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<hr><a name="events_howto"><h2>How to deal with events</h2></a>
<dl compact><dt><b>
Author: </b><dd>
Stephane Coutelas</dl><a name="events_overview"><h3>Overall view</h3></a>
<p>
To manage events, a class has to get or create a server. The server stores emitters, listeners, and events. When the server is required to pump events, it checks every emitters to get last events. All the listeners are stored in the server as a pair (type of event, listener). Thus a particular event can be handled by several listeners. It's the class which adds emitters and listeners it needs to the server.
<p>
<a name="event"><h3>CEvent</h3></a>
<p>
CEvent inherits CClassId. A predefined event is built by inheriting CEvent and using a CClassId, built whith a unique id. Existing events can be found in <a class="el" href="events_8h.html">events.h</a>.
<p>
<a name="event_listener"><h3>IEventListener</h3></a>
<p>
The interface provides a callback. A listener must implements this interface. Existing listeners can be found in <a class="el" href="event__listener_8h.html">event_listener.h</a>.
<p>
In the following example, the listener is supposed to have been added to an event server along with the event type EventCharId: <div class="fragment"><pre>CCallback cb;
server.addListener (<a class="code" href="namespaceNLMISC.html#a254">EventCharId</a>, &cb);
</pre></div> When this callback is called whith such event it prints the char : <div class="fragment"><pre><font class="keyword">class </font>CCallback : <font class="keyword">public</font> IEventListener
{
<font class="keyword">virtual</font> <font class="keywordtype">void</font> operator ()(<font class="keyword">const</font> CEvent& event)
{
CEventChar ec = (CEventChar &) event; <font class="comment">// here we know that we receive a CEventChar event</font>
printf(<font class="stringliteral">"%c"</font>, ec.Char);
}
};
</pre></div>
<p>
<a name="event_emitter"><h3>IEventEmitter</h3></a>
<p>
It's the interface which gets low-level events and posts them to the server as NEL events. An emitter must implements this interface. Existing emitters can be found in emitters.h.
<p>
<a name="event_server"><h3>CEventServer</h3></a>
<p>
A server is made of :<ul>
<li>a multimap of (CClassId, IEventListener*)<li>a list of IEventEmitter*<li>a list of CEvent*</ul>
When a call to the method pump is done, the server pumps its emitters for events. Events are stacked up in the list.
<p>
<div align="center">
<img src="event_pump.png" alt="event_pump.png">
</div>
Then, for each event, according to their id, the server applies the right callbacks stored in the multimap.
<p>
<div align="center">
<img src="event_listener.png" alt="event_listener.png">
</div>
<p>
As for emitters, both server and class know the IListener. The listener callback is the operator() which takes an event in parameter. Thus, the user defines the listener/callback he needs and adds it to the server.
<p>
<div align="center">
<img src="event_callback.png" alt="event_callback.png">
</div>
<p>
<a name="event_example"><h3>Example :</h3></a>
<p>
Here is an example of the use of the special listener CEventListenerAsync (defined in <a class="el" href="event__listener_8cpp.html">event_listener.cpp</a>).
<p>
Rq : This listener stores key states : if a key is pressed its value is on, else it's off.
<p>
<div class="fragment"><pre><font class="comment">// declaring a listener</font>
CEventListenerAsync asyncListener;
<font class="comment">// declaring the server</font>
CEventServer server;
<font class="comment">// adding an emitter to the server </font>
... <font class="comment">// here, a driver is initialized</font>
server.addEmitter(driver->getEventEmitter()); <font class="comment">//in this ex the driver provides the emitter</font>
<font class="comment">// adding the listener to the server</font>
asyncListener.addToServer(server);
<font class="comment">// events loop</font>
<font class="keywordflow">do</font>
{
<font class="comment">//pump for events</font>
server.pump();
<font class="comment">// user main function</font>
mainproc();
}
<font class="keywordflow">while</font>(!asyncListener.isKeyPush(<a class="code" href="namespaceNLMISC.html#a317a73">KeyESCAPE</a>));
<font class="comment">// removing listener from server</font>
asyncListener.removeFromServer(Server);
</pre></div>
<p>
Notes about the listener CEventListenerAsync : this listener adds two types of event to a server : EventKeyUpId and EventKeyDownId.
<p>
<div class="fragment"><pre><font class="keywordtype">void</font> CEventListenerAsync::addToServer (CEventServer& server)
{
server.addListener (<a class="code" href="namespaceNLMISC.html#a253">EventKeyUpId</a>, <font class="keyword">this</font>);
server.addListener (<a class="code" href="namespaceNLMISC.html#a252">EventKeyDownId</a>, <font class="keyword">this</font>);
}
</pre></div>
<p>
The remove method is similar to add method, we must precise both event id and listener to be removed.
<p>
<div class="fragment"><pre><font class="keywordtype">void</font> CEventListenerAsync::removeFromServer (CEventServer& server)
{
server.removeListener (<a class="code" href="namespaceNLMISC.html#a253">EventKeyUpId</a>, <font class="keyword">this</font>);
server.removeListener (<a class="code" href="namespaceNLMISC.html#a252">EventKeyDownId</a>, <font class="keyword">this</font>);
}
</pre></div>
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