net.peeweek.gameplay-ingredients/Editor/PropertyDrawers/TypeDropDownPropertyDrawer.cs

using System;
using System.Collections.Generic;
using UnityEngine;
using UnityEditor;

namespace GameplayIngredients.Editor
{
    [CustomPropertyDrawer(typeof(TypeDropDownAttribute))]
    public class TypeDropDownPropertyDrawer : PropertyDrawer
    {
        Dictionary<string, List<string>> m_AssignableTypeNames;

        public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
        {
            Type type = ((TypeDropDownAttribute)attribute).m_BaseType;
            CacheType(type);
            string TypeName = type.FullName;

            int index = m_AssignableTypeNames[TypeName].IndexOf(property.stringValue);
             
            int newVal = EditorGUI.Popup(position, index, m_AssignableTypeNames[TypeName].ToArray());
            if(GUI.changed && index != newVal)
            {
                property.stringValue = m_AssignableTypeNames[TypeName][newVal];
            }
        }

        void CacheType(Type baseType)
        {
            if (m_AssignableTypeNames == null)
                m_AssignableTypeNames = new Dictionary<string, List<string>>();

            string key = baseType.FullName;

            if (!m_AssignableTypeNames.ContainsKey(key))
                m_AssignableTypeNames.Add(key, new List<string>());

            foreach(var assembly in AppDomain.CurrentDomain.GetAssemblies())
            {
                foreach(var type in assembly.GetTypes())
                {
                    if(baseType.IsAssignableFrom(type) && !type.IsAbstract)
                    {
                        m_AssignableTypeNames[key].Add(type.Name);
                    }
                }
            }
        }
    }

}