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<h3 class="subsection"> 4.4.3 Subscription </h3>
<p>The event scheduling system is invoked by calling the
<code>csEventQueue::RegisterListener()</code> or <code>csEventQueue::Subscribe()</code>
methods (or, if you are inheriting from <samp>‘csBaseEventHandler’</samp>,
<code>csBaseEventHandler::RegisterQueue()</code>). For each of these, the arguments
of interest are the <samp>‘csEventID’</samp> and <code>csEventID[]</code> parameters.
(Note that the <code>RegisterListener()</code> form with no <samp>‘csEventID’</samp> argument
does not subscribe the listener to anything, and must be followed by a call to
<code>Subscribe()</code> if you ever want the handler to receive any events.) For
each <samp>‘csEventID’</samp> (arrays are terminated by <samp>‘CS_EVENTLIST_END’</samp>), these
functions will subscribe the event handler to that event name <em>as well as
all of its children</em>. Note, however, that it will only retrieve from the event
handler the ordering lists (the <code>Prec()</code> and <code>Succ()</code> methods) for
the event name to which it is <em>explicitly</em> subscribed; if you wish to have
one set of ordering constraints for both <samp>‘crystalspace.input.keyboard’</samp> and
<samp>‘crystalspace.input.mouse’</samp> but a different set for
<samp>‘crystalspace.input.joystick’</samp>, you <strong>should not</strong> simply subscribe to
<samp>‘crystalspace.input’</samp> and have the <code>Prec()</code> and <code>Succ()</code>
functions return different values for each of <samp>‘.keyboard’</samp>, <samp>‘.mouse’</samp>,
and <samp>‘.joystick’</samp>; rather, you should subscribe to each of those three event
names individually. This restriction helps us avoid the introduction of
hard-to-detect circular ordering rules.
</p>
<p>Also note that you should use the <code>Subscribe()</code> and <code>Unsubscribe()</code>
methods to add and remove events from the set of events a handler wishes to
receive. You should only <code>Unsubscribe()</code> from the particular
events to which you have <code>Subscribe()</code>d; the behavior is undefined
otherwise (e.g., if you subscribe to <samp>‘crystalspace.input’</samp> and
subsequently unsubscribe from <samp>‘crystalspace.input.joystick’</samp>).
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