library was invented to realise GUI applications consisting of
stand-alone components that use the Prolog database for storing the
application data. Figure
14 illustrates the flow of information using this design
|Figure 14 : Information-flow using broadcasting service|
The broadcasting service provides two services. Using the `shout' service, an unknown number of agents may listen to the message and act. The broadcaster is not (directly) aware of the implications. Using the `request' service, listening agents are asked for an answer one-by-one and the broadcaster is allowed to reject answers using normal Prolog failure.
Shouting is often used to inform about changes made to a common database. Other messages can be ``save yourself'' or ``show this''.
Requesting is used to get information while the broadcaster is not aware who might be able to answer the question. For example ``who is showing X?''.
..., broadcast_request(age_of('John', Age)),
If there is an agent (listener) that registered an `age-of' service and knows about the age of `John' this question will be answered.
?- listen(Term, (writeln(Term),fail)). ?- broadcast(hello(world)). hello(world) Yes
:- pce_begin_class(name_item, text_item). variable(id, any, get, "Id visualised"). initialise(NI, Id:any) :-> name_of(Id, Name), send_super(NI, initialise, name, Name, message(NI, set_name, @arg1)), send(NI, slot, id, Id), listen(NI, name_of(Id, Name), send(NI, selection, Name)). unlink(NI) :-> unlisten(NI), send_super(NI, unlink). set_name(NI, Name:name) :-> get(NI, id, Id), retractall(name_of(Id, _)), assert(name_of(Id, Name)), broadcast(name_of(Id, Name)). :- pce_end_class.