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  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="GUID-2C5D53A3-0211-5C14-BD7F-E89EB27BAD2A" xml:lang="en"><title>Active
Scheduler</title><shortdesc>This document describes how active objects are queued using active
schedulers.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
<p>Where multiple asynchronous services are used by a program, a <i>wait loop</i> is
required. </p>
<p>The <codeph>CActiveScheduler</codeph> class encapsulates the wait loop. </p>
<p>Nearly all threads use an active scheduler. Low-level test programs may
have to construct their own active scheduler, but all server threads and all
UI programs, have an active scheduler after they have initialized. If an active
scheduler is present, active objects may be added to it and removed from it
at will. </p>
<p>A thread may not have more than one active scheduler. </p>
<p>Active scheduler provides a non pre-emptive multi-tasking system, using
which active objects run on a single thread. </p>
<p>In Symbian platform, it is easier to write a system of co-operating active
objects than a system of co-operating threads. The run-time cost of an active
object is also significantly less than that of a thread. Creating and destroying
active objects is <i>very</i> much more efficient than creating and destroying
threads. </p>
<p>When the active scheduler executes its wait loop, the order in which active
objects are tested for scheduling is determined by their relative priorities.
Most active objects should have a priority of zero and there should be an
assumption that their relative position in the queue is irrelevant. No processing
should ever depend on the order in which active objects are scheduled. </p>
<section id="GUID-44BB8E7C-F310-42F5-B303-0D824AE86BA2"><title>See also</title> <p> <xref href="GUID-890F06C6-DE32-5EB1-BF0F-D41794F47AE1.dita">Active
objects</xref>  </p> <p> <xref href="GUID-9C36CF76-6ED2-5674-908F-A3DF8930CEB0.dita">How
to renew a request from the active scheduler</xref>. </p> </section>
</conbody></concept>