Check if a property has changed value while serialized

Hi folks

I'm still trying to get my head around the SerializedObject and SerializedProperty nonsense that Unity inspectors seem to like so much.

I've written a custom attribute, it looks something like this:

 public class MyCustomAttribute : PropertyAttribute
 {
     [SerializeField] private Object m_Value;
 
     public Object value
     {
         get {return m_Value;}
         set
         {
             if (value == m_Value)
                 return;
 
             m_Value = value;
 
             OnValueChanged();
         }
     }
 
     private void OnValueChanged()
     {
         Debug.Log("Value changed!");
     }
 }

This kind of pattern has worked flawlessly for me for years. Unfortunately, because of the way that Unity accesses the the private member directly via SerializedProperty, the inspector is able to completely bypass my setter:

 SerializedProperty valueProperty = prop.FindPropertyRelative("m_Value");
 EditorGUI.PropertyField(position, valueProperty, GUIContent.none);

How can I ensure the change is caught somewhere?

I have tried to resolve this issue by implementing ISerializationCallbackReceiver:

 #region ISerializationCallbackReceiver Methods
 
 private object m_ValueCache;
 
 /// <summary>
 ///     Called before the attribute is serialized.
 /// </summary>
 public void OnBeforeSerialize()
 {
     m_ValueCache = m_Value;
 }
 
 /// <summary>
 ///     Called after the attribute is deserialized.
 /// </summary>
 public void OnAfterDeserialize()
 {
     // We check to see if the value property was changed.
     if (m_Value != m_ValueCache)
         OnValueChanged();
 }
 
 #endregion

But I end up with a threading error:

 ArgumentException: CompareBaseObjectsInternal can only be called from the main thread.
 Constructors and field initializers will be executed from the loading thread when loading a scene.
 Don't use this function in the constructor or field initializers, instead move initialization code to the Awake or Start function.

Naturally if this was a MonoBehaviour I would put a check in Update to see if the value has changed, but since this is a PropertyAttribute I can't do that.

Does anyone know what the correct approach is here?

Thanks, Ves