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<h1>Communication between child and parent tasks</h1>
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<p>Once a child task is running, events are used to communicate between the child and
its parent.  These events can arise
automatically, or more usually by explicit program action.  They are used to trigger the execution of a
callback function in the receiving task. (Note: As with all callbacks in APLX, the actual callback is run in a <code>&#9109;WE</code> (wait for events) statement).</p>

<p>The automatic events occur when the state of the child task changes.  They are as follows (in each case, <code>&#9109;EV[6]</code> in index origin 1 will contain the
unique task ID, as returned by the <tt>taskid</tt> property):</p>

<ul>
 <li>When the state of the
     child task changes from executing to awaiting input, an <tt>onReady</tt>
     event is triggered in the parent's task object.  You can associate a callback with this, for example to pass
     the next command to the child task:


<p><pre>      ChildTask.onReady&#8592;'NEXTCMD'</pre></p>

<p>This would cause the <code>NEXTCMD</code> function
to be run when the child task is ready for the next command to be sent to it
using the <tt>Execute</tt> method.</p>
</li>

 <li>When an untrapped error
     occurs during function execution (but not desk-calculator mode) in the
     child task, an <tt>onError</tt> event is triggered in the
     parent's task object.  You can
     associate a callback with this, for example to write the error to a
     diagnostic log:

    <p><pre>      ChildTask.onError&#8592;'ERROR_HANDLER'</pre></p>


<p>This would cause the <code>ERROR_HANDLER</code>
function to be run when an error is encountered in the child task.  The error message (in the same format as <code>&#9109;ERM</code>) will be available in <code>&#9109;WARG</code> during the callback.  (If you have an <tt>onReady</tt>
callback defined as well, this will be also be triggered, after <tt>onError</tt>).</p>
</li>

 <li>When the child task is
     about to execute an expression or command (either typed by the user, or
     under program control) an <tt>onExecute</tt> event is triggered
     in the parent's task object. During the callback, <code>&#9109;WARG</code>
     contains the command or expression which is about to be executed, as a
     character vector.  This can be used
     for example to write an APL tutorial application when the parent task can
     see what the user is entering in the child-task session window.</li>
	 <p></p>

 <li>Finally, when the child
     task is about to terminate for any reason, an <tt>onClose</tt>
     event is triggered in the parent's task object.
</li>
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