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<h1>Using the child task</h1>
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<p>If the child task has a visible session window associated
with it, you can enter commands and APL expressions in it in the normal
way. Alternatively, the parent APL task
can use it as follows:</p>
<H3>Checking the status of the child task:</H3>
<p>The <tt>status</tt> read-only property
indicates the execution state of the task. It returns one of the following codes:</p>
<p><code> ¯1 </code>The task is not running (i.e. it has
not been opened, or has terminated)</p>
<p><code> 0 </code>The task is busy executing an APL
expression or command</p>
<p><code> 1 </code>The task is in desk-calculator mode,
awaiting input</p>
<p><code> 2 </code>The task is awaiting <code>⍞</code> input</p>
<p><code> 3 </code>The task is awaiting <code>⎕</code> input</p>
<p><code> 4 </code>The task is awaiting <code>⎕CC</code> input</p>
<H3>Causing
the child task to execute an expression or command</H3>
<p>The <tt>Execute</tt>
method allows the parent to cause the child task to execute an APL expression
or system command. It takes as an
argument a character string which the command to be executed (or passed as
input to <code>⍞</code> or <code>⎕)</code>.</p>
<p>The child task should be awaiting input (you can tell whether this is the case by reading the <A HREF="ch_030_030_0825.htm"><tt>status</tt></a> property, or by using the <A HREF="ch_030_050_0338.htm"><tt>onReady</tt></A> callback). If it is not, the command will be placed in a buffer and will be executed when the child task next asks for input. However, there is no queue of commands held; any existing command already in the buffer will be over-written if the child task has not yet executed it. The <tt>Execute</tt>
method returns immediately; it does not wait for the command to complete.</p>
<p>In
this example, the child task is initially ready for input. It then executes the )LOAD command, and is
busy for a short time (<tt>status</tt> property is 0). The <tt>status</tt>
property then changes back to 1 when it is ready for further input:</p>
<pre> ChildTask.status
1
ChildTask.Execute ')LOAD 10 SAMPLEEXCEL'
ChildTask.status
0
ChildTask.status
1
</pre>
<p>Note that in a real multi-tasking APLX application you would typically use 'signal'
events to allow the parent and child tasks to communicate with each other and synchronize their operations -
this is described below. The <tt>Execute</tt>
method would typically be used only when starting up the child task. </p>
<H3>Reading back the child task's session window contents</H3>
<p>The <tt>text</tt>
property of the APL object contains the text in the task's session window (assuming it is not a background task). For example, after executing the )LOAD
above you might have:</p>
<pre> CONTENTS←ChildTask.text
CONTENTS
CLEAR WS
)LOAD 10 SAMPLEEXCEL
SAVED 2002-09-12 16.43.21
APLX for Windows and Excel Sample - 18th July 2002
--------------------------------------------------
This workspace contains some functions to demonstrate how to use APLX to
access Excel spreadsheets via the Microsoft Excel OLE server.</pre>
<p><i>...
etc</i></p>
<p>It is a text vector, usually with embedded carriage return (<code>⎕R</code>) characters. The <tt>text</tt>
property returns the logical contents of the session window, even if the
session window is not visible (however it is always an empty vector for a
background task, which does not have a session window). You can also write to
this property to change the text in the session window of a child task.</p>
<H3>Uniquely identifying a child task</H3>
<p>Every task has a unique identifier. You can
read the task identifier of a child task using the read-only <tt>taskid</tt>
property. It is a scalar integer, and
can be used in event handling to identify the task:</p>
<pre> ChildTask.taskid
53429272
</pre>
<p>The <tt>taskid</tt>
property will be zero if the task is not running.</p>
<p>The child task can read its own task identifier using the <tt>taskid</tt>
property of its System object.</p>
<H3>Interrupting a child task under program control</H3>
<p>If a child task is executing (<tt>status</tt> property is 0), the <tt>Interrupt</tt>
method can be called to interrupt it. It is equivalent to a keyboard interrupt (break).</p>
<H3>Terminating a child task under program control</H3>
<p>A child task may terminate for various reasons. For example, you might use the <tt>Execute</tt> method
to cause it to execute an <code>)OFF</code> command, or it may run an APL function which
ends by executing an <code>)OFF</code>. If it has a
session window, you can end the task in the same ways as you would a top-level
task, using the File menu or by closing the session window.</p>
<p>Alternatively, a parent task can terminate the child by using the <tt>Close</tt>
method:</p>
<pre> ChildTask.Close
</pre>
<p>This closes any windows belonging to the task, releases the workspace memory, and
ends the task. The <tt>status</tt>
property reverts to <code>¯1</code>. If you wish you can then use
the <tt>Open</tt> method to start it afresh, or the <tt>Delete</tt>
method to destroy the task object altogether (or erase the last reference to it in the workspace).</p>
<p>If a parent task terminates, all its child tasks are automatically terminated as
well.</p>
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<center>Topic: <A HREF="ch.htm"><code>APLX Help</code></A> : <A HREF="ch_200.htm"><code>Multi-tasking support</code></A> : <A HREF="ch_200_040.htm"><code>Using the child task</code></A>
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