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13-3
Dynamic Data Exchange
DDE Terminology
In order to achieve a degree of understanding of the concepts of DDE, it is important
to have a grasp of some of the terminology used to describe the protocol. Two
applications co-operating in a DDE link are considered to be engaged in a DDE
conversation. The application that initiates the conversation is the client application;
and the responding application is the DDE server application. An application can be
engaged in several conversations at the same time and is capable of acting both as a
client in some conversations and as a server in others. The units of data that are
passed between client and server DDE applications are identified by a three level
hierarchy of item, topic and application name.
Each DDE conversation can be uniquely defined by the application name and topic.
As discussed in the previous example, at the beginning of a DDE conversation the
client and server agree upon the application name and topic. The application name is
usually the name of the server application. The topic is a general classification of
data within which multiple data items may be exchanged during conversation. For
applications that operate on file-based documents, the topic is usually a file name.
For other applications the topic is an application-specific name.
A DDE data item is the actual information related to the conversation topic that is
exchanged between applications. Values for the data item can be passed from the
server to the client, or from the client to the server.
Once a DDE conversation has begun, the client can establish one or more permanent
data links with the server. A data link is a communication mechanism by which the
server notifies the client that the value of a given data item has changed. The data
link is considered as permanent because this notification process continues until the
data link or the DDE conversation itself is terminated.
Hot & Warm DDE Links
There are two kinds of permanent data links, Hot and Warm. In the case of a Warm
data link, when the server application detects that the value of a data item has
changed, it will notify the client that a change has occurred, but will not actually send
the new value for that data item. The onus here is on the client application to decide
whether or not it will take the new value. Only upon receipt of a positive response
from the client will the server transmit the new value for the data item. In contrast to
this, with a Hot link the server will automatically pass onto the client the new value
for a data item immediately its value changes.
There are two methods acceptable to TeemTalk for setting up a link. The first is to use
the Edit menu Copy and Paste facility to copy DDE header information to the
clipboard, which a client application can then read and act upon appropriately. The
second is for the client application to directly transmit a WM_DDE_ADVISE
message to TeemTalk in the same format as defined for the clipboard copy.