arfoundation-samples/Assets/Scenes/FaceTracking/ToggleCameraFacingDirection.cs

using UnityEngine;
using UnityEngine.XR.ARFoundation;

namespace UnityEngine.XR.ARFoundation.Samples
{
    public class ToggleCameraFacingDirection : MonoBehaviour
    {
        [SerializeField]
        ARCameraManager m_CameraManager;

        public ARCameraManager cameraManager
        {
            get => m_CameraManager;
            set => m_CameraManager = value;
        }

        [SerializeField]
        ARSession m_Session;

        public ARSession session
        {
            get => m_Session;
            set => m_Session = value;
        }

        void Update()
        {
            if (m_CameraManager == null || m_Session == null)
                return;

            if (Input.touchCount > 0 && Input.touches[0].phase == TouchPhase.Began)
            {
                if (m_CameraManager.requestedFacingDirection == CameraFacingDirection.User)
                {
                    m_CameraManager.requestedFacingDirection = CameraFacingDirection.World;
                }
                else
                {
                    m_CameraManager.requestedFacingDirection = CameraFacingDirection.User;
                }
            }
        }
    }
}