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<h1>Signal events</h1>
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<p>The main method of communication between child and parent tasks is through explicit
'signal' events.  This mechanism allows
the child task to send a message to the parent, and vice versa. This works as
follows:</p>

<ul>
 <li>If the parent wants to
     send a signal to the child, it invokes the <tt>Signal</tt>
     method in the child task object. This causes an <tt>onSignal</tt> event to trigger in
     the System object of the child task.
	 <P></p>
	 </li>

 <li>Conversely, if the child
     task wishes to send a signal to the parent, it invokes the <tt>Signal</tt>
     method in its own System object. This causes an <tt>onSignal</tt> event to trigger in
     the child task object of the parent.</li>
</ul>

<p>In both cases, the <tt>Signal</tt> method optionally takes an
argument, which is any APL array (or an APLX overlay created using <code>&#9109;OV</code>). This is typically used to send a command to the other task, or to return
a result to it.  Any argument to the <tt>Signal</tt>
method is available to the receiving task as the <code>&#9109;WARG</code> system variable during the execution of the callback.</p>

<p>For example, suppose a child task is being used to carry out a time-consuming
calculation.  It starts by awaiting a
signal to indicate that it should do the calculation, with the argument to the
signal being the data to work on.  When
it has completed the calculation, it sends a signal back to the parent with the
answer.  The following sequence
indicates how this might be done.  </p>

<p>First we need to do some setup. The parent task attaches a callback to the child task
object:</p>

<pre>      &#8711;CALCDONE
[1]   'The answer is ' &#9109;WARG
      &#8711;
	  
      ChildTask.onSignal&#8592;'CALCDONE'
</pre>

<p>This will cause the <code>CALCDONE</code> function to be run when
the child task signals that it has a result available, and the result will thus
be printed.</p>

<p>Similarly, the child task attaches a callback to its System object, waiting for a signal
to indicate that it should carry out the calculation:</p>

<pre>      &#8711;DOCALC;RESULT;DATA
[1]   DATA&#8592;&#9109;WARG
[2]   RESULT&#8592;RUNMODEL DATA
[3]   '#' &#9109;WI 'Signal' RESULT
      &#8711;

      '#' &#9109;WI 'onSignal' 'DOCALC'
      &#9109;WE &#175;1
</pre>


<p>This will cause the <code>DOCALC</code> function to be run when the
parent signals the child, using the data passed as the argument to the <tt>Signal</tt>
method.  When the calculation is
complete, the child signals back to the parent.  The child then sits in the <code>&#9109;WE</code> event
loop waiting for signals to occur (this takes no CPU resource).</p>

<p>To carry out the calculation, the parent signals the child, and then processes
events (if it is not already running under <code>&#9109;WE</code>):</p>

<pre>      ChildTask.Signal THEDATA
      &#9109;WE &#175;1
</pre>

<p>This triggers the child task to run the <code>DOCALC</code>
function, and on completion the parent's <code>CALCDONE</code> function runs:</p>

<pre>The answer is 42</pre>

<p>In a real example, the parent task would be responding to other events (for example,
user-interface events), and there might be a number of child tasks each
simultaneously working on a different run of the model.  If you have multiple child tasks running,
you can use the task ID (<code>&#9109;EV[6]</code> in index
origin 1) to distinguish between them in the callback function.</p>

<p>There might also be a queue of signals waiting to be processed on either side. Note
that there is a limit to the number of events which can be queued, typically
around 200 events.  If this maximum is
exceeded, the oldest events are thrown away.</p>

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