How to use CAEmitterLayer on macOS in a SwiftUI app using NSViewRepresentable - swift

Question

I want to use a CAEmitterLayer within a macOS app that is based on SwiftUI.
Problem: The layer itself is perfectly visible, but it doesn’t emit any particles.
→ Here’s my demo project on GitHub
I basically built a NSViewRepresentable for a custom class EmitterNSView: NSView that handles the emitter layer itself.
final class EmitterNSView: NSView {
private let emitterLayer: CAEmitterLayer = {
let layer = CAEmitterLayer()
layer.backgroundColor = NSColor.green.withAlphaComponent(0.33).cgColor
return layer
}()
private let emitterCells: [CAEmitterCell] = {
// https://developer.apple.com/documentation/quartzcore/caemitterlayer
let cell = CAEmitterCell()
cell.name = "someParticle"
cell.birthRate = 10
cell.lifetime = 5.0
cell.velocity = 100
cell.velocityRange = 50
cell.emissionLongitude = 0.0
cell.emissionRange = CGFloat.pi * 2.0
cell.spinRange = 5
cell.scale = 1.0
cell.scaleRange = 0.25
cell.alphaSpeed = 0.25
cell.contents = NSImage(named: "whiteParticle.png")!.cgImage
cell.color = NSColor.systemPink.cgColor
cell.xAcceleration = 4
cell.yAcceleration = 3
cell.zAcceleration = 2
return [cell]
}()
override init(frame frameRect: NSRect) {
super.init(frame: frameRect)
self.wantsLayer = true
self.layer = CALayer()
self.layer?.autoresizingMask = [.layerWidthSizable, .layerHeightSizable]
self.configureEmitterLayer()
self.layer?.addSublayer(self.emitterLayer)
}
required init?(coder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
override func layout() {
super.layout()
self.configureEmitterLayer()
}
override func updateLayer() {
super.updateLayer()
self.configureEmitterLayer()
}
func configureEmitterLayer() {
self.emitterLayer.frame = self.frame
self.emitterLayer.autoresizingMask = [.layerHeightSizable, .layerWidthSizable]
self.emitterLayer.masksToBounds = false
self.emitterLayer.drawsAsynchronously = true
self.emitterLayer.emitterMode = .points
self.emitterLayer.birthRate = 2
self.emitterLayer.emitterShape = CAEmitterLayerEmitterShape.line
self.emitterLayer.emitterSize = CGSize(width: frame.width * 0.5, height: frame.height * 0.5)
self.emitterLayer.emitterPosition = CGPoint.zero
self.emitterLayer.renderMode = CAEmitterLayerRenderMode.additive
self.emitterLayer.emitterCells = self.emitterCells
self.emitterLayer.zPosition = 10
self.emitterLayer.beginTime = CACurrentMediaTime()
self.emitterLayer.speed = 1.5
self.emitterLayer.emitterCells = self.emitterCells
}
}
The fact that I can clearly see the green background of the layer within the app indicates that something is wrong with the cells? Feeling lost at this point.
Very similar implementations in UIKit work just fine.
How can I use CAEmitterLayer on macOS within a SwiftUI based app?

Answer 1

Use cgImage(forProposedRect:context:hints:) while casting NSImage to a CGImage.
cell.contents = NSImage(named:"whiteParticle.png").flatMap {
return $0.cgImage(forProposedRect: nil, context: nil, hints: nil)
}