CARenderer绘制到MTLTexture无内容，仅显示红色背景求助

我在Playground中尝试配置CARenderer将内容绘制到MTLTexture，但始终无法成功渲染内容。最终生成的图像是纯红色，黄色背景的CATextLayer完全没有被渲染出来。以下是我使用的最简代码示例：

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

关键问题修正 #

1. 纹理权限不足：原代码设置的.unknown使用权限无法满足CARenderer的写入需求，需改为[.renderTarget, .shaderRead]，确保纹理支持渲染写入和后续读取。
2. 存储模式不兼容：.private存储模式的纹理CPU无法直接访问，改用.shared模式平衡CPU/GPU访问需求。
3. 渲染流程错误：无效的addUpdate调用无法触发图层更新，需通过CATransaction提交图层状态变更，同时提前触发图层的displayIfNeeded确保内容准备完成。
4. 内存管理冗余：手动分配指针的方式既冗余又容易引发内存泄漏，直接用Metal API创建纹理更安全高效。

修正后的完整代码 #

import QuartzCore
import Metal

// 配置CATextLayer并提前准备内容
let layerTest = CATextLayer()
layerTest.frame = CGRect(x: 0, y: 0, width: 1920, height: 1080)
layerTest.string = "TEST"
layerTest.foregroundColor = UIColor.black.cgColor
layerTest.backgroundColor = UIColor.yellow.cgColor
layerTest.position = .zero
layerTest.anchorPoint = .zero
layerTest.masksToBounds = true
// 强制图层完成布局和内容渲染准备
layerTest.displayIfNeeded()

// 初始化Metal设备与上下文
let device = MTLCreateSystemDefaultDevice()!
let context = CIContext(mtlDevice: device)

// 创建符合要求的MTLTexture
let textureDescriptor = MTLTextureDescriptor.texture2DDescriptor(
    pixelFormat: .rgba8Unorm,
    width: 1920,
    height: 1080,
    mipmapped: false
)
textureDescriptor.usage = [.renderTarget, .shaderRead]
textureDescriptor.storageMode = .shared
let mtlTexture = device.makeTexture(descriptor: textureDescriptor)!

// 执行CARenderer渲染
let renderer = CARenderer(mtlTexture: mtlTexture)
renderer.bounds = layerTest.frame
renderer.layer = layerTest

// 提交事务确保图层状态生效
CATransaction.begin()
CATransaction.setDisableActions(true)
renderer.beginFrame(atTime: CACurrentMediaTime(), timeStamp: nil)
renderer.render()
renderer.endFrame()
CATransaction.commit()

// 转换为CGImage查看结果
let ciImage = CIImage(mtlTexture: mtlTexture)!
if let cgImage = context.createCGImage(ciImage, from: ciImage.extent) {
    // Playground中可添加代码显示图片，例如：
    // let image = UIImage(cgImage: cgImage)
}

内容的提问来源于stack exchange，提问作者Mike Woodworth