How do i get a callback every frame in edit mode

Questions & Answers
1

I dont want to use an Editor Window or ScriptableObject or MonoBehaviour. I just want to get a callback called every frame without having the user having to click a menu item or something like that.

2

You can use the InitializeOnLoad attribute combined with static constructors.

using UnityEditor;
using UnityEngine;

[InitializeOnLoad]
class MyClass
{
    static MyClass ()
    {
        EditorApplication.update += Update;
    }

    static void Update ()
    {
        Debug.Log("Updating");
    }
}
3

@Razgrizzz
You can call any methods you want in your Update() method, but you will need static references to them.

using UnityEditor;
using UnityEngine;
[InitializeOnLoad]
class MyClass
{
    public static List<MyObject> list;

    static MyClass ()
    {
        EditorApplication.update += Update;
        list = new List<MyObject>();
    }

     public MyClass(){ // regular constructor where we add this to static list
        MyClass.list.Add(this);
    }

    static void Update ()
    {
        Debug.Log("Updating");
        foreach(MyClass mc in list){
            mc.AnyPublicMethod();
        }         
    }

    public void AnyPublicMethod(){
        print("Dude.");
    }

}