CARenderer draws nothing into bound texture - core-animation

Question

I'm trying to setup a CARenderer to draw into a mtlTexture, but all my attempts to get this working in a playground don't draw anything.
The resulting image is solid red, the yellow layer doesn't seem to be rendered at all.
Here's the simplest version of what I've tried:
import QuartzCore
import Metal
let layerTest = CATextLayer()
layerTest.frame = .init(origin: .zero, size: .init(width: 1920, height: 1080))
layerTest.string = "TEST"
layerTest.foregroundColor = .black
layerTest.backgroundColor = CGColor(red: 1.0, green: 1.0, blue: 0.0, alpha: 1.0)
layerTest.position = CGPoint(x:0.0, y:0.0)
layerTest.anchorPoint = CGPoint(x:0.0, y:0.0)
layerTest.masksToBounds = true
let device = MTLCreateSystemDefaultDevice()!
let context = CIContext(mtlDevice: device)
let textureDescriptor = MTLTextureDescriptor.texture2DDescriptor(pixelFormat: .rgba8Unorm, width: 1920, height: 1080, mipmapped: false)
textureDescriptor.usage = [.unknown]
textureDescriptor.storageMode = .private
let bytes = UnsafeMutablePointer<UInt8>.allocate(capacity: 1920 * 1080 * 4)
//fill the buffer with red
let pattern = UnsafeMutablePointer<UInt8>.allocate(capacity: 4)
(pattern + 0).initialize(to: 255)
(pattern + 1).initialize(to: 0)
(pattern + 2).initialize(to: 0)
(pattern + 3).initialize(to: 255)
memset_pattern4(bytes, pattern, 1920 * 1080 * 4)
let mtlBuffer = device.makeBuffer(bytes: bytes, length: 1920 * 1080 * 4)!
let mtlTexture = mtlBuffer.makeTexture(descriptor: textureDescriptor, offset: 0, bytesPerRow: 1920 * 4)!
let render = CARenderer(mtlTexture: mtlTexture)
render.bounds = layerTest.frame
render.layer = layerTest
render.setDestination(mtlTexture)
render.beginFrame(atTime: CACurrentMediaTime(), timeStamp: nil)
render.addUpdate(render.bounds)
render.render()
render.endFrame()
let ciImage = CIImage(mtlTexture: mtlTexture)!
let cgImage: CGImage = context.createCGImage(ciImage, from: ciImage.extent)! //<- this is just red frame

Answer 1

I submitted this to apple DTS and got a reply:
Setting the root layer of the CARenderer requires one implicit CATransaction to transfer ownership of the layer tree to the CARenderer’s context. CARenderer frame render methods will not work correctly until this ownership transfer is complete.
To complete the current transaction we call flush() and then commit() it:
render.layer = layerTest
CATransaction.flush()
CATransaction.commit()
Adding these two lines resolved the issue for me.