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

using UnityEngine;
using UnityEditor;
using UnityEditorInternal;
using System.Linq;
using System;

namespace GameplayIngredients.Editor
{
    public class CallableReorderableList : ReorderableList
    {
        public CallableReorderableList(SerializedObject targetObject, SerializedProperty listProperty)
            : base(targetObject, listProperty, true, true, true, true)
        {
            onAddDropdownCallback = AddDropdown;
            drawHeaderCallback = DrawHeader;
            drawElementCallback = DrawElement;
        }

        void DrawHeader(Rect r)
        {
            GUI.Label(r, new GUIContent($"  {serializedProperty.displayName}", Styles.callableIcon), EditorStyles.boldLabel);
        }

        void DrawElement(Rect r, int index, bool isActive, bool isFocused)
        {
            SerializedProperty element = serializedProperty.GetArrayElementAtIndex(index);
            r.y += 2.0f;
            r.x += 4.0f;
            r.width -= 4.0f;

            EditorGUI.PropertyField(new Rect(r.x, r.y, r.width, EditorGUIUtility.singleLineHeight), element, true);
        }

        public void AddDropdown(Rect buttonRect, ReorderableList list)
        {
            var component = list.serializedProperty.serializedObject.targetObject as Component;
            var gameObject = component.gameObject;
            var propertyName = list.serializedProperty.name;

            var provider = new CallableProvider(
                gameObject,
                component,
                propertyName
                ); ;
            BrowsePopup.Show(buttonRect.position, provider);
        }

        static class Styles
        {
            public static Texture2D callableIcon = AssetDatabase.LoadAssetAtPath<Texture2D>("Packages/net.peeweek.gameplay-ingredients/Icons/Misc/ic-callable.png");
        }
    }

    public static class CallableExtensions
    {
        public static void AddCallable(this GameObject gameObject, Component component, string propertyName, Type t)
        {
            var field = component.GetType().GetFields().Where(f => f.Name == propertyName).FirstOrDefault();
            var val = field.GetValue(component) as Callable[];

            if (t != null && typeof(Callable).IsAssignableFrom(t))
            {
                var newCmp = gameObject.AddComponent(t);
                field.SetValue(component, val.Append(newCmp as Callable));
            }
            else
                field.SetValue(component, val.Append(null));
        }

        static T[] Append<T>(this T[] array, T item)
        {
            if (array == null)
            {
                return new T[] { item };
            }
            T[] result = new T[array.Length + 1];
            array.CopyTo(result, 0);
            result[array.Length] = item;
            return result;
        }
    }
}