A reactive sequence is handled just like a normal sequence, except that the driver will handle things a little different. Instead of calling get_next_item() and then driving the interface signals, the driver will wait for the interface to request a response, then call get_next_item() to get the appropriate response from the sequencer.
If you need to get some of the signal information to generate the sequence item, use a ‘null’ type of transaction that will wait for a request.