is dbms_mview.refresh assync or sync?

when I invoke dbms_mview.refresh('X')

Is the FAST REFRESH materialized view X guaranteed to be refreshed before the function returns?

This is my problem:

I have some "Real-Time" requirements. I am avoiding use ON COMMIT FAST REFRESH Materialized Views because it is impacting the transacation latency on the base tables.

Instead, I am using ON DEMMAND Fast refresh Materialized View + a refresher process which executes teh code bellow:

WHILE( true )
  LOOP
    dbms_mview.refresh('X');
    commit;
    dbms_lock.sleep( 0.01);
  end loop;

Of course, this will only works if  dbms_mview.refresh is synchronous.

If you guys can indentify any other problems with this "solution", please let me know.