Attempting to override the default UnityEvent drawer.

Drawer.cs

using UnityEngine;
using UnityEditor;
using UnityEditorInternal;
using UnityEngine.Events;
using System.Reflection;


namespace Test
{
	[CustomPropertyDrawer (typeof (UnityEventBase), true)]
	public class Drawer : UnityEventDrawer
	{
		[InitializeOnLoadMethod]
		static void OnLoad ()
		{
			Debug.LogFormat ("Attempting to disable built-in property drawer");

			/*object drawer = typeof (UnityEventDrawer).GetCustomAttributes (true).FirstOrDefault (attribute => attribute is CustomPropertyDrawer);
			typeof (CustomPropertyDrawer).GetField ("m_Type", BindingFlags.Instance | BindingFlags.NonPublic).SetValue (drawer, null);*/

			ReplaceDrawer<UnityEventBase, Drawer> ();
		}


		static void ReplaceDrawer<TType, TDrawer> ()
			where TType : class
			where TDrawer : PropertyDrawer
		{
			MethodInfo dictionarySetter = ScriptAttributeUtility.DrawerTypeForTypeField.FieldType.GetMethod ("set_Item");
			dictionarySetter.Invoke (
				ScriptAttributeUtility.DrawerTypeForType,
				new []
				{
					typeof (TType),
					ScriptAttributeUtility.DrawerKeySet.Create<TType, TDrawer>()
				}
			);
		}


		public override void OnGUI (Rect position, SerializedProperty property, GUIContent label)
		{
			base.OnGUI (position, property, label);

			GUI.Label (position, "OVERRIDDEN");

			Debug.Log ("PING");
		}
	}
}

Reflected-ScriptAttributeUtility.cs

using System;
using System.Reflection;
using UnityEditor;


namespace Test
{
	public static class ScriptAttributeUtility
	{
		public static class DrawerKeySet
		{
			static Type s_Type;


			public static Type Type
			{
				get
				{
					return s_Type ?? (s_Type = ScriptAttributeUtility.Type.GetNestedType ("DrawerKeySet", BindingFlags.NonPublic));
				}
			}


			public static object Create<TType, TDrawer> ()
				where TType : class
				where TDrawer : PropertyDrawer
			{
				object instance = Activator.CreateInstance (Type);

				Type.GetField ("drawer").SetValue (instance, typeof (TDrawer));
				Type.GetField ("type").SetValue (instance, typeof (TType));

				return instance;
			}
		}


		static Type s_Type;
		static FieldInfo s_DrawerTypeForTypeField;
		static object s_DrawerTypeForType;


		public static Type Type
		{
			get
			{
				return s_Type ?? (s_Type = typeof (EditorWindow).Assembly.
					GetType ("UnityEditor.ScriptAttributeUtility", throwOnError: true, ignoreCase: false));
			}
		}


		public static FieldInfo DrawerTypeForTypeField
		{
			get
			{
				return s_DrawerTypeForTypeField ?? (s_DrawerTypeForTypeField = Type.
					GetField ("s_DrawerTypeForType", BindingFlags.Static | BindingFlags.NonPublic));
			}
		}


		public static object DrawerTypeForType
		{
			get
			{
				if ((s_DrawerTypeForType ?? (s_DrawerTypeForType = DrawerTypeForTypeField.GetValue (null))) != null)
				{
					return s_DrawerTypeForType;
				}

				Type.GetMethod ("BuildDrawerTypeForTypeDictionary", BindingFlags.NonPublic | BindingFlags.Static).Invoke (null, null);

				return s_DrawerTypeForType = DrawerTypeForTypeField.GetValue (null);
			}
		}
	}
}

What's the point of this ? You can just use it as it is without calling the ReplaceDrawer method , here is mine :

using UnityEngine;
using System.Collections;

using UnityEditor;
using UnityEngine.Events;
using UnityEditorInternal;
using UnityEngine.UIElements;

[CustomPropertyDrawer( typeof( UnityEventBase ), true )]
public class UnityEventDrawerEx : PropertyDrawer
{
    string title = "";

    public override void OnGUI( Rect position, SerializedProperty property, GUIContent label )
    {
        //if( list == null ) Init( property );

        title = label?.text ?? "";

        property.serializedObject.Update( );

        SerializedProperty listenersArray = property.FindPropertyRelative("m_PersistentCalls.m_Calls");

        var line = position;

        for(var i = 0; i < listenersArray.arraySize; ++i)
        {
            line.height = 30f;
            line.y = position.y + 30f * i;

            var prop = listenersArray.GetArrayElementAtIndex( 0 );
            var target = prop.FindPropertyRelative("m_Target");

            EditorGUI.PropertyField( line, target, GUIContent.none );
        }

        //list.DoLayoutList();

        property.serializedObject.ApplyModifiedProperties( );
    }

    public override float GetPropertyHeight( SerializedProperty property, GUIContent label )
    {
        SerializedProperty listenersArray = property.FindPropertyRelative("m_PersistentCalls.m_Calls");

        return listenersArray.arraySize * 30f;
    }

so this

becomes this ( just realized i had a typo with the 0 index ) :

You most definitely can. There was a point to this in 2017. There is no point to it in 2021.

But thank you for taking the time to post an update for anyone else who might come across this gist :)

ah lol didn't notice the date

No worries. Thanks again for the thorough assist :)

Do you guys know how to make Unity Events show up conditionally in the inspector? Please see the last post here