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<task id="GUID-E10A3336-9C4C-59A5-B94F-6CECA92FFB9F" xml:lang="en"><title>Providing
a Custom Resolver </title><shortdesc>The plug-in framework has a default resolver that selects an appropriate
implementation to suit the client requests. If the algorithm used by the custom
resolver is inappropriate, an interface definition can provide its own custom
resolver. </shortdesc><prolog><metadata><keywords/></metadata></prolog><taskbody>
<context><p>Custom resolvers are interface implementations that implement
the <xref href="GUID-38D97F70-383E-3950-B052-CB56965B0B48.dita"><apiname>CResolver</apiname></xref> interface. They are instantiated by the framework
in response to the <xref href="GUID-1344F049-81C4-3D17-AF46-8B5584680ADB.dita#GUID-1344F049-81C4-3D17-AF46-8B5584680ADB/GUID-C7F147AC-6D77-3169-AAB4-B27262B4333B"><apiname>REComSession::CreateImplementationL()</apiname></xref> calls
that specify to use a particular custom resolver. Custom resolvers must confirm
to other aspects of interface implementations, including providing registration
resource information, and publishing its UID and factory function in an implementation
proxy table. </p><note> Additionally, in order to be trusted by ECom, the
plug-in must have the ProtServ capability set in the project file. </note><p>A
custom resolver class must derive from <xref href="GUID-38D97F70-383E-3950-B052-CB56965B0B48.dita"><apiname>CResolver</apiname></xref>.
The derived class must implement the following inherited functions: </p> </context>
<steps id="GUID-B8CC6618-FF31-5E9E-9C74-EE029F6A4A6F">
<step id="GUID-81A902BA-C325-556A-ACE4-1E945DD82731"><cmd/>
<info>Implement a factory function that takes an <xref href="GUID-C6201A47-85A9-365C-9B28-48243FA6ACD0.dita"><apiname>MPublicRegistry</apiname></xref> object
as input. <xref href="GUID-C6201A47-85A9-365C-9B28-48243FA6ACD0.dita"><apiname>MPublicRegistry</apiname></xref> provides the resolver with the
list of the implementations registered with the plug-in framework that meet
the requirements of the specified interface. </info>
</step>
<step id="GUID-39F2C21F-C917-5D40-B6AE-61CF1C8852E7"><cmd/>
<info>Implement the <xref href="GUID-38D97F70-383E-3950-B052-CB56965B0B48.dita#GUID-38D97F70-383E-3950-B052-CB56965B0B48/GUID-948455C3-D565-3D6E-ABD0-18E23C4D2C40"><apiname>CResolver::IdentifyImplementationL()</apiname></xref> function
that takes the UID of the interface for which an implementation is requested.
The resolver uses the resolution parameters and returns the UID of the best-fit
implementation. The best-fit algorithm is specific to each custom resolver. </info>
<info> <xref href="GUID-38D97F70-383E-3950-B052-CB56965B0B48.dita#GUID-38D97F70-383E-3950-B052-CB56965B0B48/GUID-948455C3-D565-3D6E-ABD0-18E23C4D2C40"><apiname>CResolver::IdentifyImplementationL()</apiname></xref> returns a list
of all matching implementations. </info>
</step>
<step id="GUID-3642618B-17BD-594F-8C0B-0985AC3B087F"><cmd/>
<info>Implement the <xref href="GUID-38D97F70-383E-3950-B052-CB56965B0B48.dita#GUID-38D97F70-383E-3950-B052-CB56965B0B48/GUID-5FDD82E1-9538-3393-A892-44D577EC4090"><apiname>CResolver::ListAllL()</apiname></xref> function. </info>
</step>
</steps>
<example id="GUID-E1354F45-99FB-5509-B9E4-9E0B9E44C6F3"><title>Custom resolver
example</title> <p>The following code depicts a sample customer resolver. </p> <codeblock id="GUID-552AC3FB-730E-54A6-91EC-EB5A6A65C0B1" xml:space="preserve">class CExampleResolver : public CResolver
    {
public:
    // Factory function: 
    static CExampleResolver* NewL(MPublicRegistry&amp; aRegistry);
    ~CExampleResolver();

    // Implement IdentifyImplementationL()
    TUid IdentifyImplementationL(TUid aInterfaceUid, 
    const TEComResolverParams&amp; aParameters) const;

// Implement ListAllL

    RImplInfoArray* ListAllL(TUid aInterfaceUid, 
    const TEComResolverParams&amp; aParameters) const;</codeblock> <p>The
following code shows a sample standard registration resource required by a
custom resolver. </p> <codeblock id="GUID-D4072E8E-DB0A-5B3B-ACC5-078F1C3FD83A" xml:space="preserve">RESOURCE REGISTRY_INFO theInfo
    {
    dll_uid = 0x10009DB3;
    interfaces = 
        {
        INTERFACE_INFO
 // interface_uid value is always 0x10009D90. 
            {
            interface_uid = 0x10009D90;
            implementations = 
                {
                IMPLEMENTATION_INFO
                    {
// uid value is specific to individual resolvers
                    implementation_uid = 0x10009DB4;

                    version_no = 1;
// unused fields
                    display_name = "";
                    default_data = "";
                    opaque_data = "";
                    }
                };
            }
        };
    }</codeblock> </example>
</taskbody></task>