TestScriptWithReorderableList.cs

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

#if UNITY_EDITOR
using UnityEditor;
using UnityEditorInternal;
#endif

public class TestScriptWithReorderableList : MonoBehaviour
{
	public string standardField;

	public SimpleStateMachine stateMachine;
}

[System.Serializable]
public class SimpleStateMachine
{
	[SerializeField]
	private int _serialId = -1;

	public SimpleStateMachineState[] states = new SimpleStateMachineState[0];
}

[System.Serializable]
public class SimpleStateMachineState
{
	public int id; // use this when saving current state etc.
	public string name;

	public override string ToString()
	{
		return name;
	}
}

#if UNITY_EDITOR
[CustomPropertyDrawer(typeof(SimpleStateMachine))]
public class SimpleStateMachineInspector : PropertyDrawer
{
	private const float FIELD_PADDING = 2f;

	private const string LIST_PROPERTY_NAME = "states";
	private const string NAME_PROPERTY = "name";
	private const string ID_PROPERTY = "id";
	private const string SERIAL_ID_PROPERTY = "_serialId";

	private ReorderableList _stateList;

	public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
	{
		var list = GetList(property);
		list.DoList(position);
	}

	public override float GetPropertyHeight(SerializedProperty property, GUIContent label)
	{
		var states = GetList(property);
		return states.GetHeight();
	}

	private ReorderableList GetList(SerializedProperty property)
	{
		if (_stateList == null)
		{
			var listProperty = property.FindPropertyRelative(LIST_PROPERTY_NAME);
			_stateList = new ReorderableList(property.serializedObject, property.FindPropertyRelative(LIST_PROPERTY_NAME));

			_stateList.drawHeaderCallback = (Rect rect) =>
			{
				EditorGUI.LabelField(rect, "States");
			};

			_stateList.drawElementCallback =
				(Rect rect, int index, bool isActive, bool isFocused) =>
				{
					rect.y += FIELD_PADDING;

					string name = listProperty.GetArrayElementAtIndex(index).FindPropertyRelative(NAME_PROPERTY).stringValue;

					listProperty.GetArrayElementAtIndex(index).FindPropertyRelative(NAME_PROPERTY).stringValue = EditorGUI.TextField(new Rect(rect.x, rect.y, rect.width, EditorGUIUtility.singleLineHeight), name);
				};

			_stateList.onAddCallback = (ReorderableList list) =>
			{
				var states = property.FindPropertyRelative(LIST_PROPERTY_NAME);

				// increment serial id
				int id = property.FindPropertyRelative(SERIAL_ID_PROPERTY).intValue;
				id++;
				property.FindPropertyRelative(SERIAL_ID_PROPERTY).intValue = id;

				int length = states.arraySize;
				states.InsertArrayElementAtIndex(length);
				states.GetArrayElementAtIndex(length).FindPropertyRelative(ID_PROPERTY).intValue = id;
				states.GetArrayElementAtIndex(length).FindPropertyRelative(NAME_PROPERTY).stringValue = "<Unnamed State>";
			};
		}

		return _stateList;
	}
}
#endif