Symbian3/SDK/Source/GUID-FE910347-7CC1-5241-B443-88AD3F5A96EF.dita
author Dominic Pinkman <Dominic.Pinkman@Nokia.com>
Thu, 11 Mar 2010 15:24:26 +0000
changeset 2 ebc84c812384
parent 0 89d6a7a84779
permissions -rw-r--r--
week 10 bug fix submission: Bug 1892, Bug 1897, Bug 1319. Also 3 or 4 documents were found to contain code blocks with SFL, which has been fixed. Partial fix for broken links, links to Forum Nokia, and the 'Symbian platform' terminology issues.
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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-FE910347-7CC1-5241-B443-88AD3F5A96EF" xml:lang="en"><title>Using
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Publish and Subscribe</title><shortdesc>This topic explains the operations that can be performed using
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publish and subscribe.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
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<ul>
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<li id="GUID-AE4CAAA9-9EB2-5AA0-ACB9-073EAAA9D2C1"><p> <xref href="GUID-FE910347-7CC1-5241-B443-88AD3F5A96EF.dita#GUID-FE910347-7CC1-5241-B443-88AD3F5A96EF/GUID-383A6127-FCE0-5B0E-BD54-3AAFEB097AC9">Creating and closing a handle to a property</xref>  </p> </li>
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<li id="GUID-3F0622EC-8490-585A-8CA8-09A3925E842E"><p> <xref href="GUID-FE910347-7CC1-5241-B443-88AD3F5A96EF.dita#GUID-FE910347-7CC1-5241-B443-88AD3F5A96EF/GUID-75BCC783-8DA2-519F-9E4A-B597BDC90E79">Defining a property</xref>  </p> </li>
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<li id="GUID-D6C38BDD-02CD-5D9F-8872-DD621D58DB85"><p> <xref href="GUID-FE910347-7CC1-5241-B443-88AD3F5A96EF.dita#GUID-FE910347-7CC1-5241-B443-88AD3F5A96EF/GUID-D942B400-37F8-5C02-AC95-2BD195D3348D">Deleting a property</xref>  </p> </li>
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<li id="GUID-ED1DE72F-9111-51A5-9AB2-D7112D947465"><p> <xref href="GUID-FE910347-7CC1-5241-B443-88AD3F5A96EF.dita#GUID-FE910347-7CC1-5241-B443-88AD3F5A96EF/GUID-3B2B63B8-EB2A-5845-A738-A10012A7C896">Publishing a property value</xref>  </p> </li>
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<li id="GUID-360F7C12-4909-52EE-9468-E40A3ABC944A"><p> <xref href="GUID-FE910347-7CC1-5241-B443-88AD3F5A96EF.dita#GUID-FE910347-7CC1-5241-B443-88AD3F5A96EF/GUID-CD731DD7-E833-5C67-9B90-0357C9552450">Retrieving a property value</xref>  </p> </li>
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<li id="GUID-57301501-34C1-5BE1-9AAB-36F97DBCF5C0"><p> <xref href="GUID-FE910347-7CC1-5241-B443-88AD3F5A96EF.dita#GUID-FE910347-7CC1-5241-B443-88AD3F5A96EF/GUID-A17955E9-FC4C-5FF9-8CC4-2425931E1DEB">Subscribing to, and unsubscribing from, a property</xref>  </p> </li>
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<li id="GUID-AAA0B4BB-4B01-5F5E-8617-6D02B36DCEF2"><p> <xref href="GUID-FE910347-7CC1-5241-B443-88AD3F5A96EF.dita#GUID-FE910347-7CC1-5241-B443-88AD3F5A96EF/GUID-EF683969-CED9-5986-938D-3D887E3B39BD">Usage patterns</xref>  </p> </li>
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<li id="GUID-E2D3A0BE-1FD9-5B56-928A-4265940E0DEA"><p> <xref href="GUID-FE910347-7CC1-5241-B443-88AD3F5A96EF.dita#GUID-FE910347-7CC1-5241-B443-88AD3F5A96EF/GUID-2B022BE9-2004-54E8-962E-5AB38DDC4222">Making efficient use of the user side API</xref>  </p> </li>
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</ul>
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<section id="GUID-383A6127-FCE0-5B0E-BD54-3AAFEB097AC9"><title>Creating and
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closing a handle to a property</title> <p>Some property operations require
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a reference to the property to be established first. This is done using the <xref href="GUID-C4776034-D190-3FC4-AF45-C7F195093AC3.dita#GUID-C4776034-D190-3FC4-AF45-C7F195093AC3/GUID-3F65DD85-6061-3370-9618-ECC0400323D1"><apiname>RProperty::Attach()</apiname></xref> member
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function. After a call to this function, the <xref href="GUID-C4776034-D190-3FC4-AF45-C7F195093AC3.dita"><apiname>RProperty</apiname></xref> object
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acts like a standard handle to a kernel resource. When this handle is no longer
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required, it can be released in the standard way by calling the inherited <xref href="GUID-727D2B62-09A9-3CBC-AB6F-591E52EC68EB.dita#GUID-727D2B62-09A9-3CBC-AB6F-591E52EC68EB/GUID-4B2775FD-AB61-3850-86C5-780CE8C97573"><apiname>RHandleBase::Close()</apiname></xref> member
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function. </p> <p>Note that releasing the handle does not cause the property
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to disappear. This only happens if the property is deleted. </p> <p>Note also
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that it is quite legitimate to attach to a property that has not been defined,
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and in this case no error will be returned either. This enables the lazy definition
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of properties as used in some of the usage patterns. </p> <codeblock id="GUID-A836D288-8511-5CED-B3F9-42B60569EE27" xml:space="preserve">// attach to the ‘counter’ property
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RProperty counter;
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TInt r=counter.Attach(KMyPropertyCat,EMyPropertyName,EOwnerThread);
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User::LeaveIfError(r);
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// use the counter object...
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// when finished, release the handle
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counter.Close();
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    </codeblock> </section>
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<section id="GUID-75BCC783-8DA2-519F-9E4A-B597BDC90E79"><title>Defining a
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property</title> <p>A property is defined using the <xref href="GUID-C4776034-D190-3FC4-AF45-C7F195093AC3.dita#GUID-C4776034-D190-3FC4-AF45-C7F195093AC3/GUID-58C54D2A-91E0-359B-AA31-69C6C4050173"><apiname>RProperty::Define()</apiname></xref> function,
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specifying the attributes of that property. </p> <p>A property does not need
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to be defined before it can be accessed. This supports programming patterns
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where both publishers and subscribers may define the property. Note, however
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that for security reasons, there are restrictions on the category that can
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be used when defining a property; see <xref href="GUID-DAF86036-CC40-5F26-9F15-2F2093F59C03.dita">security
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issues</xref> for more information. </p> <p>Once defined, a property persists
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in the kernel until the system reboots, or the property is explicitly deleted.
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Its lifetime is not tied to that of the thread or process that originally
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defined it. This means that, when defining a property, it is important to
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check the return code from the call <codeph>RProperty::Define()</codeph> to
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deal with the possibility that the property has previously been defined, but
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not deleted. </p> <p>The following code shows the definition of two properties: </p> <codeblock id="GUID-1F3E05DB-A34B-5D79-8512-52CAD86B0A2D" xml:space="preserve">enum TMyPropertyKeys={EMyPropertyCounter,EMyPropertyName};
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static _LIT_SECURITY_POLICY_PASS(KAllowAllPolicy);
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static _LIT_SECURITY_POLICY_C1(KPowerMgmtPolicy,ECapabilityPowerMgmt);
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// define first property to be integer type
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TInt r=RProperty::Define(EMyPropertyCounter,RProperty::EInt,KAllowAllPolicy,KPowerMgmtPolicy);
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if (r!=KErrAlreadyExists)
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    {
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    User::LeaveIfError(r);
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    }
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// define second property to be a byte array, allocating 100 bytes
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r=RProperty::Define(EMyPropertyName,RProperty::EByteArray,KAllowAllPolicy,KPowerMgmtPolicy,100);
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if (r!=KErrAlreadyExists)
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    {
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    User::LeaveIfError(r);
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    }
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. . .
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    </codeblock> <p>Once defined, a property value can change, but the property
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type cannot. Byte-array type properties can also change length provided the
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length does not exceed the maximum value of 512 bytes. The limit on the size
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of a property ensures some limit on RAM usage. </p> <p>The API allows byte-array
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and Unicode text type properties to be pre-allocated when they are defined.
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This means that the time taken to set the values is bounded. However, if the
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length of these property types subsequently increases, then memory allocation
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may take place, and no guarantees can then be made on the time taken to set
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them. </p> <p>There are further <xref href="GUID-DAF86036-CC40-5F26-9F15-2F2093F59C03.dita">security
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issues</xref> to be considered when defining a property. You need to provide
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two security policies - one to govern which processes can publish the property
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value, and the other to govern which processes can retrieve the property value.
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Security policies are instances of <xref href="GUID-81A285F6-3F87-3E77-9426-61BB16BC7109.dita"><apiname>TSecurityPolicy</apiname></xref> objects,
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although for efficiency reasons, you will almost always use the <codeph>_LIT_SECURITY_POLICY_...</codeph> macros
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to generate constant objects that behave like <codeph>TSecurityPolicy</codeph> objects.
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The API reference for <xref href="GUID-81A285F6-3F87-3E77-9426-61BB16BC7109.dita"><apiname>TSecurityPolicy</apiname></xref> provides far more
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detail on this. </p> <p>In this example, all processes will be allowed to
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retrieve the property value, but only those processes having the power management
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system capability (<xref href="GUID-76F4383B-4EA5-391A-A271-A8F65ED77EC0.dita"><apiname>ECapabilityPowerMgmt</apiname></xref>) will be allowed
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to publish the property value. </p> <p>Note that a process that defines a
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property does not have automatic rights of access to that property, other
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than to delete it. If the defining process also wishes to publish and/or subscribe
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to that property, then it must ensure that it satisfies the security policies
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that it itself has put in place when defining the property. </p> </section>
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<section id="GUID-D942B400-37F8-5C02-AC95-2BD195D3348D"><title>Deleting a
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property</title> <p>A defined property is deleted by calling <xref href="GUID-C4776034-D190-3FC4-AF45-C7F195093AC3.dita#GUID-C4776034-D190-3FC4-AF45-C7F195093AC3/GUID-226BC411-A2D2-30D1-BD86-9DDBD855C1E4"><apiname>RProperty::Delete()</apiname></xref>.
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Any outstanding subscriptions for this property will complete with <xref href="GUID-5E653C17-372C-32E1-A1B2-9E69A9991C40.dita"><apiname>KErrNotFound</apiname></xref>.
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Only the process with the correct secure ID is allowed to delete it. </p> <p>For
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example, extending the code fragment introduced above: </p> <codeblock id="GUID-D4CC3017-D17D-5E7C-AC57-0DBD8B789349" xml:space="preserve">enum TMyPropertyKeys={EMyPropertyCounter,EMyPropertyName};
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// define first property to be integer type
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TInt r=RProperty::Define(EMyPropertyCounter,RProperty::EInt);
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if (r!=KErrAlreadyExists)
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    {
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    User::LeaveIfError(r);
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    }
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// define second property to be a byte array, allocating 100 bytes
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r=RProperty::Define(EMyPropertyName,RProperty::EByteArray,100);
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if (r!=KErrAlreadyExists)
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    {
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    User::LeaveIfError(r);
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    }
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. . .
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// much later on
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. . .
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// delete the ‘name’ property
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r=RProperty::Delete(EMyPropertyName);
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if (r!=KErrNotFound)
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    {
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    User::LeaveIfError(r);
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    }
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    </codeblock> </section>
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<section id="GUID-3B2B63B8-EB2A-5845-A738-A10012A7C896"><title>Publishing
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a property value</title> <p>A property is published using the <codeph>RProperty::Set()</codeph> family
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of functions. Properties can be published: </p> <ul>
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<li id="GUID-3A136703-587F-55B9-9EE6-EE512845956A"><p>using a previously attached <xref href="GUID-C4776034-D190-3FC4-AF45-C7F195093AC3.dita"><apiname>RProperty</apiname></xref> handle, </p> </li>
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</ul> <p>or </p> <ul>
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<li id="GUID-126B0DC1-D4FC-520C-9583-50E64CBC7B80"><p>by specifying the property
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category and key with the new value. </p> </li>
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</ul> <p>The former mode is guaranteed to have bounded execution time, suitable
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for high-priority, real-time tasks, except when publishing a byte-array property
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that requires the allocation of a larger space for the new value. </p> <p>The
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latter mode offers no real-time guarantees. </p> <p>Property values are written
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atomically. This means that it is not possible for threads reading a property
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to get a garbled value. </p> <p>All outstanding subscriptions for a property
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are completed when the value is published, even if it is exactly the same
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as the existing value. This means that a property can be used as a simple
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broadcast notification service. </p> <p>Publishing a property that is not
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defined is not necessarily a programming error. The <codeph>Set()</codeph> functions
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just return an error. If this is not expected for any particular usage, then
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the error must be checked and processed by the caller. </p> <p>See the code
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fragment in the section <xref href="GUID-FE910347-7CC1-5241-B443-88AD3F5A96EF.dita#GUID-FE910347-7CC1-5241-B443-88AD3F5A96EF/GUID-CD731DD7-E833-5C67-9B90-0357C9552450">Retrieving
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a property value</xref>  </p> </section>
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<section id="GUID-CD731DD7-E833-5C67-9B90-0357C9552450"><title>Retrieving
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a property value</title> <p>The current value of a property is read using
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the <codeph>RProperty::Get()</codeph> family of functions. Properties can
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be retrieved: </p> <ul>
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<li id="GUID-81AD34BF-9A45-5DEE-AD20-AA908F5BF94A"><p>using a previously attached <xref href="GUID-C4776034-D190-3FC4-AF45-C7F195093AC3.dita"><apiname>RProperty</apiname></xref> handle, </p> </li>
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</ul> <p>or </p> <ul>
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<li id="GUID-1163D9F2-BDEA-55E1-B6FB-D9A353F4D53E"><p>by specifying the property
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category and key with the new value. </p> </li>
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</ul> <p>The former mode is guaranteed to have bounded execution time, suitable
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for high-priority, real-time tasks. </p> <p>The latter mode offers no real-time
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guarantees. </p> <p>Property values are read atomically. This means that it
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is not possible for threads reading a property to get a garbled value. </p> <p>Retrieving
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a property that is not defined is not necessarily a programming error. The <codeph>Get()</codeph> functions
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just return an error. If this is not expected for any particular usage, then
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the error must be checked and processed by the caller. </p> <p>Integer properties
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must be accessed using the overloads that take an integer or integer reference
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value, whereas byte-array properties can be accessed using the overloads that
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take a descriptor reference. </p> <p>The following code fragment shows publication
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and retrieval of a property. Note that it uses the idea of attaching to a
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property. Note also that it contains a race condition, especially if another
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thread is executing the same sequence to increment the ‘counter’ value. </p> <codeblock id="GUID-27B8D567-DD92-5FD6-B2E3-AF343C1116B1" xml:space="preserve">const TUid KMyPropertyCat={0x10012345};
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enum TMyPropertyKeys={EMyPropertyCounter,EMyPropertyName};
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// attach to the ‘counter’ property
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RProperty counter;
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TInt r=counter.Attach(KMyPropertyCat,EMyPropertyCounter,EOwnerThread);
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User::LeaveIfError(r);
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// publish a new name value
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TFileName n;
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RProcess().Filename(n);
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r=RProperty::Set(KMyPropertyCat,EMyPropertyName,n);
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User::LeaveIfError(r);
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// retrieve the first 10 characters of the name value
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TBuf&lt;10&gt; name;
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r=RProperty::Get(KMyPropertyCat,EMyPropertyName,name);
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if (r!=KErrOverflow)
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    {
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    User::LeaveIfError(r);
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    }
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// retrieve and publish a new value using the attached ‘counter’ property
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TInt count;
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r=counter.Get(count);
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if (r==KErrNone)
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    {
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    r=counter.Set(++count);
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    }
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User::LeaveIfError(r);
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// when finised, release the handle
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counter.Close();
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    </codeblock> </section>
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<section id="GUID-A17955E9-FC4C-5FF9-8CC4-2425931E1DEB"><title>Subscribing
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to, and unsubscribing from, a property</title> <p>Subscribing to a property
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is the act of making an asynchronous request to be notified of a change to
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that property. </p> <p>A thread makes a request for notification of a change
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to a property by calling the <xref href="GUID-C4776034-D190-3FC4-AF45-C7F195093AC3.dita#GUID-C4776034-D190-3FC4-AF45-C7F195093AC3/GUID-21A2FEB7-AAF1-3AC0-84B9-AB30E6CFFF99"><apiname>RProperty::Subscribe()</apiname></xref> member
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function on an already attached property object. Only one subscription request
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can be outstanding at any one time for a <xref href="GUID-C4776034-D190-3FC4-AF45-C7F195093AC3.dita"><apiname>RProperty</apiname></xref> instance. </p> <p>An
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outstanding subscription request can be cancelled by calling the <xref href="GUID-C4776034-D190-3FC4-AF45-C7F195093AC3.dita#GUID-C4776034-D190-3FC4-AF45-C7F195093AC3/GUID-326D75D3-9D54-3BD2-8B78-FAD1EEA1A1A5"><apiname>RProperty::Cancel()</apiname></xref> member
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function. This is unsubscribing from the property. </p> <p>Subscribing to
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a property is a single request to be notified when the property is next updated,
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it does not generate an ongoing sequence of notifications for every change
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to that property's value. Neither does it provide the caller with the new
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value. In essence, the act of notification should be interpreted as “Property
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X has changed” rather than “Property X has changed to Y”. This means that
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the new value must be explicitly retrieved, if required. As a result, multiple
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updates may be collapsed into one notification, and subscribers may not have
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visibility of all intermediate values. </p> <p>This might appear to introduce
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a window of opportunity for a subscriber to be out of synchronisation with
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the property value – in particular, if the property is updated again before
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the subscriber thread has had the chance to process the original notification.
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However, a simple programming pattern, outlined in the second example below
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ensures this does not happen. The principle is that, before dealing with a
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subscription completion event, an active object should re-issue the subscription
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request. </p> <p>Note that if the property has not been defined, then a subscription
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request does not complete until the property is subsequently defined and published.
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Note that the request will complete with <xref href="GUID-213DE05E-24F7-3E94-9B35-F4A72B3EBFD8.dita"><apiname>KErrPermissionDenied</apiname></xref> if
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the subscribing process does not have sufficient capability as defined by
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the <xref href="GUID-81A285F6-3F87-3E77-9426-61BB16BC7109.dita"><apiname>TSecurityPolicy</apiname></xref> object supplied by the process defining
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the property. </p> <p>If the property is already defined, then the request
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completes immediately with <xref href="GUID-213DE05E-24F7-3E94-9B35-F4A72B3EBFD8.dita"><apiname>KErrPermissionDenied</apiname></xref> if the
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subscribing process does not have sufficient capability. </p> <codeblock id="GUID-F8AB71E9-1EB4-5EFB-9D1F-38625F782BA6" xml:space="preserve">const TUid KMyPropertyCat={0x10012345};
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enum TMyPropertyKeys={EMyPropertyCounter,EMyPropertyName};
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// attach to the ‘counter’ property
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RProperty counter;
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TInt r=counter.Attach(KMyPropertyCat,EMyPropertyCounter,EOwnerThread);
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User::LeaveIfError(r);
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// wait for the previously attached ‘counter’ property to be updated
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TRequestStatus s;
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counter.Subscribe(s);
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User::WaitForRequest(s);
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// Notification complete, retrieve the counter value.
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TInt count;
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counter.Get(count);
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. . .
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    </codeblock> <codeblock id="GUID-5E726809-0060-5E76-A0D5-4AE73155724A" xml:space="preserve">const TUid KMyPropertyCat={0x10012345};
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enum TMyPropertyKeys={EMyPropertyCounter,EMyPropertyName};
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// Active object that tracks changes to the ‘name’ property
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class CPropertyWatch : public CActive
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    {
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    enum {EPriority=0};
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public:
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    static CPropertyWatch* NewL();
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private:
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    CPropertyWatch();
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    void ConstructL();
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    ~CPropertyWatch();
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    void RunL();
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    void DoCancel();
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private:
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    RProperty iProperty;
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    };
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CPropertyWatch* CPropertyWatch::NewL()
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    {
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    CPropertyWatch* me=new(ELeave) CPropertyWatch;
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    CleanupStack::PushL(me);
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    me-&gt;ConstructL();
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    CleanupStack::Pop(me);
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    return me;
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    }
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CPropertyWatch::CPropertyWatch()
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    :CActive(EPriority)
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    {}
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void CPropertyWatch::ConstructL()
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    {
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    User::LeaveIfError(iProperty.Attach(KMyPropertyCat,KMyPropertyName));
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    CActiveScheduler::Add(this);
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    // initial subscription and process current property value
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    RunL();
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    }
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CPropertyWatch::~CPropertyWatch()
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    {
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    Cancel();
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    iProperty.Close();
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    }
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void CPropertyWatch::DoCancel()
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    {
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    iProperty.Cancel();
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    }
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void CPropertyWatch::RunL()
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    {
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    // resubscribe before processing new value to prevent missing updates
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    iProperty.Subscribe(iStatus);
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    SetActive();
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    // property updated, get new value
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    TFileName n;
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    if (iProperty.Get(n)==KErrNotFound)
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        {
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        // property deleted, do necessary actions here...
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        NameDeleted();
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        }
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    else
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        {
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        // use new value ...
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        NameChanged(n);
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        }
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    }
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    </codeblock> </section>
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<section id="GUID-EF683969-CED9-5986-938D-3D887E3B39BD"><title>Usage patterns</title> <p>There
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are three usage patterns that can easily be identified, labelled as: standard
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state, pure event distribution, and speculative publishing. </p> <p id="GUID-0DFB902E-FCE1-52CF-8523-C7AE7B6768CE"><b>Standard state</b> </p> <p>This
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pattern is used for events and state that are known to be used widely in the
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system. Examples of this might be battery level and signal strength, which
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are important in every phone. </p> <p>The publisher calls <xref href="GUID-C4776034-D190-3FC4-AF45-C7F195093AC3.dita#GUID-C4776034-D190-3FC4-AF45-C7F195093AC3/GUID-58C54D2A-91E0-359B-AA31-69C6C4050173"><apiname>RProperty::Define()</apiname></xref> to
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create the appropriate property. For byte array or text properties, a size
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sufficient for all possible values should be reserved. An error of <xref href="GUID-D1D25122-F2B8-3C78-8599-84905BFD47B8.dita"><apiname>KErrAlreadyExists</apiname></xref> should
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be ignored. The publisher then publishes the property values as, and when,
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appropriate. If the <codeph>RProperty::Set()</codeph> call fails, this should
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be treated as a serious error, since it indicates that important system state
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is not getting through. Appropriate action might include panicking or rebooting
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the system. Subscribers will use <xref href="GUID-C4776034-D190-3FC4-AF45-C7F195093AC3.dita#GUID-C4776034-D190-3FC4-AF45-C7F195093AC3/GUID-21A2FEB7-AAF1-3AC0-84B9-AB30E6CFFF99"><apiname>RProperty::Subscribe()</apiname></xref> to
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request notification, and <codeph>RProperty::Get()</codeph> to retrieve the
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property value on notification. </p> <p>The memory to store the property value
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will be permanently allocated, even if it turns out that no-one in the system
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needs that value. This does ensure that the value can always be published,
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even if the system is in an out of memory situation. For this reason, this
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approach should be limited to widely used and important state. The <xref href="GUID-FE910347-7CC1-5241-B443-88AD3F5A96EF.dita#GUID-FE910347-7CC1-5241-B443-88AD3F5A96EF/GUID-01254366-696D-598F-9670-6A178F4E69F7">Speculative publishing</xref> pattern offers an approach for dealing with
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less important state. </p> <p id="GUID-F2A60E64-C279-5E08-8AA8-52DF1EEA32BD"><b>Pure event distribution</b> </p> <p>This
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pattern is used when events need to be distributed, not values. </p> <p>The
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publisher of the event simply uses an integer property, and calls RProperty::Set()
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with any value. Even if the value of the property is not changed by this operation,
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all subscribers will be notified that a Set() has occurred, and by implication
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that the related event has occurred. </p> <p>Subscribers will be able to detect
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that an event has occurred, but will get no other information. The minimum
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possible memory is wasted on storage for the dummy value. </p> <p id="GUID-01254366-696D-598F-9670-6A178F4E69F7"><b>Speculative publishing</b> </p> <p>This
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pattern is used when it is not known whether a value will be of interest to
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others or not. Unlike the <xref href="GUID-FE910347-7CC1-5241-B443-88AD3F5A96EF.dita#GUID-FE910347-7CC1-5241-B443-88AD3F5A96EF/GUID-0DFB902E-FCE1-52CF-8523-C7AE7B6768CE">standard
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state</xref> pattern, the publisher of the event does not call <xref href="GUID-C4776034-D190-3FC4-AF45-C7F195093AC3.dita#GUID-C4776034-D190-3FC4-AF45-C7F195093AC3/GUID-58C54D2A-91E0-359B-AA31-69C6C4050173"><apiname>RProperty::Define()</apiname></xref> to
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create the property. Instead, it simply calls <codeph>RProperty::Set()</codeph> as
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appropriate, and ignores any <xref href="GUID-5E653C17-372C-32E1-A1B2-9E69A9991C40.dita"><apiname>KErrNotFound</apiname></xref> error. </p> <p>When
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other code in the system, i.e. a potential subscriber, is interested in the
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state, it calls <xref href="GUID-C4776034-D190-3FC4-AF45-C7F195093AC3.dita#GUID-C4776034-D190-3FC4-AF45-C7F195093AC3/GUID-58C54D2A-91E0-359B-AA31-69C6C4050173"><apiname>RProperty::Define()</apiname></xref> to create the property
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   359
and allocate the memory for the value. An error of <xref href="GUID-D1D25122-F2B8-3C78-8599-84905BFD47B8.dita"><apiname>KErrAlreadyExists</apiname></xref> should
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be ignored, as this only indicates that some other code in the system is also
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interested in the value and has already created the property. </p> <p>The
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   362
subscriber then calls <xref href="GUID-C4776034-D190-3FC4-AF45-C7F195093AC3.dita#GUID-C4776034-D190-3FC4-AF45-C7F195093AC3/GUID-21A2FEB7-AAF1-3AC0-84B9-AB30E6CFFF99"><apiname>RProperty::Subscribe()</apiname></xref> and <codeph>RProperty::Get()</codeph> as
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   363
usual to interact with the property. On the first <codeph>Get()</codeph>,
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   364
the subscriber may retrieve the property default value (zero, or a zero length
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   365
descriptor). This must be substituted with a sensible default value for the
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   366
property in question. </p> <p>Using this pattern, no memory is wasted on properties
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   367
that have no subscribers, while the publisher code is simpler as there is
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   368
no need for configuration as to which properties to publish. </p> <p>The publisher,
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   369
however, wastes some time attempting to publish unneeded values, but this
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   370
should not be an issue unless the value is very frequently updated. </p> <p>Where
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   371
events are published very infrequently, the subscriber could have a dummy
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   372
value for a long time, until the next publish event updates the value. Often
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   373
this is not a problem as a default value can be substituted. For example a
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   374
full/empty indicator for a battery level, none for signal strength etc. This
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   375
pattern is unlikely to be useful if there is no suitable default value. </p> </section>
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   376
<section id="GUID-2B022BE9-2004-54E8-962E-5AB38DDC4222"><title>Making efficient
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   377
use of the user side API</title> <p>While the Publish and Subscribe API, as
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   378
represented by <xref href="GUID-C4776034-D190-3FC4-AF45-C7F195093AC3.dita"><apiname>RProperty</apiname></xref> is designed to be efficient, there
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   379
are certain usage patterns that can improve performance. </p> <p><b>Use
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   380
the attached version of the API calls if possible</b> </p> <p>If you intend
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   381
to call <codeph>Set()</codeph> or <codeph>Get()</codeph> repeatedly, it is
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   382
preferable to use the attached forms of the calls. The attached forms are
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   383
the ones that do not take a UID/Key parameter, and that can only be called
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   384
after calling <xref href="GUID-C4776034-D190-3FC4-AF45-C7F195093AC3.dita#GUID-C4776034-D190-3FC4-AF45-C7F195093AC3/GUID-3F65DD85-6061-3370-9618-ECC0400323D1"><apiname>RProperty::Attach()</apiname></xref>. The attached variants
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   385
are constant time operations, and execute much faster than the corresponding
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   386
unattached versions. </p> <p><b>Only
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   387
preallocate space if the publishing is time critical</b> </p> <p>For byte-array
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   388
and text properties, it is possible to pre-allocate space for the data. Doing
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   389
this results in <codeph>Set()</codeph> operations that do not exceed the preallocated
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   390
space, avoiding the need to do a memory allocation. However, if the data is
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   391
shorter than the reserved space, the excess is wasted. Since <codeph>Set()</codeph> automatically
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   392
extends the data area if needed, then the only reason to pre-allocate space
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   393
is if the <codeph>Set()</codeph> operation has to be real-time, i.e. has to
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   394
have known execution time. </p> <p><b>Always
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   395
consider using Speculative Publishing</b> </p> <p>Even in situations where
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   396
the <xref href="GUID-FE910347-7CC1-5241-B443-88AD3F5A96EF.dita#GUID-FE910347-7CC1-5241-B443-88AD3F5A96EF/GUID-0DFB902E-FCE1-52CF-8523-C7AE7B6768CE">Standard
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   397
State</xref> pattern may seem appropriate, speculative publishing may be a
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   398
better choice, specially for low-level components that know little about how
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diff changeset
   399
they are used or what the wider system configuration may be. The onus is then
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   400
on the UI/Policy layer to ensure that the appropriate properties are defined
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diff changeset
   401
early on in device boot according to policy rules it can define. This ensures
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diff changeset
   402
that the policy layers in the system maintain control and can implement a
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diff changeset
   403
wide variety of policies. </p> <p>Standard state is only relevant for properties
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diff changeset
   404
that are essential to every Symbian device. Battery level probably falls into
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diff changeset
   405
this category, signal strength may well not. </p> <p><b>Define the expected update frequency</b> </p> <p>When a property is changed,
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diff changeset
   406
all subscribers are notified. This leads to their threads running to service
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diff changeset
   407
the notification. If a property changes value frequently, it would be wasteful
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diff changeset
   408
for subscribers to perform substantial processing for each notification. </p> <p>Take
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diff changeset
   409
a property representing signal strength as an example. Potentially, this could
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diff changeset
   410
be updated several times a second. If a change in value were only used to
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   411
update the UI signal bar, it would not be harmful. However, if it were used
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diff changeset
   412
by many entities for serious processing (e.g. polling for email, sending unsent
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diff changeset
   413
SMSes, re-connecting to the internet), then such frequent updates would have
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diff changeset
   414
a severe effect on battery life. </p> <p>Nevertheless, it is obviously desirable
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diff changeset
   415
for many parts of a phone OS to know about the state of network coverage,
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and to take appropriate action. In cases like this, it may be worth the publisher
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defining multiple properties with associated update characteristics. For example,
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raw signal strength (updated &gt; 1 time/sec), periodic signal strength (updated
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once every 10s) and network coverage (updated only when moving between some
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signal and none). Each subscriber can then monitor the appropriate notification
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and so reduce the number of threads that run when the underlying value changes. </p> </section>
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</conbody></concept>