DDE
Dynamic Data Exchange is a feature of Windows that allows two programs to
share data or send commands directly to each other. DDE can be thought of
as a direct conversation between two application programs. In most cases,
one application is providing some form of data (either text or graphics)
to another application. The application that is the source of the data is called
the "server" and the application that is receiving the data is called the "client".
Each data item that a server application can provide has a unique identifier
consisting of three parts, a DDE Application Name, a DDE Topic, and a DDE Item Name.
The DDE Application Name is almost always the executable filename for the server
application (without the .EXE extension). The DDE Topic typically identifies a group or
category of data in the server application and each data item that a server can provide
has a unique DDE Item Name. Thus, the Application Name, Topic, and Item Name identify
the exact source of the data in a server application that is to be linked.
(Similar to a telephone number with the three parts: Area Code, Exchange and Number.)
DDE links are always initiated in the client application. The client initiates a
DDE link by broadcasting a message containing a DDE Application Name, A DDE Topic,
and optionally a DDE Item to all other applications currently running. If a server
application is running that can provide the data, it responds to the "DDE initiate"
and the Windows operating system opens a "link" between the two applications. Fortunately,
most Windows programs that support DDE insulate the user from the low level details of
establishing DDE links and simply allow you to specify the Application Name, Topic, and
Item Name for a specific piece of data and the link is then automatically established
for you by your application program.
Either application involved in a DDE conversation can terminate the link. Some
applications have menu options that allow you to selectively terminate any open
DDE links. Closing either of the linked applications also causes all links
between the two programs to be terminated.
DDE also allows a client application to send commands to a server. The types of
DDE commands, if any, that a server program can accept will vary depending on the
application and should be well documented in the application's user's manual along
with the DDE Application Name and Topic required by client programs to establish the
DDE link to the server command processor.
