BallSpecAssetEditor.cs

using UnityEditor;
using UnityEngine.UIElements;

namespace Breakout.Specs.Editor
{
    [CustomEditor(typeof(BallSpecAsset))]
    public class BallSpecAssetEditor : UnityEditor.Editor
    {
        private VisualElement root;
        private VisualTreeAsset tree;

        private void OnEnable()
        {
            root = new VisualElement();

            tree = AssetDatabase.LoadAssetAtPath<VisualTreeAsset>(
                "Assets/Scripts/Specs/Editor/BallSpecAssetTemplate.uxml");

            var style = AssetDatabase.LoadAssetAtPath<StyleSheet>(
                "Assets/Scripts/Specs/Editor/BallSpecAssetStyles.uss");

            root.styleSheets.Add(style);
        }

        public override VisualElement CreateInspectorGUI()
        {
            root.Clear();
            tree.CloneTree(root);
            return root;
        }
    }
}