a.swift

private func setupAVSession() {
  captureSession.beginConfiguration()
  captureSession.sessionPreset = .high

  defer {
    captureSession.commitConfiguration()
  }

  // input
  guard
    let backCamera = AVCaptureDevice.default(.builtInWideAngleCamera, for: .video, position: .back),
    let input = try? AVCaptureDeviceInput(device: backCamera),
    captureSession.canAddInput(input)
  else {
    return
  }

  captureSession.addInput(input)

  // output
  let output = AVCaptureVideoDataOutput()

  guard captureSession.canAddOutput(output) else {
    return
  }

  captureSession.addOutput(output)
  output.setSampleBufferDelegate(self, queue: DispatchQueue(label: "MyQueue"))
  output.alwaysDiscardsLateVideoFrames = true

  // connection
  let connection = output.connection(with: .video)
  connection?.videoOrientation = .portrait

  // preview layer
  cameraLayer.videoGravity = AVLayerVideoGravity.resizeAspectFill
  view.layer.addSublayer(cameraLayer)
  view.layer.addSublayer(overlayLayer)
}