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+++ b/Symbian3/SDK/Source/GUID-30201A05-C6CE-5D34-9BDF-CDA4EE44878D.dita	Wed Mar 31 11:11:55 2010 +0100
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+<?xml version="1.0" encoding="utf-8"?>
+<!-- Copyright (c) 2007-2010 Nokia Corporation and/or its subsidiary(-ies) All rights reserved. -->
+<!-- This component and the accompanying materials are made available under the terms of the License 
+"Eclipse Public License v1.0" which accompanies this distribution, 
+and is available at the URL "http://www.eclipse.org/legal/epl-v10.html". -->
+<!-- Initial Contributors:
+    Nokia Corporation - initial contribution.
+Contributors: 
+-->
+<!DOCTYPE concept
+  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
+<concept id="GUID-30201A05-C6CE-5D34-9BDF-CDA4EE44878D" xml:lang="en"><title>ECom
+Architecture</title><prolog><metadata><keywords/></metadata></prolog><conbody>
+<p>In C++, the existence of abstract base classes and virtual functions allows
+the programs to call, or access interfaces without knowing the actual implementation.
+This mechanism gives a flexibility of writing the implementation independent
+of the interface. The implementations are known as Plug-ins. </p>
+<p>When an application wishes to use a plug-in, an object is to be instantiated
+to perform the required processing. The object specifics are not known until
+run-time. The general characteristics of the processing are known, and these
+are presented as an interface API. </p>
+<p>In early versions of Symbian platform, implementations were provided by <xref href="GUID-6482956D-C3A3-5B68-A37F-4250C85B0D82.dita">polymorphic interface DLLs</xref>.
+All the frameworks within the Symbian platform that needed to use plug-ins
+had to provide own mechanisms for the clients to discover and instantiate
+the implementations. The above method resulted in duplication of functionality. </p>
+<p>The Plug-in (ECom) Framework introduces a generic framework that provides
+a single mechanism to: </p>
+<ul>
+<li id="GUID-6460F0E0-26A3-5038-B8C2-0F4706547600"><p>Register and discover
+interface implementations </p> </li>
+<li id="GUID-4045CA55-1286-5ACD-A532-240667125469"><p>Select an appropriate
+implementation </p> </li>
+<li id="GUID-78AE02BB-13BE-5DA0-B2F3-CC376654589B"><p>Provide version control
+for plug-ins </p> </li>
+</ul>
+<section id="GUID-561303EB-F0EA-4BB5-B7BB-3AE31176E79C"><title>Essentials of a Plug-In System</title> <p>A client wishes
+to access an object to perform some processing. The specifics of this object
+are not known until run-time. The general characteristics of the processing
+are known, and are defined in an interface, but several variants of required
+processing could exist, which are provided by implementations that support
+the interface. </p> <p>There are four clearly-defined roles in such a system. </p> <ul>
+<li id="GUID-B77969AC-EE5C-53C1-8906-07CF3B2BD11A"><p>The <b>Client</b> that
+wishes to access services. </p> </li>
+<li id="GUID-2E175B1C-7A50-5A00-BBF6-C54F2FC71FFA"><p>The <b>Interface API</b> that
+defines how to request services. </p> </li>
+<li id="GUID-C8F67E1A-2175-5093-BCFD-06BD90EF59C6"><p>The <b>Interface Implementation </b> that
+provides the required processing. </p> </li>
+<li id="GUID-A6817508-C5D1-5A7A-B50A-667272292362"><p>The <b>Framework</b> that
+provides the clients with the required access to the implementations. </p> </li>
+</ul> <p>The relationships can be represented as follows: </p> <fig id="GUID-185BDADA-4576-5175-B53B-03C2633E9EA0">
+<title>              Plug-in relationships            </title>
+<image href="GUID-F78F12AE-64CA-50BD-A715-8F254FBDB793_d0e179619_href.png" placement="inline"/>
+</fig> <p>The instantiation mechanism forms the backbone of such a system,
+and is responsible for providing the services that identify, and load the
+correct interface implementation at run-time. ECom is such a framework. </p> <p>We'll
+now look at the system from the perspectives in turn of each of the interface
+client, interface implementation, and interface definition, before summarising
+the functions of the ECom. </p> </section>
+</conbody></concept>
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