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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>⎕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>⎕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> ∇CALCDONE
[1] 'The answer is ' ⎕WARG
∇
ChildTask.onSignal←'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> ∇DOCALC;RESULT;DATA
[1] DATA←⎕WARG
[2] RESULT←RUNMODEL DATA
[3] '#' ⎕WI 'Signal' RESULT
∇
'#' ⎕WI 'onSignal' 'DOCALC'
⎕WE ¯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>⎕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>⎕WE</code>):</p>
<pre> ChildTask.Signal THEDATA
⎕WE ¯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>⎕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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