Listen to AdvancedDropdown selection cancellation in Unity 2022.1 using Reflection (actual as per 2022.1.0a15)

DropdownSelectionCanceledExample.cs

namespace CodeStage.Gists.Editor
{    
    using System;
    using System.Reflection;
    using UnityEditor.IMGUI.Controls;
    using UnityEngine;
    
    public class DropdownSelectionCanceledExample : AdvancedDropdown
    {
        //...
        
        private void SubscribeToSelectionCancel()
        {
            var field = typeof(AdvancedDropdown).GetField("m_WindowInstance", BindingFlags.Instance | BindingFlags.NonPublic);
            if (field == null)
            {
                Debug.LogError($"Couldn't find {nameof(AdvancedDropdown)}.m_WindowInstance field!");
                return;
            }

            var windowInstance = field.GetValue(this);
            if (windowInstance == null)
            {
                Debug.LogError($"Couldn't get {nameof(windowInstance)}!");
                return;
            }

            var selectionCanceledEventInfo = windowInstance.GetType().GetEvent("selectionCanceled", BindingFlags.Instance | BindingFlags.Public);
            if (selectionCanceledEventInfo == null)
            {
                Debug.LogError("Couldn't find AdvancedDropdownWindow.selectionCanceled event!");
                return;
            }

            Action handler = OnSelectionCanceled;
            Delegate convertedHandler = Delegate.CreateDelegate(selectionCanceledEventInfo.EventHandlerType, handler.Target, handler.Method);
            selectionCanceledEventInfo.AddEventHandler(windowInstance, convertedHandler);
        }

        private void OnSelectionCanceled()
        {
            Debug.Log("An AdvancedDropdown closed without selection.");   
        }
    }
}