GenericPopupContent

// Example:
// List<Type> items = GetAllSubclasses(typeof(ScriptableObject)).ToList();
// GenericPopupContent<Type>.Show(items, OnScriptableObjectSelected, type => type.Name);
// private void OnScriptableObjectSelected(Type selectedType)
// {
//  _type = selectedType;
//  Repaint();
// }

using System;
using System.Collections.Generic;
using System.Linq;

using UnityEditor;

using UnityEngine;

namespace Game.EditorWindows
{
    public class GenericPopupContent<T> : PopupWindowContent
    {
        private string _searchFilter = "";
        private readonly List<T> _items;
        private readonly Action<T> _onSelectCallback;
        private readonly Func<T, string> _nameSelector;

        private Vector2 _scrollView;

        private GenericPopupContent(List<T> items, Action<T> onSelect, Func<T, string> nameSelector = default)
        {
            _nameSelector = nameSelector ?? (t => t.ToString());
            _items = items;
            _onSelectCallback = onSelect;
        }

        public static void Show(List<T> items, Action<T> onSelect, Func<T, string> nameSelector = default)
        {
            var content = new GenericPopupContent<T>(items, onSelect, nameSelector);

            PopupWindow.Show(new Rect(Event.current.mousePosition, Vector2.zero), content);
        }

        public override void OnGUI(Rect rect)
        {
            EditorGUILayout.Space();
            _searchFilter = EditorGUILayout.TextField(_searchFilter, (GUIStyle)"SearchTextField");
            EditorGUILayout.Space();

            _scrollView = EditorGUILayout.BeginScrollView(_scrollView);

            IEnumerable<T> query = from item in _items
                let itemName = _nameSelector(item)
                where Contains(itemName)
                where GUILayout.Button(itemName)
                select item;

            foreach(T item in query)
            {
                _onSelectCallback?.Invoke(item);
                editorWindow.Close();
            }

            EditorGUILayout.EndScrollView();
        }

        private bool Contains(string itemName) => itemName.ToLower().Contains(_searchFilter.ToLower());
    }
}