myClassEditorMultiple

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

[CustomEditor(typeof(myClass))]
[CanEditMultipleObjects]
public class myClassEditorMultiple : Editor
{
    SerializedProperty showVariables;
    SerializedProperty myInt;
    SerializedProperty myFloat;
    SerializedProperty myGameobject;
    SerializedProperty myColor;


    private void OnEnable()
    {
        showVariables = serializedObject.FindProperty("showVariables");
        myInt = serializedObject.FindProperty("myInt");
        myFloat = serializedObject.FindProperty("myFloat");
        myGameobject = serializedObject.FindProperty("myGameobject");
        myColor = serializedObject.FindProperty("myColor");
    }

    public override void OnInspectorGUI()
    {
        serializedObject.Update();


        showVariables.boolValue = EditorGUILayout.Toggle("Show variables", showVariables.boolValue);

        using (var showVariablesGroup = new EditorGUILayout.FadeGroupScope(Convert.ToSingle(showVariables.boolValue)))
        {
            if (showVariablesGroup.visible == true)
            {
                EditorGUILayout.DelayedIntField(myInt, new GUIContent("Integer"));

                EditorGUILayout.Slider(myFloat, 0.0f, 10.0f, new GUIContent("Float slider"));

                EditorGUILayout.ObjectField(myGameobject, new GUIContent("A gameobject"));

                EditorGUILayout.ColorField(new GUIContent("Color"), myColor.colorValue);
            }
        }

        serializedObject.ApplyModifiedProperties();
    }
}