A utility class for working with Unity's Undo class. Mainly contains UndoPerformed and RedoPerformed events

UndoUtility.cs

using System;
using System.Collections.Generic;
using System.Reflection;
using UnityEditor;
using Object = UnityEngine.Object;

/// <summary>
/// Additional utilities for use with <see cref="Undo"/>.
/// </summary>
[InitializeOnLoad]
public static class UndoUtility
{
    private static Action<List<string>, List<string>> _getRecords;
    private static List<string> _undoRecords = new List<string>();
    private static List<string> _redoRecords = new List<string>();

    /// <summary>
    /// Callback that is triggered after a undo was executed.
    /// </summary>
    public static event Action UndoPerformed;

    /// <summary>
    /// Callback that is triggered after a redo was executed.
    /// </summary>
    public static event Action RedoPerformed;

    static UndoUtility()
    {
        Undo.undoRedoPerformed += OnUndoRedoPerformed;
    }

    private static void OnUndoRedoPerformed()
    {
        GetRecords(_undoRecords, _redoRecords);
        
        // All "UndoRecordsCount" and "RedoRecordsCount" should be changed to be unique to your tool.
        int previousUndoCount = SessionState.GetInt("UndoRecordsCount", 0);
        int previousRedoCount = SessionState.GetInt("RedoRecordsCount", 0);

        if (previousUndoCount < _undoRecords.Count && previousRedoCount > _redoRecords.Count)
        {
            RedoPerformed?.Invoke();
        }
        else
        {
            UndoPerformed?.Invoke();
        }

        SessionState.SetInt("UndoRecordsCount", _undoRecords.Count);
        SessionState.SetInt("UndoRecordsCount", _redoRecords.Count);
    }

    public static void GetRecords(List<string> undoRecords, List<string> redoRecords)
    {
        if (_getRecords == null)
        {
            var paramTypes = new Type[] { typeof(List<string>), typeof(List<string>) };
            var method = typeof(Undo).GetMethod("GetRecords", BindingFlags.NonPublic | BindingFlags.Static, Type.DefaultBinder, paramTypes, null);
            _getRecords = (Action<List<string>, List<string>>)method.CreateDelegate(typeof(Action<List<string>, List<string>>));
        }

        _getRecords(undoRecords, redoRecords);
    }
}