CARendererMetal.swift

import UIKit
import MetalKit

class ViewController: UIViewController {
    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        
        let imageView = UIImageView(image: UIImage(named: "landscape.jpeg"))
        imageView.frame = CGRect(x: 0, y: 200, width: 350, height: 197)
        view.addSubview(imageView)
        
        DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(3)) {
            // CARenderer outputs 350 🤕
            let texture = imageView.snapshot(device: MTLCreateSystemDefaultDevice()!)!
            print(texture.width)
            
            let image = UIImage(ciImage: CIImage(mtlTexture: texture)!)
            print(image.size.width * image.scale)
            
            // UIView.drawHierarchy outputs 1,050 as expected 🥳
            let rendererFormat = UIGraphicsImageRendererFormat.default()
            rendererFormat.opaque = true

            let renderer = UIGraphicsImageRenderer(size: imageView.bounds.size, format: rendererFormat)

            let image2 = renderer.image { context in
                imageView.drawHierarchy(in: imageView.bounds, afterScreenUpdates: false)
            }
            
            print(image2.size.width * image2.scale)
        }
    }
}

extension UIView {
    func snapshot(device: MTLDevice) -> MTLTexture? {
        let width = Int(bounds.width)
        let height = Int(bounds.height)
        
        let desc = MTLTextureDescriptor.texture2DDescriptor(pixelFormat: .rgba8Unorm, width: width, height: height, mipmapped: false)
        desc.usage = [.renderTarget, .shaderRead, .shaderWrite]
        
        let texture = device.makeTexture(descriptor: desc)!
        let renderer = CARenderer(mtlTexture: texture)
                
        renderer.layer = layer
        renderer.bounds = CGRect(x: 0, y: 0, width: width, height: height)
                    
        CATransaction.flush()
        
        renderer.beginFrame(atTime: CACurrentMediaTime(), timeStamp: nil)
        renderer.addUpdate(CGRect(x: 0, y: 0, width: width, height: height))
        renderer.render()
        renderer.endFrame()
        
        return texture
    }
}