Custom CIFilter on CALayer does NOT work

We are trying to create a custom CIFilter to add on top of our CALayer's. How ever only the default CIFilters seem to work on a CALayer.

We created a small new project on the ViewController.swift we added:

import Cocoa
import CoreImage

class ViewController: NSViewController {

	override func viewDidLoad() {
		super.viewDidLoad()
		
		// Create some layers to work with! (square with gradient color)
		let mainLayer = CALayer()
		let shapeLayer = CAShapeLayer()
		let gradientLayer = CAGradientLayer()
		gradientLayer.colors = [NSColor.red.cgColor, NSColor.white.cgColor, NSColor.yellow.cgColor, NSColor.black.cgColor]
		
		shapeLayer.path = CGPath(rect: CGRect(x: 0, y: 0, width: 500, height: 500), transform: nil)
		shapeLayer.fillColor = CGColor.black
		
		gradientLayer.frame = CGRect(x: 0, y: 0, width: 500, height: 500)
		gradientLayer.mask = shapeLayer
		
		gradientLayer.setAffineTransform(CGAffineTransform(translationX: 50, y: 50))
		mainLayer.addSublayer(gradientLayer)
		
		mainLayer.filters = []
		self.view.layer?.addSublayer(mainLayer)

		// Register the custom filter
		CustomFilterRegister.register()
		
		// Test with a normal image file, WORKS!
//		if let image = NSImage(named: "test"), let cgImage = image.cgImage(forProposedRect: nil, context: nil, hints: nil) {
//			if let filter = CIFilter(name: "CustomFilter") {
//				filter.setValue(CIImage(cgImage: cgImage), forKey: kCIInputImageKey)
//				let output = filter.outputImage
//				// WORKS! Image filtered as expected!
//			}
//		}
		
		// Does NOT work. No change in color of the layer!
		if let filter = CIFilter(name: "CustomFilter") {
			filter.name = "custom"
			mainLayer.filters?.append(filter)
		}

		// This works: mainLayer and sublayers are blurred!
//		if let filter = CIFilter(name: "CIGaussianBlur") {
//			filter.name = "blur"
//			mainLayer.filters?.append(filter)
//		}


	}
}
}

We created a simple custom CIFilter to give it a first try before we start building our custom CIFilter.

class CustomFilter: CIFilter {
	
	// Error in xcode if you don't add this in!
	override class var supportsSecureCoding: Bool {
		return true
	}
		
	@objc dynamic var inputImage: CIImage?
	@objc dynamic var inputSaturation: CGFloat = 1
	@objc dynamic var inputBrightness: CGFloat = 0
	@objc dynamic var inputContrast: CGFloat = 1
	override func setDefaults() {
		inputSaturation = 1
		inputBrightness = 0
		inputContrast = 2
	}
	
	override public var outputImage: CIImage? {
		guard let image = inputImage else {
			return nil
		}
		return image.applyingFilter("CIPhotoEffectProcess")
			.applyingFilter("CIColorControls", parameters: [
				kCIInputSaturationKey: inputSaturation,
				kCIInputBrightnessKey: inputBrightness,
				kCIInputContrastKey: inputContrast
			])
	}
}

class CustomFilterRegister: CIFilterConstructor {
	static func register() {
		CIFilter.registerName(
			"CustomFilter", constructor: CustomFilterRegister(),
			classAttributes: [
				kCIAttributeFilterCategories: [kCICategoryBlur, kCICategoryVideo, kCICategoryStillImage]
			])
	}
	func filter(withName name: String) -> CIFilter? {
		switch name {
		case "CustomFilter":
			return CustomFilter()
		default:
			return nil
		}
	}
}

In the ViewController we added code to test with a normal image. This DOES work so the filter seems to be ok. We also tried a default CIGaussianBlur and that does work on the CALayer.

We are lost as to what is needed to get a custom CIFilter working with CALayer, and can't seem to find any information on it.

Please note that we are NOT looking for this type of CIFilter or a different way to get the filters result. We need a custom CIFilter to work on a CALayer.