Sample Application Demonstrates Using DDEML

• Microsoft Windows Software Development Kit (SDK) for Windows versions 3.1 and 3.0

SUMMARY

DMLDEMO is a file in the Microsoft Software Library that demonstrates how two applications for the Windows environment can communicate using the Dynamic Data Exchange Management Libraries (DDEML). DMLDEMO demonstrates the following types of transactions:

• Connect
• Request
• Asynchronous Request
• Hot Advise Loop
• Warm Advise Loop

• Microsoft Download Service (MSDL)

        Dial (206) 936-6735 to connect to MSDL
        Download DMLDEMO.EXE (size: 76247 bytes) 
  

• Internet (anonymous FTP)

        ftp ftp.microsoft.com
        Change to the \SOFTLIB\MSLFILES directory
        Get DMLDEMO.EXE (size: 76247 bytes) 
  

   dmldemo -d

MORE INFORMATION

The remainder of this article details the five transaction types.

Connect Transaction

The client calls the DdeConnect function to connect to the server and initiate a conversation. In the DMLDEMO application, the DdeConnect function starts a general conversation, which deals with text and bitmaps, and a SYSTEM conversation, which supports the system topic.

Request Transaction

When the client requires information from the server, the client calls the DdeClientTransaction function, specifying XTYP_REQUEST as the value for the uType parameter. The DMLDEMO client uses a request transaction to retrieve the number and type of bitmaps to display. The request transaction is used in both synchronous and asynchronous contexts.

Asynchronous Transaction

The client starts an asynchronous transaction with the server to request information that may not be ready at the time the client makes the request.

In the DMLDEMO application, the client starts an asynchronous transaction when the user chooses Asynchronous Transaction from the Transactions menu. When the server receives the request (as an XTYP_REQUEST transaction), it displays a dialog box that asks the user how many copies of the window bitmap the client should display. After the user dismisses the dialog box, the server returns a data handle containing the number the user selected from its callback function. When the server has completed the transaction, the DDEML sends an XTYP_XACT_COMPLETE to the client application's callback function. The client then displays the requested number of bitmaps.

In this case, an asynchronous transaction is required to prevent the transaction from timing out before the user has a chance to close the dialog box.

Hot Advise Loop

When a hot advise loop is established between a client and a server, the server notifies the client (through an XTYP_ADVDATA transaction) each time a certain piece of data changes. The notification includes a handle to the changed data. When a hot advise loop is active in the DMLDEMO application, the server notifies the client that the bitmap has changed (this notification includes a handle to the changed bitmap) every time the server's bitmap is changed. (To change the bitmap, choose Change Bitmap from the server application's menu.) The client application then displays the bitmap.

Warm Advise Loop

When a warm advise loop is established between a client and a server, the server notifies the client that the specified data has changed, but the server does not send the changed data to the client. In the DMLDEMO application, when the client receives a notification from the server, the client brings up a message box asking the user if the client's window should be updated with the new bitmap from the server. If the user chooses the "Yes" button, the client requests the changed bitmap from the server using the DdeClientTransaction function, specifying XTYP_REQUEST as the value of the uType parameter.