Symbian3/SDK/Source/GUID-CCCEC352-3A49-534C-9551-A4CAE1A3475E.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-CCCEC352-3A49-534C-9551-A4CAE1A3475E" xml:lang="en"><title>How
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to handle a single asynchronous request</title><shortdesc>This document describes how to handle a single asynchronous request.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
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<p>A thread that requests an asynchronous service from another thread can
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continue processing but eventually reaches a point where it must wait until
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the asynchronous request is complete before it can resume processing.</p>
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<p>Typically, the requester performs the following sequence:</p>
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<ul>
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<li id="GUID-0A1C409D-80A4-5EC0-AC59-A3D6C7E5228D"><p>creates an object of
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type <codeph>TRequestStatus</codeph> to monitor the state of the request.</p> </li>
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<li id="GUID-BBCD758B-2B52-5D83-9C13-FC0204600563"><p>calls the asynchronous
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provider's request function, passing the <codeph>TRequestStatus</codeph> object;
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all asynchronous functions are prototyped to take a <codeph>TRequestStatus</codeph> parameter.</p> </li>
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<li id="GUID-E874E08B-19EF-5887-A7CC-DED65B5A84B4"><p>uses the operating system
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function <codeph>User::WaitForRequest()</codeph> to allow the thread to wait
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for completion of the request.</p> </li>
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</ul>
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<codeblock id="GUID-6425F8F3-7196-56C7-9886-F599EEA9E07D" xml:space="preserve">TRequestStatus status;                        // Request status object
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someProvider.IssueRequest(parameters,status); // Issue the request
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...
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User::WaitForRequest(status);                 // Wait for completion
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if (status==KErrNone)
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    {
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    /* success */
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    }
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else if (status==KErrXxx)
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    {
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    /* check for some error */
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    }
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else // check for other error conditions etc.
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...</codeblock>
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<section id="GUID-E8B90FAF-1891-49EE-8915-D9265F8884AB"><title>Notes</title> <ul>
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<li id="GUID-99618D33-6281-5C32-B54B-4451F6228C85"><p>there is no special
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name for a request function; the request performed depends on the class, the
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function name and the parameters passed.</p> </li>
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<li id="GUID-F1D8C498-3A72-5C02-892B-8DB1792D651F"><p>when the request is
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complete, an integer is stored in the request status object to convey additional
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information about the completion of the function. The meaning of the completion
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code varies from function to function. Additionally, information returned
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by the request may be returned to reference parameters passed to the request
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function.</p> </li>
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<li id="GUID-E87D9AED-C897-5F19-8E34-A43C2E7AFF35"><p>The integer completion
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code must not be <codeph>KErrPending</codeph>. By convention, <codeph>KErrNone</codeph> indicates
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no error and <codeph>KErrCancel</codeph> indicates a cancelled request. Other
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values may be used for specific types of error.</p> </li>
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</ul> </section>
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</conbody></concept>