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<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>