Example of how to expose a field from a class reference and have it update the inspector at an 'appropriate' framerate

UpdateInspectorBehaviour.cs

using UnityEngine;

public class UpdateInspectorBehaviour : MonoBehaviour
{
	public class CustomValueClass
	{		
		public  int  _MyUpdatingValue;
	}

	public CustomValueClass _CustomValueClass;

	private void OnEnable ()
	{
		_CustomValueClass = new CustomValueClass();
	}

	private void OnDisable ()
	{
		_CustomValueClass = null;
	}

	void Update ()
	{
		_CustomValueClass._MyUpdatingValue++;
	}
}

UpdateInspectorBehaviourEditor.cs

using UnityEditor;

/// <summary>
/// Example of how to expose a field from a class reference and have it update the inspector at an 'appropriate' framerate.
/// In practice though its easier to create a duplicate field in the monoBehaviour, update its value and let Unity deal with it.
/// However this is still useful code to know.
/// </summary>
[CustomEditor(typeof(UpdateInspectorBehaviour))] 
public class UpdateInspectorBehaviourEditor : Editor
{	
	private int	thisSample = 0;
	private int	lastSample = int.MaxValue;
	

	bool UpdatePerFrameInfo ()
	{		
		lastSample = thisSample;

		UpdateInspectorBehaviour myTarget = (UpdateInspectorBehaviour) target;
		thisSample = (null != myTarget._CustomValueClass ? myTarget._CustomValueClass._MyUpdatingValue : 0);

		// Returns if we need a repaint
		return  EditorApplication.isPlaying && thisSample != lastSample;		
	}
	
	// If return is true will refresh Inspector once per frame. Best method as avoids editor overhead getting too large.
	public override bool RequiresConstantRepaint ()
	{			
		bool requiresRepaint = UpdatePerFrameInfo();

		return requiresRepaint;  
	}	

	public override void OnInspectorGUI()
	{
		EditorGUILayout.LabelField("MyUpdatingValue", thisSample.ToString("D6"));	
		EditorGUILayout.Space();
		DrawDefaultInspector();
    }	
}