DDE servers respond to connection requests containing a service name and a topic name. The service name of a server is the same for any conversation whereas the topic name may vary from conversation to conversation, and identifies the context of the conversation. Typically, valid topics correspond to open documents within the application, so the set of valid topics varies from time to time. In addition, all servers implement a topic called "
", which contains a standard set of items that can be read.
The LispWorks DDE interface supports three types of topics:
A general topic is an instance of a user-defined topic class. The actual set of topics available may vary from time to time as the application is running.
A dispatching topic has a fixed name, and is available at all times that the server is running. It supports a fixed set of items, and each of these items has Lisp code associated with it to implement these items.
The system topic is provided automatically by the LispWorks DDE interface. However, a mechanism is provided to extend the functionality of the system topic by handling additional items.