COM & .NET Component Services

The Event Class

A publisher object fires an event at COM+ (to be delivered to the subscribers) using an event class . The event class is a COM+ provided implementation of the sink interfaces the publisher can fire the events at. The implementation is synthesized by COM+, based on a type library you provide. This library contains the interface definitions and stipulates which CoClass implements them. COM+ uses the same CoClass definition for its implementation of the event classes. To publish an event, the publisher first CoCreates the event class (the publisher has to know the event class CLSID) and then fires the events at its interfaces.

For example, suppose an object wants to fire events at the sink interface IMySink, using an event class called MyEventClass. IMySink is defined as:

interface IMySink : IUnknown
{
   HRESULT OnEvent1(  );
   HRESULT OnEvent2(  );
};

The publisher code looks like:

HRESULT hres = S_OK;

IMySink* pMySink = NULL;

hres =: =:CoCreateInstance(CLSID_MyEventClass,NULL,CLSCTX_ALL,IID_IMySink,
                           (void**)&pMySink);
ASSERT(SUCCEEDED(hres));

hres = pMyEvent->OnEvent1(  );
ASSERT(hres == S_OK);

pMyEvent->Release(  );

Compare the simplicity on the publisher side to classic COM connection points—the publisher does not have to manage lists of subscribers. All the publisher has to do is create an event class and fire the event on it.

Figure 9-2 illustrates the interaction between the publisher, the event class, COM+, and the subscribers. The client creates the event class (Step ...