Metal-backed CoreImage Renderer

Renderer.swift

import UIKit
import CoreImage
import Metal

public class Renderer {
    
    public enum Error: Swift.Error {
        case failedToCreateRenderer
        case infiniteRenderRect
        case failedToRenderImage
    }
    
    /// Having access to this is useful if you want to use a MTKView to display the output.
    public let queue: MTLCommandQueue
    
    /// Does the rendering.
    private let context: CIContext
    
    public init() throws {
        guard
            let device = MTLCreateSystemDefaultDevice(),
            let queue = device.makeCommandQueue()
        else {
            throw Error.failedToCreateRenderer
        }
        
        self.queue = queue
        context = CIContext(mtlCommandQueue: queue)
    }
    
    /// Renders a CIImage using a GPU-backed CIContext.
    ///
    /// - Parameter image: The `CIImage` to render.
    /// - Parameter rect: The area of the image to render.
    /// - Returns: A UIImage of the render.
    ///
    /// - Throws: A `Renderer.Error` if the image failed to render or the image extent was infinite and no render rect was specified.
    ///
    public func render(image: CIImage, inRect rect: CGRect? = nil) throws -> UIImage {
        let renderRect = rect ?? image.extent
        guard renderRect != .infinite else {
            throw Error.infiniteRenderRect
        }
        guard let cgImage = context.createCGImage(image, from: renderRect) else {
            throw Error.failedToRenderImage
        }
        return UIImage(cgImage: cgImage)
    }
}