public PXR_Request <T> OnComplete(PXR_Message <T> .Callback callback)
        {
            if (msgCallback != null)
            {
                throw new UnityException("Attempted to attach multiple handlers to a Request.  This is not allowed.");
            }

            msgCallback = callback;
            PXR_Callback.AddRequest(this);
            return(this);
        }
 public Request OnComplete(PXR_Message.Callback callback)
 {
     callback_ = callback;
     PXR_Callback.AddRequest(this);
     return(this);
 }