OnEnable/Disable on property drawers

The best method I could think of would be to use a custom MonoBehaviour (with the ExecuteInEditMode annotation) with the sole purpose of checking for and sending that event.

You'd need to somehow tell if and when the inspector was changed by something in the editor (either directly by the user or by some other script, or even the editor itself), and then you'd need to check to see if the script that the inspector was checking had any components in them that had property drawers associated with them.

There's likely a much better way to do this, assuming you reverse engineered the internal workings of the Unity Editor, but I wouldn't suggest that, for a pretty large number of reasons. Not least of which is that it would be horrifically unreadable and hackish.

I wouldn't worry about making an OnEnable method that works like the one in Editor scripts. Frankly, a single if statement checking to see if you've initialized something too expensive to do every frame would be a pretty inexpensive addition to the "OnGUI" or even the "GetPropertyHeight" methods. However, keep in mind that GetPropertyHeight (at least in my experience) is called at least 3 times per OnGUI call, and would be a lot less efficient to be doing checks in.

@Cobo3 Is has been a while. But I am dealing with this issue right now. Did you find an elegant solution for this?

 [CustomPropertyDrawer( typeof( Example ) )]
 public class ExamplePropertyDrawer : PropertyDrawer
 {
     public ExamplePropertyDrawer() : base ()
     {
         Debug.Log( "ExamplePropertyDrawer Constructor" );
     }
     
     ~ExamplePropertyDrawer()
     {
         // Note: This is called by the GC; and not necessarily on inspector selection change.
         Debug.Log( "ExamplePropertyDrawer Destructor" );
     }
 }

This is what I've been using, seems to be working just fine.

However if you need a solution that fires for absolutely every property drawer individually:

 private bool initialized = false;
  
 public override void OnGUI( Rect position, SerializedProperty property, GUIContent label )
 {
     if ( !initialized )
     Init( property );
 }
 
 private void Init( SerializedProperty property )
 {
     initialized = true;
 }