MonoImageOperation.swift

import UIKit

class MonoImageOperation: Operation {

    var inputImage: UIImage?
    var outputImage: UIImage?

    init(inputImage: UIImage) {
        self.inputImage = inputImage
    }

    override public func main() {
        if self.isCancelled {
            return
        }
        outputImage = applyMonoEffectTo(image: inputImage)
    }

    private func applyMonoEffectTo(image: UIImage?) -> UIImage? {
        guard let image = image,
            let ciImage = CIImage(image: image),
            let mono = CIFilter(name: "CIPhotoEffectMono",
                                 withInputParameters: [kCIInputImageKey: ciImage])
        else { return nil }

        let ciContext = CIContext()
        guard let monoImage = mono.outputImage,
            let cgImage = ciContext.createCGImage(monoImage, from: monoImage.extent)
        else { return nil }

        return UIImage(cgImage: cgImage)
    }
}