/usr/lib/swi-prolog/doc/Manual/broadcast.html

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<h2 id="sec:broadcast"><a id="sec:A.4"><span class="sec-nr">A.4</span> <span class="sec-title">library(broadcast): 
Broadcast and receive event notifications</span></a></h2>

<a id="sec:broadcast"></a>

<p><a id="idx:broadcast:1786"></a><a id="idx:messageservice:1787"></a><a id="idx:blackboard:1788"></a><a id="idx:agent:1789"></a><a id="idx:daemon:1790"></a>The <code>library(broadcast)</code> 
library was invented to realise GUI applications consisting of 
stand-alone components that use the Prolog database for storing the 
application data. <a class="fig" href="broadcast.html#fig:broadcast">Figure 
12</a> illustrates the flow of information using this design

<p><a id="fig:broadcast"></a><div style="text-align:center"><img src="broadcast.gif"></div>
<div class="caption"><b>Figure 12 : </b>Information-flow using 
broadcasting service</div>

<p>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.

<p>Shouting is often used to inform about changes made to a common 
database. Other messages can be ``save yourself'' or ``show this''.

<p>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 <var>X</var>?''.

<dl class="latex">
<dt class="pubdef"><a id="broadcast/1"><strong>broadcast</strong>(<var>+Term</var>)</a></dt>
<dd class="defbody">
Broadcast <var>Term</var>. There are no limitations to <var>Term</var>, 
though being a global service, it is good practice to use a descriptive 
and unique principal functor. All associated goals are started and 
regardless of their success or failure, <a id="idx:broadcast1:1791"></a><a class="pred" href="broadcast.html#broadcast/1">broadcast/1</a> 
always succeeds. Exceptions are passed.</dd>
<dt class="pubdef"><a id="broadcast_request/1"><strong>broadcast_request</strong>(<var>+Term</var>)</a></dt>
<dd class="defbody">
Unlike <a id="idx:broadcast1:1792"></a><a class="pred" href="broadcast.html#broadcast/1">broadcast/1</a>, 
this predicate stops if an associated goal succeeds. Backtracking causes 
it to try other listeners. A broadcast request is used to fetch 
information without knowing the identity of the agent providing it. C.f. ``Is 
there someone who knows the age of John?'' could be asked using

<pre class="code">
        ...,
        broadcast_request(age_of('John', Age)),
</pre>

<p>If there is an agent (<em>listener</em>) that registered an `age-of' 
service and knows about the age of `John' this question will be 
answered.</dd>
<dt class="pubdef"><a id="listen/2"><strong>listen</strong>(<var>+Template, 
:Goal</var>)</a></dt>
<dd class="defbody">
Register a <em>listen</em> channel. Whenever a term unifying
<var>Template</var> is broadcasted, call <var>Goal</var>. The following 
example traps all broadcasted messages as a variable unifies to any 
message. It is commonly used to debug usage of the library.

<pre class="code">
?- listen(Term, (writeln(Term),fail)).
?- broadcast(hello(world)).
hello(world)
true.
</pre>

</dd>
<dt class="pubdef"><a id="listen/3"><strong>listen</strong>(<var>+Listener, 
+Template, :Goal</var>)</a></dt>
<dd class="defbody">
Declare <var>Listener</var> as the owner of the channel. Unlike a 
channel opened using <a id="idx:listen2:1793"></a><a class="pred" href="broadcast.html#listen/2">listen/2</a>, 
channels that have an owner can terminate the channel. This is commonly 
used if an object is listening to broadcast messages. In the example 
below we define a `name-item' displaying the name of an identifier 
represented by the predicate <a id="idx:nameof2:1794"></a><span class="pred-ext">name_of/2</span>.

<pre class="code">
:- pce_begin_class(name_item, text_item).

variable(id,    any,    get, "Id visualised").

initialise(NI, Id:any) :-&gt;
        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) :-&gt;
        unlisten(NI),
        send_super(NI, unlink).

set_name(NI, Name:name) :-&gt;
        get(NI, id, Id),
        retractall(name_of(Id, _)),
        assert(name_of(Id, Name)),
        broadcast(name_of(Id, Name)).

:- pce_end_class.
</pre>

</dd>
<dt class="pubdef"><a id="unlisten/1"><strong>unlisten</strong>(<var>+Listener</var>)</a></dt>
<dd class="defbody">
Deregister all entries created with <a id="idx:listen3:1795"></a><a class="pred" href="broadcast.html#listen/3">listen/3</a> 
whose <var>Listener</var> unify.</dd>
<dt class="pubdef"><a id="unlisten/2"><strong>unlisten</strong>(<var>+Listener, 
+Template</var>)</a></dt>
<dd class="defbody">
Deregister all entries created with <a id="idx:listen3:1796"></a><a class="pred" href="broadcast.html#listen/3">listen/3</a> 
whose <var>Listener</var> and <var>Template</var> unify.</dd>
<dt class="pubdef"><a id="unlisten/3"><strong>unlisten</strong>(<var>+Listener, 
+Template, :Goal</var>)</a></dt>
<dd class="defbody">
Deregister all entries created with <a id="idx:listen3:1797"></a><a class="pred" href="broadcast.html#listen/3">listen/3</a> 
whose <var>Listener</var>,
<var>Template</var> and <var>Goal</var> unify.</dd>
<dt class="pubdef"><a id="listening/3"><strong>listening</strong>(<var>?Listener, 
?Template, ?Goal</var>)</a></dt>
<dd class="defbody">
Examine the current listeners. This predicate is useful for debugging 
purposes.
</dd>
</dl>

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swi-prolog-nox 6.6.4-2ubuntu1 / usr / lib / swi-prolog / doc / Manual / broadcast.html
