<concept id="GUID-8A7B837D-4069-5364-A596-686EEBAE351D" xml:lang="en"><title>Plug-In Framework Overview</title><prolog><metadata><keywords/></metadata></prolog><conbody>

<p>The Plug-in (ECom) framework is a generic framework that provides

a single mechanism to register and discover plug-ins and load an appropriate

plug-in. </p>

<section id="GUID-495BDDE0-C473-47F1-9FC2-AB73B96FD19F"><title>Purpose</title> <p>In early versions of Symbian platform,

plug-ins were provided by polymorphic interface DLLs.

All the frameworks within the Symbian platform that needed to use

plug-ins had to provide their own mechanisms for the clients to discover

and instantiate the implementations. This resulted in duplication

of functionality. </p> <p>The Plug-In framework is intended to provide

a common and system-wide mechanism for instantiating a dynamically

determined component. </p> <p>Three distinct users of the plug-in

framework are: </p> <ul>

<li id="GUID-B72B2ED5-545F-54BA-89F3-9F014A17D9D8"><p>Framework or

Application designers </p> <p>These users need to understand interface

definition design and platform security implications. </p> </li>

<li id="GUID-F0961823-257F-553D-B58E-8061ECA03FCD"><p>Plug-in writers </p> <p>These users need to understand interface implementation, DLL

creation and platform security implementation issue. </p> </li>

<li id="GUID-939853D9-4C52-5C6E-9804-E0594DE2B152"><p>Plug-in users </p> <p>These users need to understand that the plug-in framework helps

in selecting the plug-in but sometimes may need to specify which implementation

the framework should choose. </p> </li>

</ul> </section>

<section id="GUID-7AECE2EC-36C4-4225-A55F-02E9030CAEF4"><title>Required background</title> <p> <b>Run-time Binding</b>: This is called dynamic binding or late binding. The linking of

a routine or object is performed during runtime based on the requirements. </p> <p> <b>DLL (Dynamic Link Library)</b>: Is a piece of code (a library)

that is linked to a program at runtime rather than during the build

process. The code inside a DLL can be shared by several clients at

the same time without being duplicated in the mobile memory. </p> <p>In addition , the user should be aware of the<xref href="GUID-9E4D75C0-D797-5541-8E52-3C6D154CC74A.dita"> Platform Security

Considerations</xref> of the plug-in framework architecture. </p> </section>

<section id="GUID-4848DB3A-356F-4E82-8DD6-8E71D0392B70"><title>Key concepts and terms</title> <dl>

<dlentry>

<dt> Plug-in</dt>

<dd><p>In computing, a plug-in(extension) consists of a computer program

that interacts with a host application(a web browser or an email client)

to provide a very specific function on demand. </p> <p>Applications

support plugins to: </p> <ul>

<li id="GUID-E5F6B556-5957-50CE-9309-1013335A3911"><p>Enable third-party

developers to create capabilities to extend an application. </p> </li>

<li id="GUID-F882F8D5-A744-5079-9A15-DC5DD166C8BC"><p>Reduce the size

of an application. </p> </li>

<li id="GUID-6FC7254E-3CCF-58A2-810A-0B750CA178D5"><p>Separate source

code from an application. </p> </li>

</ul> </dd>

</dlentry>

<dlentry>

<dt>Plug-in System</dt>

<dd><p>A client wishes to access a plug-in. The plug-in details are

known only at run-time. The general characteristics of the plug-in

are defined in an interface. Multiple variants of the plug-in can

co-exist. </p> <p>The four clearly-defined roles of a plug-in system

are: </p> <ul>

<li id="GUID-30A0D998-79B7-5039-B4B7-602C6C74F08B"><p>The <b>Client</b> that wishes to access services. </p> </li>

<li id="GUID-8ED5CDA4-F17A-5BB2-A657-76ECE561844A"><p>The <b>Interface

Definition</b> that defines how to request services. </p> </li>

<li id="GUID-C371204A-5D56-52B9-AD55-D69BCCA5812B"><p>The <b>Interface

Implementation/Plug-in </b> that provides the required processing. </p> </li>

<li id="GUID-E89C937D-0DA1-5CDD-BDFC-6F4E3FD40FD2"><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-47C384EC-4F9E-55EA-B50B-7DAD9E990865">

<title>                   Plug-in relationships                  </title>

<image href="GUID-CBD78250-3EF4-5DF0-B356-19996E15BBBD_d0e220201_href.png" placement="inline"/>

</fig> <p>The instantiation mechanism is responsible for providing

the services that identify, and load the correct implementation at

run-time. </p> </dd>

</dlentry>

<dlentry>

<dt> Interface</dt>

<dd><p>Is an abstraction that an entity provides of itself to the

outside. This separates the methods of external communication from

internal operation. It allows internal modification and this modification

does not affect the outside entities interaction with the interface.

An interface can provide multiple abstractions. </p> </dd>

</dlentry>

<dlentry>

<dt> Interface API / Interface Definition</dt>

<dd><p>Is a template for an interface implementation. </p> </dd>

</dlentry>

<dlentry>

<dt>Interface Client/Client</dt>

<dd><p>Is an application that uses the interface definition. </p> </dd>

</dlentry>

<dlentry>

<dt>Interface Implementation Collection</dt>

<dd><p>Is a collection of interface implementations/plug-ins. </p> </dd>

</dlentry>

<dlentry>

<dt>Registry</dt>

<dd><p>Is a collection of plug-ins registered with the framework. </p> </dd>

</dlentry>

</dl> </section>

<section id="GUID-861B53D9-1928-45FB-97D0-D3DBCB31EC4F"><title>Architecture</title> <p>The plug-in framework employs

the client server architecture. The client session is a singleton

per process thread. </p> <fig id="GUID-7B34EF3B-5C56-5CA8-ABC0-60AB4F5652A1">

<title>              Plug-in framework architecture            </title>

<image href="GUID-2E3A91F9-5538-5E23-8D23-CE97B677A668_d0e220278_href.jpg" placement="inline"/>

</fig> <p>The functional components of the plug-in framework are: </p> <ul>

<li id="GUID-57437172-8AE4-5355-8776-E14C3298A6A1"><p>The <b> Client

/Interface Client</b> that invokes the available services via the

interface definition. </p> </li>

<li id="GUID-807D6C07-9202-549D-9250-17806FBE1C40"><p>The <b>Server

/ Framework</b> controls access to a registry of available implementations

that is used for interface implementation resolution and loading. </p> <p>The server facilitates: </p> <ul>

<li id="GUID-5A87C3FF-3576-5EAD-BD49-E2B498A09340"><p>Addition and

removal of interface implementations. </p> <p>The addition, removal

or modifications to an implementation is notified to the server. The

server updates the changes in the registry. </p> </li>

<li id="GUID-7D2957EE-C6BF-5687-84AA-A44C1EFBFD42"><p>Search/Access

mechanisms. </p> <p>The registry data is maintained as a hierarchical

arrangement of implementations. The complete data is scanned and matching

interface implementation list is produced as a result of search. </p> </li>

<li id="GUID-B44DA453-D953-515C-8B6C-D6F315CD03BD"><p>Resolution mechanisms. </p> <p>The best fit implementation from the resultant search is identified. </p> </li>

<li id="GUID-1ABE83FD-E2DE-5A72-BBEA-F6F216961C9D"><p>Load control. </p> <p>Controls access to a single instance of the available implementations.

The implementation identification information is used to load the

appropriate collection. </p> </li>

</ul> </li>

</ul> </section>

<section id="GUID-D9E09CF7-97F8-44F9-9557-63C1E84BD43A"><title>APIs</title> <table id="GUID-1B7014F2-7210-554B-BE78-35560FF68FE0">

<tgroup cols="2"><colspec colname="col0"/><colspec colname="col1"/>

<thead>

<row>

<entry>API</entry>

<entry>Description</entry>

</row>

</thead>

<tbody>

<row>

<entry><p> <xref href="GUID-1344F049-81C4-3D17-AF46-8B5584680ADB.dita"><apiname>REComSession</apiname></xref>  </p> </entry>

<entry><p>Resolves, instantiates, and destroys interface class implementations

requested by the plug-in framework clients. </p> </entry>

</row>

</tbody>

</tgroup>

</table> </section>

<section id="GUID-6B41C7BF-CF5C-47FF-9EC8-C23CBFE0D0AF"><title>Typical uses</title> <p>The plug-in framework allows: </p> <ul>

<li id="GUID-593D6FAD-EF95-53E8-B4C2-A60EF4A9DB1A"><p>Framework developers

to write interface definitions and extend interfaces. </p> </li>

<li id="GUID-DD0929A4-F9E4-5B70-8D3D-7D5697A09C27"><p>Plug-in writer

to implement the interfaces. </p> </li>

<li id="GUID-272F59F3-8EF6-58C4-9F61-9532D1D0EC46"><p>Plug-in user

to use the interfaces. </p> </li>

</ul> </section>

</conbody><related-links>

<link href="GUID-41D80358-473F-5B50-A09A-4F5601EAC4A9.dita"><linktext>Framework

Developer Tutorials</linktext></link>

<link href="GUID-57F29787-B425-53EA-9DB4-372B77543584.dita"><linktext>Plug-in

Writer's Tutorials</linktext></link>

<link href="GUID-DE9CB4F7-5129-5450-A6D6-9B0BA253FC81.dita"><linktext>Plug-in

User's Tutorials</linktext></link>

</related-links></concept>