+ /* If non-NULL this function is called from thread context if the
+ * state changes. The old state is found in thread_info.state. */
+ void (*state_change) (pa_source_output *o, pa_source_output_state_t state); /* may be NULL */
+
+ /* If non-NULL this function is called before this source output
+ * is moved to a source and if it returns false the move
+ * will not be allowed */
+ bool (*may_move_to) (pa_source_output *o, pa_source *s); /* may be NULL */
+
+ /* If non-NULL this function is used to dispatch asynchronous
+ * control events. */
+ void (*send_event)(pa_source_output *o, const char *event, pa_proplist* data);
+
+ /* If non-NULL this function is called whenever the source output
+ * volume changes. Called from main context */
+ void (*volume_changed)(pa_source_output *o); /* may be NULL */
+
+ /* If non-NULL this function is called whenever the source output
+ * mute status changes. Called from main context */
+ void (*mute_changed)(pa_source_output *o); /* may be NULL */
+