Symbian3/SDK/Source/GUID-1BD987D8-B018-51B7-80B2-4E1988841BC2.dita
author Dominic Pinkman <Dominic.Pinkman@Nokia.com>
Thu, 21 Jan 2010 18:18:20 +0000
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<?xml version="1.0" encoding="utf-8"?>
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<!-- Copyright (c) 2007-2010 Nokia Corporation and/or its subsidiary(-ies) All rights reserved. -->
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<!-- This component and the accompanying materials are made available under the terms of the License 
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"Eclipse Public License v1.0" which accompanies this distribution, 
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and is available at the URL "http://www.eclipse.org/legal/epl-v10.html". -->
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<!-- Initial Contributors:
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    Nokia Corporation - initial contribution.
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Contributors: 
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-->
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<!DOCTYPE concept
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  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
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<concept id="GUID-1BD987D8-B018-51B7-80B2-4E1988841BC2" xml:lang="en"><title>How
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to cancel requests</title><shortdesc>This document describes how to cancel a previously scheduled request.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
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<p>The <codeph>CActive</codeph> class provides the <codeph>Cancel()</codeph> member
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function to cancel an active object’s request.</p>
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<p>If no request has been issued, <codeph>Cancel()</codeph> does nothing,
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otherwise it calls <codeph>DoCancel()</codeph>. This is a pure virtual function
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that must be provided by derived classes and which handles specific cancellation
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as required by the service provider. <codeph>Cancel()</codeph> then waits
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for the completion of that request and sets the active request flag to false.</p>
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<p>The service provider must guarantee that the cancellation operation completes
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in a very short period of time.</p>
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<p>For example, a specialized timer active object, derived from <codeph>CTimer</codeph> which
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is derived from <codeph>CActive</codeph>, might implement this as:</p>
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<codeblock id="GUID-2454885F-9410-5E9F-A72E-618E78D32500" xml:space="preserve">void CTimedMessenger::DoCancel()
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    {
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      // Base class
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    CTimer::DoCancel(); 
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      // Reset variable - needed if the object is later re-activated
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    iTicksDone = 0;
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    ...
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    }</codeblock>
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<p>Note that an active object's destructor should ensure that any outstanding
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requests are cancelled. If the class implements a DoCancel() function, then
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the destructor must call the <codeph>Cancel()</codeph> member function. For
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example:</p>
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<codeblock id="GUID-1C265422-8C45-5ED2-A2F8-DFF68C08BF9C" xml:space="preserve">CTimedMessenger::~CTimedMessenger()
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    {
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    Cancel();
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    }</codeblock>
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</conbody></concept>