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    10 <!DOCTYPE concept

    11   PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">

    12 <concept id="GUID-2C5D53A3-0211-5C14-BD7F-E89EB27BAD2A" xml:lang="en"><title>Active

    13 Scheduler</title><shortdesc>This document describes how active objects are queued using active

    14 schedulers.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

    15 <p>Where multiple asynchronous services are used by a program, a <i>wait loop</i> is

    16 required. </p>

    17 <p>The <codeph>CActiveScheduler</codeph> class encapsulates the wait loop. </p>

    18 <p>Nearly all threads use an active scheduler. Low-level test programs may

    19 have to construct their own active scheduler, but all server threads and all

    20 UI programs, have an active scheduler after they have initialized. If an active

    21 scheduler is present, active objects may be added to it and removed from it

    22 at will. </p>

    23 <p>A thread may not have more than one active scheduler. </p>

    24 <p>Active scheduler provides a non pre-emptive multi-tasking system, using

    25 which active objects run on a single thread. </p>

    26 <p>In Symbian platform, it is easier to write a system of co-operating active

    27 objects than a system of co-operating threads. The run-time cost of an active

    28 object is also significantly less than that of a thread. Creating and destroying

    29 active objects is <i>very</i> much more efficient than creating and destroying

    30 threads. </p>

    31 <p>When the active scheduler executes its wait loop, the order in which active

    32 objects are tested for scheduling is determined by their relative priorities.

    33 Most active objects should have a priority of zero and there should be an

    34 assumption that their relative position in the queue is irrelevant. No processing

    35 should ever depend on the order in which active objects are scheduled. </p>

    36 <section id="GUID-44BB8E7C-F310-42F5-B303-0D824AE86BA2"><title>See also</title> <p> <xref href="GUID-890F06C6-DE32-5EB1-BF0F-D41794F47AE1.dita">Active

    37 objects</xref>  </p> <p> <xref href="GUID-9C36CF76-6ED2-5674-908F-A3DF8930CEB0.dita">How

    38 to renew a request from the active scheduler</xref>. </p> </section>

    39 </conbody></concept>