Defer and Usurp with regular Max/MSP attributes

Hi there,

I am working on a complex UI external that has a 'numelems' attribute.

The attribute is implemented as follows:

CLASS_ATTR_LONG(c,               "numelems",      ATTR_FLAGS_NONE, t_myobject, numelems);
CLASS_ATTR_ACCESSORS(c,           "numelems",      NULL, myobject_numelems_set);
CLASS_ATTR_LABEL(c,               "numelems",      ATTR_FLAGS_NONE, "Number of Elements");
CLASS_ATTR_DEFAULT_SAVE_PAINT(c,   "numelems",      ATTR_FLAGS_NONE, "4");
CLASS_ATTR_FILTER_CLIP(c,       "numelems",      1, 1024);
CLASS_ATTR_ORDER(c,                "numelems",      ATTR_FLAGS_NONE, "1");

The attribute uses a custom setter because every time a new 'numelems' is set, several memory allocations, event generations and data calculations are performed. This becomes especially slow when the number of elements is set to a high value. The problem is that in a Max patcher the 'numelems' attribute could be set repeatedly by the user at high priority. Obviously, this should be avoided at all costs since it could potentially bring the Max scheduler to a halt.

Immediately I am thinking that something needs to be deferred... :)

So, learning from the good ol' Jitter, I would like to give the attribute getter and setter respectively the DEFER_LOW and USURP_LOW behavior. That's because I want both the latest high priority value when the next 'numelems' change is triggered at low priority and no low priority queue backlog from generating more events at high priority than can be processed at low priority. Obviously that goes hand in hand with a deferred getter to preserve the order of execution in case the attribute is queried by the user.

Now, I see ATTR_GET_DEFER_LOW | ATTR_SET_USURP_LOW flags in ext_obex.h, however it says they are "placeholders for potential future functionality".
Has this functionality been already implemented as of Max 6.0.4 ?
If not, what would be the way to achieve the same without using attribute flags?
Should I create custom getter and setter and put a defer_low() in my getter and a qelem() in my setter?

Any comment/advice is highly appreciated.

Thank you.

- Luigi