DDE has many potential uses in real-time data acquisition applications. Consider the example of a DDE-based, real-time system for tracking portfolios. Two hypothetical PM applications cooperate in this example. One application, named the collector, is a specialized interface that draws data from an online data service. The other application is a spreadsheet. Both applications use the DDE protocol. The following figure shows the sample spreadsheet layout:
A B C D
1 Stock Shares Price Extension
2 ABCD 1000 148 148000
3 EFGH 2000 26 52000
4 IJKL 200 24 4800
5 MNOP 2000 93 186000
6 390800
Without DDE, this spreadsheet could be updated by using the clipboard to manually copy numbers from the screen display of the collector application to the spreadsheet. This would require screen sharing or switching between applications. The user also would have to pay close attention to the price data, then undertake the data exchange personally whenever the price data changes.
With DDE, this system could be much more automatic, providing the spreadsheet with the current values for multiple data items, without user intervention. DDE enables the user to set up an exchange between the two applications that updates the spreadsheet whenever a change occurs in the value of specified stocks. After this connection is established, the cell values in the spreadsheet always reflect the most current data available from the collector. This system facilitates the timely analysis of real-time data.
The usefulness of the DDE protocol is not restricted to specialized real-time data-acquisition applications. Productivity software, in general, can benefit significantly from the protocol. For example, a monthly report is prepared using word processor, and the report includes graphs generated in a separate business-graphics package. Without DDE, someone must manually copy and paste each month's new graphs into each month's report. With DDE, the word processor can establish a permanent link to the graphics application so that any changes made to the graphs are reflected in the word-processing document, either automatically or on request.
The following diagram shows a detailed view of the workings of the DDE protocol and describes the collector and spreadsheet interaction and illustrates the forwarding of stock quotes from the collector application to the spreadsheet. For simplicity, this example is limited to the exchange of quotes for a single stock, ABCD.
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� Spreadsheet Collector �
� (Client) (Server) �
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. Start application
.
Start application .
.
Load stock-portfolio document .
.
Calls WinDdeInitiate which .
sends WM_DDE_INITIATE 컴컴컴컴컴컴컴�.
.
.
. Accepts and calls
. WinDdeRespond
. with positive
.컴컴컴컴컴컴컴컴컴 WM_DDE_INITIATEACK
.
.
Calls WinDdePostMsg which .
posts WM_ADVISE 컴컴컴컴컴컴컴컴컴컴�.
.
Request to send info each time .
data item ABCD changes; send .
in format DDEFMT_TEXT .
.
.
. Records request in
. database and calls
. WinDdePostMsg which posts
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.
.
. Whenever data changes
. for item ABCD, calls
. WinDdePostMsg which posts
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.
Retrieves information from .
shared-memory object indicated .
by pointer in WM_DDE_DATA .
.
Updates value of ABCD in .
spreadsheet .
.
When ready to end updates, .
calls WinDdePostMsg which posts .
WM_DDE_UNADVISE 컴컴컴컴컴컴컴컴컴컴�.
.
. Removes request from
. database and calls
. WinDdePostMsg which posts
.컴컴컴컴컴컴컴컴컴컴컴컴컴 WM_DDE_ACK
.
To end DDE transaction, .
calls WinDdePostMsg which posts .
WM_DDE_TERMINATE 컴컴컴컴컴컴컴컴컴컴.
.
. Responds by calling
. WinDdePostMsg which posts
.컴컴컴컴컴컴컴컴컴컴 WM_DDE_TERMINATE
The collector DDE server application is started first. Typically, applications designed to operate as dedicated DDE servers have a user interface for initialization, and then run minimized. As part of the initialization process, the collector DDE server application performs the necessary tasks (such as entering passwords and testing) to ensure that it can provide data to clients.
The spreadsheet is started next, and the stock-portfolio document is loaded. At this time, the spreadsheet calls WinDdeInitiate, which sends a WM_DDE_INITIATE message to all top-level frame windows, that is, frame windows that have HWND_DESKTOP as their parent. The WM_DDE_INITIATE message is a request to initiate an exchange with an application on a specified topic-in this case, STOCKS. An application can accept this message by responding with a positive WM_DDE_INITIATEACK message, or decline the message by passing the message on to WinDefWindowProc. If no application accepts the request, the spreadsheet assigns an error value to the external reference and its DDE activity concludes.
If the collector acknowledges the request, the spreadsheet can use the newly established exchange to request that the collector provide continuous updates on a specified data item. To make this request, the spreadsheet posts a WM_DDE_ADVISE message to the collector (actually, to a window within the collector that acts as the message recipient for DDE messages), indicating that updates must be sent every time there is a new value available for the data item, ABCD, and that the updates should be in a particular format-for example, DDEFMT_TEXT.
Upon receiving this message, the collector application records the request in its database and posts a WM_DDE_ACK message to the spreadsheet. From then on, whenever the collector receives a new ABCD stock quote, it posts a WM_DDE_DATA message to the window in the spreadsheet that initiated the exchange. Each of these messages carries a pointer to a shared-memory object that contains the data, rendered in the requested format. When the spreadsheet receives such a message, it retrieves the data from the referenced memory object and uses the data to update the value of the cell containing the external reference.
The periodic updates continue until the spreadsheet document is closed. At that point, the spreadsheet application posts a WM_DDE_UNADVISE message to the collector application, indicating that further updating is unnecessary. Upon receipt of this message, the collector application removes the corresponding data request from its database and posts a positive WM_DDE_ACK message back to the spreadsheet.
Finally, unless the spreadsheet initiates other data exchanges under this same topic, it posts a WM_DDE_TERMINATE message to the collector application, indicating the end of the DDE transaction. The collector application responds with a WM_DDE_TERMINATE message.
Note: At any time during the transaction, both the spreadsheet and collector are free to post a WM_DDE_TERMINATE message to the other application.