How to get Editor Window to update immediately in Editor Mode/Play Mode

Trying to make an editor window that will show the distance between two selected gameobjects. I used`OnDrawGizmosSelected()` which calls some stuff and successfully calculates the distance. The distance is correct when I log it (OnDrawGizmosSelected called a few times per second when object is selected)

 private void OnDrawGizmosSelected()
 {
         Distance.CheckSelect();
 }

This updates some static variable in my Distance class.

My editor window reads this static variable in OnGUI, but OnGUI is only called sometimes, whereas I want the Distance to be reflected immediately on the window as a read only variable.

 private void OnGUI()
 {
         EditorGUI.BeginDisabledGroup(true);
         EditorGUILayout.FloatField("Distance: ", Distance.distance);
         EditorGUI.EndDisabledGroup();
 }

How do I get my EditorWindow to update instantly in both edit and play mode?