Files in "Editor" directory: - MonoBehaviourCustomEditor.cs

ExposeMethodInUnityEditorAttribute.cs

using System;

[AttributeUsage(AttributeTargets.Method)]
public class ExposeMethodInUnityEditorAttribute : Attribute
{
}

ExposeMethodInUnityEditorPlayingOnlyAttribute.cs

using System;

[AttributeUsage(AttributeTargets.Method)]
public class ExposeMethodInUnityEditorPlayingOnlyAttribute : Attribute
{
}

MonoBehaviourCustomEditor.cs

using UnityEngine;
using UnityEditor;
using System.Reflection;
using System;

namespace Project.Template.Editor
{
    [CanEditMultipleObjects]
    [CustomEditor(typeof(MonoBehaviour), true)] 
    public class MonoBehaviourCustomEditor : UnityEditor.Editor
    {
        public override void OnInspectorGUI()
        {
            DrawDefaultInspector();
            
            var type = target.GetType();
            
            foreach (var method in type.GetMethods(BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Instance))
            {
                var editorAttributes = method.GetCustomAttributes(typeof(ExposeMethodInUnityEditorAttribute), true);
                if (editorAttributes.Length > 0)
                {
                    DrawButton(method.Name, type, target);
                }
                else if (Application.isPlaying)
                {
                    var editorPlayingOnlyAttributes = method.GetCustomAttributes(typeof(ExposeMethodInUnityEditorPlayingOnlyAttribute), true);
                    if (editorPlayingOnlyAttributes.Length > 0)
                    {
                        DrawButton(method.Name, type, target);
                    }
                }
            }
        }

        private static void DrawButton(string methodName, Type type, object target)
        {
            if (GUILayout.Button(methodName))
            {
                type.GetMethod(methodName).Invoke(target, null);
            }
        }
    }
}