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<span id="Agendas"></span><div class="header">
<p>
Next: <a href="Scripts.html" accesskey="n" rel="next">Scripts</a>, Up: <a href="Scripting.html" accesskey="u" rel="up">Scripting</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index.html" title="Index" rel="index">Index</a>]</p>
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<hr>
<span id="Agendas-1"></span><h4 class="subsection">2.5.1 Agendas</h4>
<p>To schedule a task to be performed later, an “agenda” is used.
There is a default, global agenda that is ready to be used, or
additional agendas may be created for different purposes. The
following example prints the text “hello” when the agenda has
advanced to time unit 10.
</p>
<div class="example">
<pre class="example">(at 10 (display "hello\n"))
</pre></div>
<p>Most of the time it is more convenient to schedule tasks relative to
the current time. This is where <code>after</code> comes in handy:
</p>
<div class="example">
<pre class="example">(after 10 (display "hello\n"))
</pre></div>
<p>Time units in the agenda are in no way connected to real time. It’s
up to the programmer to decide what agenda time means. A simple and
effective approach is to map each call of the update procedure
(see <a href="Kernel.html">Kernel</a>) to 1 unit of agenda time, like so:
</p>
<div class="example">
<pre class="example">(define (update dt)
(update-agenda 1))
</pre></div>
<p>It is important to call <code>update-agenda</code> periodically, otherwise
no tasks will ever be run!
</p>
<p>In addition to using the global agenda, it is useful to have multiple
agendas for different purposes. For example, the game world can use a
different agenda than the user interface, so that pausing the game is
a simple matter of not updating the world’s agenda while continuing to
update the user interface’s agenda. The current agenda is dynamically
scoped and can be changed using the <code>with-agenda</code> special form:
</p>
<div class="example">
<pre class="example">(define game-world-agenda (make-agenda))
(with-agenda game-world-agenda
(at 60 (spawn-goblin))
(at 120 (spawn-goblin))
(at 240 (spawn-goblin-king)))
</pre></div>
<dl>
<dt id="index-make_002dagenda">Procedure: <strong>make-agenda</strong></dt>
<dd><p>Return a new task scheduler.
</p></dd></dl>
<dl>
<dt id="index-agenda_003f">Procedure: <strong>agenda?</strong> <em>obj</em></dt>
<dd><p>Return <code>#t</code> if <var>obj</var> is an agenda.
</p></dd></dl>
<dl>
<dt id="index-current_002dagenda">Procedure: <strong>current-agenda</strong></dt>
<dt id="index-current_002dagenda-1">Procedure: <strong>current-agenda</strong> <em>agenda</em></dt>
<dd><p>When called with no arguments, return the current agenda. When called
with one argument, set the current agenda to <var>agenda</var>.
</p></dd></dl>
<dl>
<dt id="index-with_002dagenda">Syntax: <strong>with-agenda</strong> <em>agenda body …</em></dt>
<dd><p>Evaluate <var>body</var> with the current agenda set to <var>agenda</var>.
</p></dd></dl>
<dl>
<dt id="index-agenda_002dtime">Procedure: <strong>agenda-time</strong></dt>
<dd><p>Return the current agenda time.
</p></dd></dl>
<dl>
<dt id="index-update_002dagenda">Procedure: <strong>update-agenda</strong> <em>dt</em></dt>
<dd><p>Advance the current agenda by <var>dt</var>.
</p></dd></dl>
<dl>
<dt id="index-schedule_002dat">Procedure: <strong>schedule-at</strong> <em>time thunk</em></dt>
<dd><p>Schedule <var>thunk</var>, a procedure of zero arguments, to be run at
<var>time</var>.
</p></dd></dl>
<dl>
<dt id="index-schedule_002dafter">Procedure: <strong>schedule-after</strong> <em>delay thunk</em></dt>
<dd><p>Schedule <var>thunk</var>, a procedure of zero arguments, to be run after
<var>delay</var>.
</p></dd></dl>
<dl>
<dt id="index-schedule_002devery">Procedure: <strong>schedule-every</strong> <em>interval thunk [n]</em></dt>
<dd><p>Schedule <var>thunk</var>, a procedure of zero arguments, to be run every
<var>interval</var> amount of time. Repeat this <var>n</var> times, or
indefinitely if not specified.
</p></dd></dl>
<dl>
<dt id="index-at">Syntax: <strong>at</strong> <em>time body …</em></dt>
<dd><p>Schedule <var>body</var> to be evaluated at <var>time</var>.
</p></dd></dl>
<dl>
<dt id="index-after">Syntax: <strong>after</strong> <em>delay body …</em></dt>
<dd><p>Schedule <var>body</var> to be evaluated after <var>delay</var>.
</p></dd></dl>
<dl>
<dt id="index-every">Syntax: <strong>every</strong> <em>interval body …</em></dt>
<dt id="index-every-1">Syntax: <strong>every</strong> <em>(interval n) body …</em></dt>
<dd><p>Schedule <var>body</var> to be evaluated every <var>interval</var> amount of
time. Repeat this <var>n</var> times, or indefinitely if not specified.
</p></dd></dl>
<p>It is also possible to schedule things that are not dependent on how
much time passes. The agenda will periodically poll to see if any
registered conditions are met.
</p>
<dl>
<dt id="index-call_002dwhen">Procedure: <strong>call-when</strong> <em>pred thunk</em></dt>
<dd><p>Call <var>thunk</var> sometime in the future when <var>pred</var> is satisfied
(returns a value other than <code>#f</code>.)
</p></dd></dl>
<hr>
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