How do i get a callback every frame in edit mode

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");
     }
 }

@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.");
     }
 
 }