Swift UIImage Extension that generates a grayscale copy of the original image

UIImageToGrayscale.swift

//
//  UIImageToGrayscale.swift
//
//  Created by João Santos on 27/11/2018.
//  Copyright © 2018 João Santos. All rights reserved.
//

import UIKit

extension UIImage {
    
    func grayscaleImage() -> UIImage? {
        guard let cgImage = self.cgImage, let filter = CIFilter(name: "CIPhotoEffectTonal") else { return nil }

        filter.setDefaults()
        filter.setValue(CIImage(cgImage: cgImage), forKey: kCIInputImageKey)
        
        guard let outputImage = filter.outputImage, let imageRef = CIContext(options: nil).createCGImage(outputImage, from: outputImage.extent) else { return nil }

        return UIImage(cgImage: imageRef)
    }
    
}