
snowEmitter.name = "snowEmitter"
snowEmitter.zPosition = 10.0
snowEmitter.emitterPosition = CGPoint(x: 0, y: 0)
snowEmitter.renderMode = CAEmitterLayerRenderMode.backToFront
snowEmitter.emitterShape = CAEmitterLayerEmitterShape.circle
snowEmitter.emitterZPosition = -43.00
snowEmitter.emitterSize = CGSize(width: 160, height: 160)
snowEmitter.velocity = 20.57
snowEmitter.emitterMode = CAEmitterLayerEmitterMode.points
snowEmitter.birthRate = 10
let snowFlakesCell2 = CAEmitterCell()
snowFlakesCell2.emissionRange = .pi
snowFlakesCell2.lifetime = 10.0
snowFlakesCell2.birthRate = 4.0
snowFlakesCell2.velocity = 2.0
snowFlakesCell2.velocityRange = 100.0
snowFlakesCell2.yAcceleration = 300.0
snowFlakesCell2.contents = NSImage(named: "Ball_green.png")!.cgImage
snowFlakesCell2.magnificationFilter = convertFromCALayerContentsFilter(CALayerContentsFilter.nearest)
snowFlakesCell2.scale = 0.72
snowFlakesCell2.scaleRange = 0.14
snowFlakesCell2.spin = 0.38
snowFlakesCell2.spinRange = 0
snowEmitter.emitterCells = [snowFlakesCell2]
rootLayer!.addSublayer(snowEmitter)
}
}
fileprivate func convertFromCALayerContentsFilter(_ input: CALayerContentsFilter) -> String {
return input.rawValue
}

Answer 1

extension NSImage {
var cgImage: CGImage {
get {
let imageData = self.tiffRepresentation
let source = CGImageSourceCreateWithData(imageData as! CFData, nil)
let maskRef = CGImageSourceCreateImageAtIndex(source!, 0, nil)
return maskRef!
}
}
}
I've solved the problem after adding the code.