Providing Services Through a Site

Services are groups of related interfaces. Typically, a service is defined for actions that are performed by a site. The service itself is defined by the site, but the interfaces that make up the service may be implemented by other objects on the site.

The service architecture provides a flexible, general means of making features accessible to contained objects. The flexibility comes from several important aspects of the design:

Interfaces that are part of a service may be implemented on the site object or on a different object.
The object that implements a service may implement other interfaces in addition to those that are part of the service.
The same interface may be part of more than one service.
Services may designate some interfaces as optional.

The following figure shows a hypothetical service implementation:

The figure shows four interfaces. The IServiceProvider interface provides access to services. All sites that support services implement this interface. ActiveX designers and other contained objects call the QueryService method of IServiceProvider to get pointers to the interfaces they want to use.

The other three interfaces illustrate hypothetical service implementations. The following table shows the various combinations of services that can be defined. Each service has a unique service identifier (SID), similar to an interface identifier (IID). The SID is a globally unique identifier (GUID) that identifies the service in calls to QueryService.

Service	Service and Interface Identifiers	Description
Montana service	SID_SMontana	Provides access to the Montana interfaces.
	IID_IMontana	Montana interface.
Washington service	SID_SWashington	Provides access to the Washington interfaces.
	IID_IWashington	Original Washington interface.
Washington2 service	SID_SWashington2	Second version of the Washington service.
	IID_IWashington	Original Washington interface. Same as IID_IWashington in SID_SWashington.
	IID_IWashington2	Second version of the Washington interface, with different features.

In the figure, the Montana service is implemented on the site object, and the two Washington services are implemented on a separate object. The following configurations also are possible:

All three services can be on the site object.
All three services can be on the same helper object.
The Montana service can be on one helper object and the Washington services can be on a different helper object.

Because the IWashington interface is the same in both SWashington and SWashington2, both of these services can be implemented on the same object.

This design allows ActiveX designers and other embedded objects straightforward access to useful features of their containers. The container, in turn, can configure these features efficiently. All configurations are transparent to the ActiveX designer, and different containers may organize the services differently.