- * @parameter_type: (out): the parameter type, or %NULL if none needed
- * @state_type: (out): the state type, or %NULL if stateless
- * @state_hint: (out): the state hint, or %NULL if none
- * @state: (out): the current state, or %NULL if stateless
+ * @parameter_type: (out) (allow-none): the parameter type, or %NULL if none needed
+ * @state_type: (out) (allow-none): the state type, or %NULL if stateless
+ * @state_hint: (out) (allow-none): the state hint, or %NULL if none
+ * @state: (out) (allow-none): the current state, or %NULL if stateless