Advanced MFC Programming

Chapter 16. Context Sensitive Help
hszAppName Handle of the item name, must be obtained by calling function
::DdeCreateStringHandle()
The server must check if it supports the topic and the specific item, as well as the data format. If it
supports all of them, the server must prepare data using the required format and send the data back to client.
Preparing Data
One way to send data to the client is to prepare data in server’s local buffers, then create a handle for
this data, and send the handle to the client. The data handle can be obtained by calling function
::DdeCreateDataHandle(…), which has the following format:
HDDEDATA DdeCreateDataHandle
(
DWORD idInst,
LPBYTE pSrc,
DWORD cb,
DWORD cbOff,
HSZ hszItem,
UINT wFmt,
UINT afCmd
);
The data can also be sent through a pointer. We will discuss this in a later section.
The following table explains the meanings of the above parameters:
Parameter
idInst
pSrc
cb
cbOff
hszItem
wFmt
afCmd
Meaning
Instance identification obtained from DDE initialization
Pointer to the buffers which hold the data
The length of the data in bytes
Specify the offset from the beginning of the buffer
String handle, indicating the item name
Indicates the data format
The creation flags which indicates who is responsible for freeing the data
We can use standard clipboard format such CF_TEXT, CF_DIB to pass data. If we define a special data
format, we must register it before passing the data.
Receiving Data
Because data is not sent directly, the client must obtain the required data from the handle first. For
synchronous transmission mode, the data handle will be returned directly from function
::DdeClientTransacton(). In case the transaction is not successful, a NULL value will be returned.
After receiving the handle, the client must first call function ::DdeAccessData(…) to access the data.
After the data is processed, function ::DdeUnaccessData(…) must be called to “unaccess” the data. If the
data is created with HDATA_APPOWNED flag, the client should not free the data. Otherwise, it can release the
data by calling function ::DdeFreeDataHandle(…).
Samples
In the sample application, an item “Time” is supported by the server under “Topic” topic name. When
the client request an XTYPE_REQUEST transaction on this item, the server get the current system time and
send it to the client. The client then displays the time in one of its edit box.
Compared with the samples in the previous section, a new edit box and a button labeled “Request” are
added to the dialog box (Figure 15-3). The edit box is read-only, which will be used to display the time
obtained from the server. The button is used to let the user initiate the request transaction.
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