C++Builder 5 Developers Guide

If you recall from Chapter 16, for our InventoryKeeper DLL to share information with the various clients (the Honda factory and dealerships), Windows Messaging was used for notification through a PostMessage() call. On reception of a notification from the DLL, a client, such as Dealer or Factory , would than make a function call back into the DLL to retrieve additional information.

With COM, we can do away with PostMessage() calls and client polling by setting up a server to inform clients of new information establishing an outgoing interface specifically called a dispinterface , which is short for dispatch interface. In a moment, we will see how a dispinterface can be implemented using the Type Library Editor.

For this to work, a client, not the server, will need to implement the outgoing interfaces established by the server. Specifically, the client will need an object that implements a dispinterface . Such an object is called an event sink .

In short, an event sink is a COM object on the client-side that implements an outgoing interface ( dispinterface ) associated to a COM object within a server. The client can then connect its event sink to the server through a connection point and start receiving events. The server fires events to the client through the methods of the sink object interface. Figure 17.15 illustrates how it works.