com.misaki.hdrp-toon/Editor/Inspector/View/BoxLightAdjustmentView.cs

using UnityEditor;
using UnityEngine;
using UnityEngine.UIElements;

namespace Misaki.HdrpToon.Editor
{
    [CustomEditor(typeof(BoxLightAdjustment))]
    public class BoxLightAdjustmentView : UnityEditor.Editor
    {
        [SerializeField]
        private VisualTreeAsset _visualTreeAsset;

        public override VisualElement CreateInspectorGUI()
        {
            if (_visualTreeAsset == null)
            {
                return base.CreateInspectorGUI();
            }

            var root = new VisualElement
            {
                dataSource = target
            };

            var visualTreeElement = _visualTreeAsset.Instantiate();

            root.Add(visualTreeElement);

            return root;
        }
    }
}